Final Report for the

OTAG Air Quality Analysis Workgroup

 

VOLUME II: SUMMARY AND INTEGRATION OF RESULTS

 

 

 

 

INTRODUCTION

 

Background

 

During the summer months, high ground-level ozone concentrations are observed within and downwind of many of the large urban areas in the eastern US. Peak hourly average concentrations exceed the current National Ambient Air Quality Standard (NAAQS) for ozone of 0.12 ppm and peak 8-hour average concentrations rise above the recently proposed revised ozone NAAQS level of 0.08 ppm. Figure 1 shows nonattainment areas with respect to the current 1-hour NAAQS; Figure 2 shows those areas with design values in excess of the proposed 8-hour standard. A significant feature of ozone concentrations in the East is that, in contrast to other parts of the country, ozone concentrations well in excess of the 30 - 40 ppb tropospheric background level are observed throughout most of the eastern U.S., including locations outside of the major urban nonattainment areas. As a result of these large-scale elevated ozone events and the proximity of nonattainment areas to one another, many states have found that NAAQS attainment cannot be achieved with any reasonable level of local emission control measures and therefore that a regional, multi-state emission control strategy is necessary. The Ozone Transport Assessment Group (OTAG) was formed to deal with this issue.

 

 

Air Quality Analysis Work Group Activities

 

Within OTAG’s Modeling and Assessment Subgroup, the Air Quality Analysis Workgroup (AQAWG) was formed to evaluate and perform analyses of air quality and meteorological data relevant to OTAG’s mission. Data analyses conducted by AQAWG members complement work performed under the auspices of the Regional and Urban Scale Modeling Workgroup. AQAWG members contributed numerous analyses as listed in Table 1. Some of the analyses were based on ozone and precursor data from intensive field measurement programs such as the Nashville/Middle Tennessee Ozone Study and the NARSTO-Northeast study that provide detailed spatial and temporal coverage of ozone, ozone precursor and meteorological conditions both at the surface and aloft during a limited number of individual ozone episode events. Other analyses were based on research-grade ozone and precursor monitoring programs conducted over multi-year periods at a limited number of locations such as the SOS/SCION network. Still other analyses were based on the routine ozone and meteorological data which is collected at regular time intervals at a large number of sampling sites located throughout the OTAG domain. The latter two types of analyses were designed to identify major features of the ozone climatology within the OTAG domain which provide insights into ozone/precursor relationships and transport issues. A major advantage of these results is that, in contrast to modeling studies and analyses based on intensive measurements of individual ozone episodes, they provide information on ozone, ozone/precursor relationships, and ozone and precursor transport over the full range of meteorological conditions in the OTAG domain.

 

Analyses contributed by AQAWG members were presented and discussed during monthly AQAWG meetings. Comments made during these meetings frequently resulted in significant revisions and additions. Formal written reports are currently available for many of the analyses. Most of these reports have been posted on the AQAWG world wide web site (http://capita.wustl.edu/OTAG) for widespread distribution and additional discussion and review. Based on these reviews, summaries of many of the analyses were prepared. These summaries, which appear in the final section of this volume, include brief discussions of study purpose, methods, results, technical limitations, and the implications of the study’s findings for OTAG. Draft summaries were provided to the study authors for review and comment before being finalized. Information contained in the final reviews was then used as the basis for an integrated assessment of analysis results as presented in the next section.

 

Unfortunately, due to resource constraints, it was not possible to prepare summaries for all of the analyses which were presented to or discussed by the Workgroup. Table 1 indicates which analyses were summarized and which were not. Although the following discussion is based primarily on the analyses for which summaries were prepared, references are made in some cases to major results from some of the other studies. It should be recognized, however, that these results were not subjected to the same AQAWG review process as those from studies for which summaries are provided here, although in some cases these studies may have been reviewed by other groups.

 

 

Overview of Regional Ozone and Transport in the Eastern United States

 

The regional nature of elevated ozone episodes in the eastern U.S. has long been recognized (see, for example, NRC, 1991; OTC, 1994; SOS, 1996; Husar, 1996). A review of measurement studies and statistical data analyses conducted by the National Research Council (NRC) (NRC, 1991) identified elevated ozone events on spatial scales exceeding 600,000 km² lasting from a few days to as long as a week with concentrations exceeding at least 80 ppb for a major portion of each day. The most wide spread regional episodes were found to be associated with slow moving high pressure weather systems that produced high temperatures, light winds, limited vertical mixing, and mostly clear skies. Air movement around the high, the slow (typically eastward or northeastward) migration of the high, and the influence of transient weather features on the fringes of the high can result in transport of elevated ozone and precursor concentrations over significant distances. Within this regional "sea" of elevated ozone, imbedded urban ozone plumes consisting of significantly higher concentrations can be found. Surface and aloft observations have shown that these urban plumes can be distinguished over a period of 12 hours covering an area of up to 8,000 km² (NRC, 1991).

 

Given the observational evidence cited above, it is clear that transport mechanisms acting to produce the large-scale regional ozone excess above tropospheric background in the eastern U.S. have the potential to contribute significantly to exceedances of the current 1-hour or proposed 8-hour ozone standards. The direction and spatial extent of transport and the relative contribution of transported ozone and precursors to individual ozone exceedances is highly variable. On the one hand, some episodes, such as the July 11-15, 1995 Nashville episode analyzed by Meagher (1996) may be entirely local and not involve any transport. During this episode, ozone concentrations in downtown Nashville reached 138 ppb, more than 55 ppb above the daily maximum value at any surrounding site and about 80 ppb above values near the boundary of the metropolitan area. On the other hand, as indicated by Blumenthal et al. (1997), a number of one day and multi-day transport mechanisms have been observed along the coastal plain in the Northeast, some or all of which may be involved in any given ozone exceedance episode. These mechanisms include:

  Near surface flows acting within the first few hundred meters above ground level where surface frictional, radiative and sensible heating processes are important. In coastal areas, this includes offshore, over water, and onshore flows; orographic effects (drainage flows, blocking, channeling) are also important in areas with significant terrain.

 

Boundary layer synoptic flows occurring within the daytime planetary boundary layer but above the surface layer at heights (typically 800 - 1500 m) where surface influences are less important and synoptic scale pressure gradient forces are relatively more important.

 

Channeled nighttime flows in low level jets which form under stable conditions just above the nocturnal boundary layer (typically 200 - 800 m above sea level under episode conditions in the Northeast) and are enhanced by the channeling effects of major terrain features (such as the Appalachians in the Northeast).

  Within any given episode, each of the above flow features may be characterized by different directions, average speeds, and temporal and spatial variations. Surface flows are typically more sluggish and more localized than the synoptic flows and exhibit a strong diurnal pattern. In most cases, winds are calm or near calm during the nighttime and early morning hours under episode conditions. Surface flows are thus primarily involved in near-field transport whereas the synoptic flows are capable of transporting material for two or more days over considerably larger distances. The relatively high speed channeled nighttime flows can move material long distances overnight close to the surface where it is available to be mixed down to the surface as the mixing depth increases the next morning. Based on surface and upper air observations of winds and back trajectory calculations during Northeastern ozone episodes, Blumenthal and co-workers conclude that near surface transport along the urban corridor can cover distances of 50 - 250 km from morning until evening, boundary layer synoptic flows can transport material 200 - 600 km across the Appalachians in a 24-hour period, and channeled nighttime flows such as those observed by Korc et al. (1997) can transport material 200 - 400 km overnight.

 

While not all of the three basic flow regimes described above will be important at all locations, some or all may play important roles in ozone and precursor transport in various nonattainment regions throughout the OTAG domain. Thus, the factors contributing to transport are quite complex. The AQAWG conducted analyses designed to identify the relative contributions of these transport mechanisms to ozone exceedances at different locations in the OTAG domain.

 

 

INTEGRATED SUMMARY OF WORKGROUP ANALYSES

 

Analyses conducted by the AQAWG provide information in six key areas:

  The spatial and temporal pattern of ozone in the OTAG domain.

 

The transport of ozone and ozone precursors within the eastern U.S.

 

The effectiveness of VOC and NO_x emission control strategies in reducing ozone concentrations.

 

The potential impact of an 8-hour average ozone standard on transport and attainment issues.

 

The degree to which ozone episodes selected for photochemical modeling of regional control strategies are representative of the range of conditions associated with high ozone events. Evaluation of the accuracy of UAM-V photochemical model predictions of base-case ozone and precursor concentrations.

  An integrated summary of AQAWG analysis results as they pertain to these six key areas is provided in the following sections. This is followed by a discussion of recommendations for future analyses. Summaries of individual analyses are provided in the final section of this volume.

 

 

Spatial and temporal pattern of ozone in the OTAG domain

 

To better understand the temporal and spatial patterns of regional ozone levels associated with transport impacts in the eastern U.S., an integrated set of ozone monitoring data from both the primarily population-oriented AIRS network (consisting of the National Ambient Monitoring Station and the State and Local Ambient Monitoring Station networks) and several rural networks (CASTNet, IMPROVE, SCION, EMEFS) was constructed. Analyses of this database revealed the following key features:

  Average one-hour daily maximum ozone concentrations in the "corners" of the OTAG domain (south Florida, south Texas Gulf Coast, Upper Midwest/northern Plains, and northern Main) are 30 - 40 ppb which is generally consistent with northern hemisphere background levels. Concentrations along much of the southern portion of the western OTAG boundary average slightly higher (55 - 65 ppb). It is unclear why values are higher in this location but these results could reflect a relatively larger influence of anthropogenic emissions on the sparse monitoring network in this region. Concentrations along the eastern seaboard from Maryland to New York (which is primarily an outflow boundary) are higher, reaching above 70 ppb in the New York/New Jersey area.

 

Average concentrations in the "corners" of the OTAG domain boundaries tend to be spatially uniform and exhibit relatively little day-to-day variability, consistent with a tropospheric background source.

 

Seasonal mean daily maximum 1-hour concentrations within the OTAG domain range from 80 ppb near urban areas to 40 ppb near the domain boundaries. The domain average seasonal mean daily maximum 1-hour concentration is approximately 60 ppb. Seasonal mean daily maximum 1-hour concentrations are highest in the Washington - New York corridor with "hot spots" near several other major metropolitan areas. In addition, a broad region of 65 - 70 ppb averages covers the area just north of the Ohio River.

  Although no effort was made to estimate the flux of ozone and precursors across the OTAG domain boundary, these observations suggest that, with the possible exception of the international border along the Quebec-Windsor corridor, air masses entering the OTAG domain contain little or no ozone burden above tropospheric background levels (and precursor fluxes into the domain are likely to be equally negligible). Thus, ozone concentrations above approximately 35 ppb observed within the OTAG domain must be generated by sources within the domain. Due to the pervasive influence of anthropogenic activities, present-day monitoring data cannot provide any information on the likely range of concentrations which would be observed in the OTAG domain in the absence of any anthropogenic influences. While seasonal mean daily maximum concentrations along some parts of the OTAG boundary where anthropogenic influences are minimized are in the 30 - 40 ppb range as noted above, climatic conditions and biogenic inventories within the central portion of the domain differ markedly from those in these boundary regions and may conspire to produce somewhat higher "natural" background ozone concentrations. On the other hand, some modeling estimates of pre-industrial ozone levels suggest that lower values might be expected in the absence of any anthropogenic influence (e.g., Levy et al., 1997) but these have not been reviewed for this report.

 

Additional findings from the spatial and temporal pattern analyses include:

  On low ozone days (10th percentile of the concentration distribution), concentrations are similar in urban and rural areas, are below about 30 ppb in the "corners" of the domain, and remain slightly higher (42 - 47 ppb) over a broad area in the central portion of the domain.

 

On high ozone days (90th percentile of the concentration distribution), concentrations exceed 100 ppb along the Northeast corridor and near other major metropolitan areas. Concentrations are also above about 90 ppb from the Ohio River northward through central Indiana and much of Ohio and western Pennsylvania in contrast to the lower values seen in most of the surrounding regions.

  The total number of exceedances of a 1-hour average 120 ppb concentration level across the entire OTAG domain increases during the course of the week from Monday through Friday and then declines sharply on the weekends. On Sundays, the number of exceedances are one third of the number on Fridays. Large decreases are also observed in the 90th percentiles of the daily maximum 1-hour concentrations. The largest Sunday decreases in the 90th percentiles (more than 15 ppb lower than on Fridays) are observed near the major metropolitan areas whereas 90th percentiles on Sundays are slightly higher (5 ppb) in a band stretching along and somewhat to the south of the Ohio River Valley. This feature may be the result of differences in the weekly pattern of emissions in this region as compared to those in major urban areas located outside of this region.

 

  Transport Analyses

 

Several analyses conducted by the AQAWG focussed on identifying the spatial and temporal extent and magnitude of the transport of ozone and precursors. Measurements of ozone, precursors, and meteorological conditions conducted as part of both the 1995 SOS Nashville/Middle Tennessee ozone study, and the 1995 NARSTO Northeast field study were analyzed for this purpose. In addition, long-term data sets of routine air quality and meteorological measurements were used to examine spatial ozone and temperature correlation patterns and to compute air parcel trajectories. Daily back trajectories ending at 23 monitoring sites located throughout the OTAG domain were computed using seven summers (1989 - 1995) of three-dimensional gridded wind fields and variations in the trajectory patterns with ozone concentrations at the receptor sites were analyzed. Principal findings of these analyses are summarized below.

 

One day transport of ozone within plumes from urban areas and major power plants has been demonstrated using aircraft data (NRC, 1991 and references therein). In addition, impacts of distinct power plant plumes and urban plumes have been noted at several rural sites in the Southeast (Edgerton and Hartsell, 1996). Aircraft measurements of NO_x, NO_z, and O₃ made within the Cumberland and Johnsonville power plant plumes located near Nashville (Meagher, 1996) indicated that, during these summer daytime measurement periods, essentially all of the NO_x is converted to NO_z within 30 - 100 km of the source. Thus, ozone production due to NO_x in the plume ceases within 100 km of the power plant. These results differ somewhat from those the earlier Tennessee Plume Study in which aircraft measurements of the Cumberland plume conducted in August, 1978 indicated ozone production continuing beyond 110 km to at least 160 km (Gillani and Pleim, 1996). Differences in the distance estimates between these two studies may be related to differences in mean plume transport wind speeds, as well as other meteorological and air quality factors. It is important to note that, under the right conditions, power plant plumes may travel relatively long distances overnight with little loss of NO_x and thus be available to participate in photochemical reactions at distant locations on the following day. Trainer (1993) shows that ambient O₃/NO_z concentration ratios provide a measure of the efficiency of ozone production, i.e. essentially the amount of ozone produced during the lifetime of each NO₂ molecule. Planetary boundary layer measurements of O₃/NO_z ratios from aircraft and surface sites made within the power plant plumes and the Nashville urban emissions plume indicate that ozone formation efficiency as measured by this ratio is 65 - 70 percent greater in the urban plume as compared to the power plant plumes (Meagher, 1996). Thus, on a per unit emissions basis, urban area source NO_x emissions may be more important to same day ozone formation than elevated point source NO_x emissions.

 

Conditions during ozone episodes are conducive to longer transport distances over one to two day periods in the northeastern portion of the OTAG domain (coinciding with higher average wind speeds and a more organized flow pattern) than in the southeastern portion of the domain where average wind speeds are lower and the flow pattern is less well organized. Evidence of this can be found in the "source regions of influence" and residence time analyses conducted by Schichtel and Husar (1997, 1996), and in the residence time analysis, autocorrelations, and lagged inter-regional correlations computed by Poirot and Wishinski (1996a,b,c).

 

Analyses of the July 11-15 ozone episode in Nashville (Meagher, 1996) indicate that this episode was primarily a result of local emissions - the area of high ozone (with concentrations up to 80 ppb above a 60 - 70 ppb regional background) was restricted to the immediate metropolitan area and did not extend away from it in any direction. However, the frequency of such "home grown" episodes vs. regional episodes was not examined.

 

Porter et al. (1996) present spatial correlation results for the short-term (deseasonalized) ozone component that suggest a spatial scale on the order of 560 - 640 km (350 - 400 miles) for same-day formation and transport. Schichtel and Husar’s source regions of influence analysis indicates a scale of 300 - 600 km for one day transport times and 450 - 1000 km for a two day transport times in the northern half of the OTAG domain. In the southern half of the domain, one day scales were found to be somewhat less: 200 - 400 km (one day) and 300 - 800 km (two day). Similarly, analysis of typical transport distances and trajectory residence times for days in the upper 20 percent of daily maximum ozone in each portion of the OTAG domain show relatively large transport distances and short residence times in the Northeast and short transport distances and long residence times in the South. Interpretation of these results is subject to two important caveats:

  1. Given the somewhat weak but nevertheless significant correlations of Porter’s short-term ozone component with temperature and the likely influence of other meteorological factors on day-to-day variations in the short-term ozone component, it is not possible to distinguish the degree to which the spatial scales implied by this analysis are representative of actual ozone and precursor transport or merely representative of correlations in meteorological conditions between locations.

 

2. The spatial scales obtained from Schichtel and Husar’s source regions of influence analysis represent the typical distances air parcels are estimated to travel over one and two day intervals and provide no information on the relative contributions of precursor sources to ozone concentrations at the given downwind distances.

 

  Trajectory model results do not show actual spatial transport scales but do indicate potential source regions along the pathways of air parcels associated with above average ozone at receptor sites. Back trajectory analyses conducted by Poirot and Wishinski (1996a,b,c) indicate that air parcels originating outside of the OTAG domain are associated with below average ozone at various locations throughout the OTAG domain. In contrast, air parcels associated with above average ozone concentrations at these receptor sites predominantly originate from the heart of the OTAG domain. Remarkably, this was found to be the case no matter where in the domain the receptor site is located. Of course, it should be noted that the trajectory analysis does not take into consideration atmospheric chemical processes, the injection of precursor emissions, or the deposition of material along the trajectory path. Therefore, while these results provide consistent circumstantial evidence that emissions from the central portion of the domain contribute in some way to above average ozone events throughout the domain, the relative contribution of precursor emissions along various portions of the trajectory paths leading to the receptor sites (i.e., emissions originating near the upwind end of the trajectory vs. emissions originating near the receptor site) are not known.

 

 

Development of Control Strategies

 

Comparisons of historical trends in ozone and precursor concentrations with trends in precursor emission levels can provide insights into the relative effectiveness of control strategies. Unfortunately, trends in the annual extreme ozone concentrations of interest are sensitive to the influences of interannual variations in meteorological conditions, movement of monitoring sites, missing data, and random fluctuations in concentrations which can make identification of consistent trends difficult. Furthermore, high quality data on ambient precursor trends is extremely limited and estimates of precursor emissions are subject to potentially significant biases. Despite these difficulties, several important features can be noted in the trends of ozone and precursor emissions between the early to mid-1980s and the mid-1990s as described below.

  After adjusting for meteorological variations, ozone concentrations decreased significantly in all of the major metropolitan areas in the Northeast urban corridor from Washington to Boston between 1984 and 1995. Total declines were less in Washington and Baltimore than in the other cities (Chinkin et al., 1996; Rao et al., 1995).

  Porter et al. (1996) showed declines in deseasonalized, temperature adjusted daily maximum 1-hour ozone concentrations between 1985 and 1995 at a majority of monitoring sites throughout the OTAG domain although increases were noted for a significant minority of sites. No inter-regional differences in trends can be discerned from the Porter et al. (1996) report. However, a similar analysis of 1983 - 1992 trends (Rao et al., 1995) for the northern half of the OTAG domain showed statistically significant downward trends at most northeastern sites. Trends were also exclusively or predominantly negative in all of Pennsylvania, Ohio, West Virginia, Kentucky, North Carolina, Wisconsin, Iowa, Illinois, and Indiana. Mixed or positive trends were noted in Michigan, Maryland, Tennessee, and Missouri.

 

Wolf and Korsog (1994) showed that ozone trends based on various peak hourly statistics (such as the annual maximum 1-hour value and the 95th percentile of 1-hour values) between 1980/1981 and 1993 showed declines in most northeastern cities (one monitoring site was selected to represent each city). Negative trends were also found in Detroit, Chicago, and Atlanta but a positive trend was noted in Grand Rapids. Trends computed for Boston were somewhat unreliable due to missing data from 1980 - 1983. These trends were not adjusted for variations in meteorological conditions.

 

In contrast to trends based on peak 1-hour statistics, Wolf and Korsog (1994) found trends based on running 8-hour averages to be zero or positive in Detroit, Grand Rapids, Atlanta, Washington, Philadelphia, and Baltimore. Chinkin et al. (1996) report lower percent reductions in number of exceedances of a 100 ppb threshold than for higher thresholds. Thus, trends in exceedances of an 8-hour, 80 ppb threshold are generally less negative than trends in exceedances of a 1-hour, 120 ppb threshold. In some cases the 8-hour trends are positive as compared to negative 1-hour trends.

 

EPA’s annual trends report (EPA, 1994) includes total NO_x, VOC, and CO emissions by EPA Region for 1985 - 1994. VOC emissions declined sharply in all regions after 1988, reaching a total reduction of 10 - 15 percent below 1985 levels by 1991. In contrast, NO_x emissions, while decreasing by 5 percent between 1989 and 1991 in Region I, increased by more than 5 percent in regions IV and V. Thus, the largest ozone declines occurred over approximately the same geographic area where simultaneous VOC and NO_x reductions occurred. In general, ozone trends were less negative or of mixed signs in the South and Midwest, areas which experienced increases in NO_x emissions. Chinkin et al. (1996) compared correlation coefficients for NO_x and VOC emissions vs. ambient ozone for metropolitan areas in the Northeast and found somewhat better correlations between VOC and ozone as compared to NO_x although the statistical significance of these comparisons was not explored. The correlation coefficients were low overall, indicating that annual emission changes fail to explain a significant percentage of the variability in meteorologically adjusted ozone trends.

  The above results do not lend themselves to formulation of a definitive conclusion regarding causal relationships between changes in VOC and NO_x emissions and ambient ozone concentrations. When looking at emissions and ozone trends from the mid-1980s to the mid-1990s for various ozone trend measures at various locations, the most consistently (although by no means universally) identifiable feature is the simultaneous decline in peak ozone concentrations and VOC emissions between 1988 and 1991. This result by itself, however, does not mean that the observed decrease in ozone is attributable in whole or even in part to the drop in VOC, although it is reasonable to conclude that VOC decreases contributed at least partially to the ozone decline outside of biogenic dominated southern locations. It should be noted that comprehensive analyses of ambient NOx or VOC concentration trends which might validate the emission trends have not been conducted. Rich Poirot of the Vermont Department of Environmental Conservation has noted examples of apparent discrepancies between ambient NO and NO₂ concentration trends, nitrate deposition trends, and NO_x emission trends which warrant further investigation. In particular, EPA (1995) reported a three percent increase in national total NO_x emissions between 1985 and 1994 while over the same period, the national composite NO₂ concentrations decreased by nine percent.

 

It should also be noted that future ozone/precursor trend relationships may differ from those of the past. For example, if the ozone peak downwind of an urban area results from the superposition of an ozone plume generated under VOC limited conditions by the urban area with a regional background "cloud" of elevated ozone formed largely under NO_x limited conditions, then VOC reductions, such as those which occurred between the mid-1980s and mid-1990s, may reduce the total ozone peak by reducing the urban area’s contribution. However, as the peak is lowered, the regional, NO_x limited contribution to the total peak becomes relatively more important. Thus, future VOC reductions may be less effective at reducing ozone concentrations than in the past. This effect would be magnified for exceedances of an 8-hour average ozone standard as compared to a 1-hour standard since the 8-hour averages are less sensitive to sharp 1-hour concentration peaks downwind of urban areas.

 

 

Impact of an 8-Hour Ozone Standard

 

EPA’s proposed revisions to the ozone NAAQS is based on an 8-hour average daily maximum concentration instead of the 1-hour daily maximum measure used in the current standard. Although closely related, spatial and temporal patterns of peak 8-hour averages, the role of transport, and the response of 8-hour values to control strategies are somewhat different than for one hour averages. AQAWG analyses of these issues produced the following principal findings:

  Over the OTAG domain as a whole, spatial ozone patterns for 1-hour and 8-hour daily maximum concentrations are similar in that areas of high and low 1-hour concentrations are also regions of high and low 8-hour concentrations.

  Eight-hour average daily maximum concentrations range from 75 - 95 % of the corresponding 1-hour daily maximums. The biggest differences are near major metropolitan areas with pronounced diurnal ozone cycles while the smallest differences are in remote rural locations which exhibit relatively weak diurnal ozone cycles. Thus, exceedances of a given daily maximum 8-hour average threshold will be relatively more frequent further away from the urban areas as the diurnal cycle flattens out.

  No analyses of the relative effectiveness of various control strategies for 1-hour vs. 8-hour ozone have been conducted by the AQAWG. Relative effectiveness is dependent on the complex interaction of many factors and can be expected to vary significantly from one situation to the next.

 

 

Representativeness of OTAG Episodes

 

In issue of concern in the interpretation of control strategy modeling results is the degree to which the set of episodes selected for modeling represent the full range of conditions under which ozone episodes occur. If certain types of episodes are not well represented by the selected episodes, model results may give a misleading picture of control strategy impacts. The AQAWG examined the representativeness of episodes modeled by the OTAG Regional and Urban Scale Modeling Workgroup ("OTAG episodes") from two perspectives:

 

1. How does the spatial distribution of ozone on OTAG episode days compare with that observed on high ozone days in general?


 
2. How do forward and backwards trajectories computed as part of the "source regions of influence" and "trajectory residence time" analyses discussed above compare between OTAG episode days and high ozone days in general?

 

While the general spatial patterns of daily maximum ozone concentrations during the modeled episodes in 1991, 1993, and 1995 were similar to the pattern of 90th percentile daily maximum concentrations for all days during the 1991-1995 ozone seasons, there are clear differences from episode to episode (see Figure 3). Generally speaking, the 1991 and 1995 episodes are more similar to each other than the 1993 episode which was characterized primarily by high ozone levels in the South with lower values elsewhere. Similar results are seen in the comparisons of transport vectors and normalized residence times between the modeled episodes in each year (Figure 4). Each episode is characterized by different regions of high residence times, with the 1993 episode differing markedly from the other two with high residence times in the South and low residence times in the North.

 

 

OTAG Model Evaluation

 

Although the major model evaluation studies for OTAG were conducted by the Regional and Urban Scale Modeling Workgroup, the AQAWG also conducted several diagnostic comparisons of observed and predicted ozone and ozone precursor concentrations which shed light on the suitability of the OTAG photochemical model results for evaluation of regional control strategy impacts (Hartsell and Edgerton, 1996; Edgerton, 1997; Morris et al., 1997; Imhoff, 1996; Lurmann et al., 1997). The principal findings of these analyses are discussed below.

 

Ozone: Comparisons of observed surface ozone concentrations during the July, 1995 episode with concentrations predicted by the OTAG UAM-V runs for a suburban and a rural site near Nashville revealed a fair amount of scatter but no bias. However, comparisons aloft indicated that the model consistently under predicted ozone concentrations above the mixed layer. There are several possible reasons for this and the current analyses do not allow us to distinguish among them.

 

Comparisons of observed and modeled ozone for nine rural sites in the South and Northeast for the July 1995 episode revealed a tendency for the model to over-estimate the frequency and duration of ozone values in the 80-100 ppb range for these rural sites. If confirmed by analyses for a larger group of sites, this finding would indicate a potential problem with using OTAG results to provide boundary conditions for urban scale modeling.

 

Isoprene: Comparisons of observed mid-afternoon isoprene concentrations at monitoring sites located in various parts of the OTAG domain with predicted surface layer isoprene concentrations for corresponding model grid cells revealed a tendency for the model to over predict isoprene. More than three-fourths of the sites examined in these studies exhibited biases greater than ±25% and nearly half the sites exhibited mean biases of greater than ±50% of mean observed values. Nearly all the biases over ±50% were over predictions. No geographic pattern in the magnitude or sign of the bias is evident in the results and the magnitude of the over predictions greatly exceed the level of bias one might expect from comparing a near surface concentration with a (50 m) vertically averaged value for the model grid cell. These results point to a potentially significant problem with the UAM-V/BEIS2 modeling system: if isoprene emissions in the OTAG inventory are significantly overestimated (for whatever reason), the over representation of biogenic VOCs would cause the model to underestimate the expected response to anthropogenic VOC controls. In addition, the model would tend to be overly sensitive to NO_x controls in all but the largest urban areas.

 

Other precursors: An analysis of CO/NO_Y and VOC/NO_Y ratios at a Nashville urban site during 1995 found good agreement between measured and modeled ratios, indicating consistency between the modeled and ambient CO/NO_X and VOC/NO_X emission ratios around this site. The intercepts at zero NO_Y from these analyses are indicative of the background concentrations of CO and VOC: the analyses indicated that the model tended to under estimate the CO background and over estimate the VOC background at this site. The discrepancy between the apparent background for VOC of 90 ppb in the model compared to near zero in the ambient data possibly indicates that a source of VOC (but not CO and NO_Y) is overestimated in the model. The tendency for the model to under estimate the CO background at this urban site is surprising given that the model had a tendency to over predict CO at two rural sites in Georgia and Tennessee: however, these findings are not necessarily inconsistent. The tendency to over predict CO at the rural sites may be indicative of emissions or atmospheric mixing problems in this region.

 

Chemistry: Comparisons of observed and predicted ozone production efficiencies based on O₃/NO_Z ratios showed good agreement at rural site near Nashville in 1995; both observed and predicted O₃/NO_Z ratios were about 6 suggesting ozone formation was NO_X limited. At a Nashville suburban site, there was more variability in both the observed and (particularly) modeled O₃/NO_Z ratios, making it difficult to draw conclusions about model performance for this location. The average observed O₃/NO_Z ratio (about 3) was lower for the suburban site than the rural site, suggesting ozone formation was relatively less NO_X limited and more VOC limited at the suburban site. Similarly, a comparison of modeled and observed O₃/NO_Y relationships revealed good agreement across 9 rural sites in the South and Northeast in 1995; both observed and predicted O₃/NO_Y ratios were about 9 suggesting ozone formation was NO_X limited. The good performance of the model chemistry for O₃/NO_Y (and O₃/NO_Z) relationships under NO_X limited conditions supports the use of the model for evaluating regional control strategies. This finding is particularly relevant to OTAG because ozone formation is NO_X limited over much of the OTAG domain.

 

 

Recommendations for Additional Analyses

 

Further work is recommended in a number of different areas as outlined below. The infrastructure consisting of the community of analysts brought together by the OTAG process and associated communications capabilities (regular meetings, conference calls, web sites, mailing lists) has proven to be a valuable resource for conducting policy-relevant research and efforts should be made to maintain this infrastructure in the future. Specific areas recommended for future inquiry include:

  Prepare summaries of meteorological conditions associated with the build up and dissipation of ozone episodes in different portions of the OTAG domain and contrast with conditions during non-episode days. Of particular interest are differences in surface and upper-air pressure fields which could be used to help interpret the trajectory analysis results.

  Perform a comparison of weekday vs. weekend concentration differences for 1-hour vs. 8-hour daily maximums. This would shed light on any differences in response to a control strategy that mimics the weekday/weekend emissions cycle.

 

Examine weekday/weekend differences in diurnal ozone and precursor patterns.

 

Evaluate the ability of the photochemical model to reproduce recent observed trends in ozone concentrations.

 

Expand the trajectory analyses to include simulation of atmospheric chemical processes and deposition.

  Several AQAWG members also suggested that the current ozone, ozone precursor, and meteorological monitoring network be reconfigured and enhanced to better address the issue of regional elevated ozone events, ozone and precursor transport, and photochemical model evaluation. Currently, there is a lack of routine air quality and meteorological data suitable for analysis of transport over a broad range of episode conditions. In particular, the current monitoring network is primarily focussed on the larger metropolitan areas whereas spatial coverage in less populated areas or along boundaries of political jurisdictions is quite limited. The current monitoring network is also primarily geared towards monitoring ozone attainment status and trends. Measurements of ozone aloft or of ozone precursors are quite limited. Such measurements are needed both for transport assessments and to better evaluate the performance of photochemical models used to estimate control strategy impacts.

 

 

Table 1. Technical analyses considered by the Air Quality Analysis Workgroup.

	Summary?	Written Report Available?	WWW Document?
1. Spatial and temporal pattern of ozone and precursors
Spatial Pattern of Daily Maximum Ozone (Husar, 1996a)	Y	Y	Y
Spatial Pattern of Daily Maximum 8-Hour Ozone (Husar, 1996b)	Y	Y	Y
Weekly Pattern of Ozone (Husar, 1996c)	Y	Y	Y
Spectral Decomposition of O3 time Series (Porter et al., 1996)	Y	Y	Y
Seasonal Pattern of Ozone over the OTAG Region. (Husar, 1997)	N	Y	Y
Historical Perspective on the Climatological Potential for "Local" Pollution Episodes (Poirot and Wishinski, 1997)	N	Y	Y
Representativeness of 1995 Ozone Season (Chinkin et al, 1996)	Y	Y	---
Review of Ozone Trend Studies (Morris, 1996)	Y	Y	---
Nashville/Middle TN Ozone Study (Meagher, 1996)	Y	Y	---
2. Transport of ozone and precursors
Air Trajectory Analysis of Long-Term Ozone Climatology (Poirot and Wishinski, 1996a,b,c; Wishinski and Poirot, 1996)	Y	Y	Y
Source Regions of Influence (Schichtel and Husar, 1997, 1996)	Y	Y	Y
Ozone/Tracer/NOy Relationships at Three SOS SCION Sites (Edgerton and Hartsell, 1996)	Y	Y	Y
Analysis of Ozone, NOy, and Tracer Data from a Site in South-Central PA (Edgerton, 1997)	Y	Y	Y
Analysis of low-level jets using NARSTO Northeast data (Korc et al., 1997)	N	N	---
Source Apportionment Model Results (Morris, 1997)	N	Y	---
Transport impact distance analysis (Rao, 1996)	N	N	---
Intra-Annual and High Frequency Variations at SOS-SCION sites (Vukovich, 1996)	N	Y	---
Transport and Mixing Phenomena Related to Ozone Exceedances in the Northeast U.S. (Blumenthal et al., 1997)	N	Y	Y
Classification of Ozone Episodes for Four Southern Cities According to Transport Characteristics (Hudischewskyj and Douglas, 1997)	N	Y	Y
3. Model performance evaluation
Model/Measure comparisons at nine regional sites (Hartsell and Edgerton, 1996)	Y	Y	Y
Ambient Monitoring Sites for OTAG (time series) Model Evaluation (Poirot, 1996)	Y	Y	Y
Observed/Predicted isoprene comparison (Edgerton, 1997; Morris et al., 1997)	Y	Y	Y
Comparison of SOS Nashville Data to OTAG 1995 Base Model (Imhoff, 1996)	Y	---	---
Evaluation of the UAM-V Model Performance in the Northeast Region for OTAG Episodes (Lurmann et al., 1997)	N	Y	Y
4. Episode representativeness
Episode "Representativeness" (preliminary view from backward trajectory perspective) (Poirot and Schichtel, 1996)	N	N	N
Comparison of ozone distributions during OTAG modeled episodes with climatological distributions (Husar, 1997b)	N	N	N

 

 

 

 

FIGURES

 

 

 

Figure 1: Areas not in attainment of the current 1-hour National Ambient Air Quality Standard for ozone.

 

Figure 2. Areas not in attainment of the proposed 8-hour, 0.08 ppm national ambient air quality standard for ozone.

 

Figure 3. Comparison of average daily maximum ozone concentrations for days in the 1991, 1993, and 1995 OTAG modeled episodes with 90th percentile daily maximum values from the 1991 - 1995 ozone seasons (Source: Husar, 1997).

 


Figure 4. Transport vectors and normalized residence times over the Eastern U.S. for pollutants with one day lifetimes during the a) 1991, b) 1993, c) 1995 OTAG modeled episodes and d) the combined ‘91, ‘93, and ‘95 episodes (Source: Schichtel and Husar, 1997).

 

 

 

SUMMARIES OF INDIVIDUAL AQAWG ANALYSES

 

A brief summary and critical review was prepared for most of the AQAWG analyses identified in Table 1. These summaries provide a description of the purpose, methodology, and major findings of each study and are intended as a resource for quick reference in lieu of turning to the full study report(s). These summaries also include brief discussions of each study’s limitations and the scientific and policy implications for the issues being addressed by the OTAG. The summaries of findings, limitations, and implications were used as the basis for the integrated assessment presented above. Summaries are presented on the following pages in the order in which projects are listed in Table 1.

 

Spatial Pattern of Daily Maximum Ozone Over the OTAG Region

 

 

 

Participants: Rudy Husar, CAPITA, Washington University, St. Louis, MO

 

Reference: Spatial Pattern of Daily Maximum Ozone Over the OTAG Region, Rudy Husar 16 September 1996 (http://capita.wustl.edu/otag/Reports/otagspat/otagspat.html).

 

Purpose: Identify spatial patterns in daily maximum 1-hour ozone concentrations over the OTAG domain.

 

Method: A five year (1991-1995) data set of June-August daily maximum 1-hour ozone concentrations for the eastern U.S. was obtained by combining data from AIRS, CASTNET, and several other monitoring networks. All sites with at least 25 percent complete data were included in the analysis (thus, some of the sites used may only have data for less than a single season) . The final data consisted of a total of 715 monitoring sites. Six of these sites were discarded as a result of quality control procedures. Average concentrations and various percentiles of the concentration distribution were calculated for each monitoring site and contour plots prepared for each summary statistic. Specific emphasis was placed on the spatial distribution of the mean, 10, 50, and 90th percentiles.

 

Findings: Principal findings of this analysis were:

 

1. Mean daily maximum values range from a value close to 40 ppb (which is within the range of northern hemisphere tropospheric background values) in the "corners" of the OTAG domain (south Florida, south Texas Gulf Coast, upper Midwest/northern Plains, and northern Maine), up to about 80 ppb in the vicinity of some major urban centers. Mean values along the southern portion of the western boundary in Texas, Oklahoma, and Nebraska are about 10 ppb higher than those in the corners of the domain.

 

2. Mean daily maximum concentrations are highest in the Washington-New York urban corridor, with elevated concentration "hot spots" also observed in the vicinity of several other (but, surprisingly, not all) major urban centers.

 

3. A large region of elevated (65 - 70 ppb) mean daily maximum concentrations is evident along a band stretching across southern Indiana and Ohio, just north of the Ohio River.

 

4. East-west sections across the center of the domain show concentrations increasing consistently from west to east.

 

5. In the industrial Midwest, from Missouri through Illinois, Indiana, Ohio, Kentucky, and the Virginias, low (10 percentile) concentrations range from 42-47 ppb, about 15 ppb above values observed along the edges of the OTAG domain. However, in contrast to seasonal mean values, tenth percentile concentrations in urban areas are indistinguishable from their surroundings.

 

6. 90th percentile concentrations are highest in urban areas, including those urban areas which did not show mean concentrations elevated above regional background values. Elevated 90th percentile values are also found over a large region covering southern Indiana and southern Ohio.

7. Differences between 90th and 10th percentiles show that daily maximum ozone concentrations are most variable in urban areas and least variable in the corners of the OTAG domain. This is consistent with notion that monitored values in the corners of the domain represent broad-scale background ozone levels.

 

 

Limitations: These results are dependent on the spatial distributions of monitors, the spatial interpolation methods used, and the accuracy, precision, and completeness of the data. Quantitative estimates of spatial interpolation errors applicable to the AIRS/CASTNet data subset of this study are discussed by Falke (1996) but the impact of these errors are not discussed in connection with the principal findings of this study. A complete spatial analysis of interpolation errors for the integrated data set used in the spatial analysis would be needed to confirm some the findings noted above. In particular, since urban, near-urban, and rural sites were weighted equally in the interpolation procedure, it is possible that the interpolation from urban or near urban sites (which make up most of the network) produced a bias in the results in some locations. Also, since extreme concentrations (e.g., 90th percentiles) exhibit larger spatial gradients than mean or median values, the interpolation procedure may give a less accurate picture of the spatial pattern of extreme events as compared to average events. It should also be noted that the author uses a 25 percent data completeness criteria for including monitors in the analysis rather than the more commonly used 75 percent criteria. No analysis of the effect of the less stringent completeness criteria on uncertainties in computed summary statistics is provided. This is particularly of concern for the more extreme (i.e., 10 and 90) percentiles and the exceedance statistics. Application of the completeness criteria to the integrated five year data set rather than year-by-year is also a potential concern since missing values are unlikely to be randomly distributed over the five-year period and individual years are known to differ markedly in mean and peak ozone concentrations due to variations in meteorological conditions. Thus, blocks of missing values at key groups of monitoring sites could be biasing the results in unknown ways. Finally, it should also be noted that many or all of the days contributing to the low 10th percentile concentrations observed in many of the urban areas may have been impacted by NO_x titration under conditions leading to high NO_x concentrations and limited photochemical ozone production. This is in contrast to the authors conclusion that the low urban 10th percentile values reflect well ventilated conditions under which urban and surrounding rural concentrations are similar in magnitude. Additional analysis of the conditions contributing to the 10th percentile values will be needed to resolve this question.

 

Scientific Implications: Any realistic model of ozone/precursor relationships in the OTAG domain should be able to reproduce (or at least be consistent with) the principal features of the monitoring data described in this analysis. Of particular importance are the tropospheric background levels found in the corners of the domain and the apparent narrowing and shift to the right (i.e., towards higher concentrations) in the distribution of daily maximum values in southern Ohio and Indiana.

 

Policy Implications: Human activities must be responsible for some or most of the ozone excess in the central portions of the eastern U.S. over hemispheric background levels. Furthermore, these results suggest that precursor sources (presumably major NO_x point sources) along the Ohio River Valley are associated with a broad region of elevated ozone levels not associated with any specific major urban centers. These results by themselves, however, do not indicate whether or not this broad region of elevated ozone concentrations is of concern from the stand point of limiting exceedances of the ambient ozone standard. Nor do these results by themselves indicate whether or not control of the major NO_x sources geographically associated with this region will have a beneficial effect on reducing exceedances in the OTAG domain.

 

 

Pattern of 8-Hour Daily Maximum Ozone Over the OTAG Domain

 

 

Participants: Rudy Husar, Washington University, St. Louis, MO

 

Reference: Pattern of 8-Hour Daily Maximum Ozone and Comparison with the 1-Hour Standard. Rudy Husar, 16 September 1996 (http://capita.wustl.edu/otag/Reports/).

 

Purpose: Compare and contrast the spatial patterns of daily maximum 8-hour and 1-hour ozone concentrations over the OTAG region.

 

Method: A five year (1991-1995) data set of June-August hourly average ozone concentrations for the eastern U.S. was obtained by combining data from AIRS, CASTNET, and several other monitoring networks. All sites with at least 25 percent complete data were included in the analysis (thus, some of the sites used may only have data for less than a single season). The final data consisted of a total of 715 monitoring sites. Six of these sites were discarded as a result of quality control procedures. Daily maximum 1-hour and 8-hour averages were extracted for each day with sufficient data and means, 10, 50, 90th percentiles, and exceedance counts for various 1-hour and 8-hour thresholds were computed.

 

Results: Comparisons of daily maximum 1-hour values with 8-hour values for the same day indicated that the 8-hour to 1-hour ratios were lowest in urban areas where high peak 1-hour ozone concentrations are observed and approaches 1:1 at rural remote sites. For example, the average ratio at Greenwich, CT (adjacent to New York City) is 0.82 with ratios corresponding to daily maximum 1-hour values above 100 ppb averaging about 0.75. Other locations exhibit ratios in between these extremes. For the OTAG region as a whole, the average ratio is 0.86 with slightly higher values in the northwestern portion of the region and slightly lower values in the southwestern portion. Exceedances of a 1-hour, 120 ppb level are most common in the northeastern urban corridor and near other major urban centers. The spatial pattern of 8-hour exceedances of 102 ppb is similar. Moving to a 1-hour, 94 ppb or an 8-hour, 80 ppb level broadens the geographic extent of the exceedances to include a band of eight or more exceedance days per year just north of the Ohio river stretching from Illinois to Ohio and on into western Pennsylvania. Although the overall spatial patterns of 1-hour and 8-hour exceedances are similar, examination of a difference plot (8-hour exceedances of 80 ppb minus 1-hour exceedances of 94 ppb) indicates that the 8-hour exceedances are relatively more frequent within those broad areas north of the line separating Kentucky and Virginia from Tennessee and North Carolina which are away from the direct influence of a major urban area. With the exception of North Carolina, differences in exceedances are much less pronounced south of this line. Interestingly, Dallas and Houston, TX show quite different responses, with 8-hour exceedances relatively more common in Dallas and less common in Houston.

 

Maps of 90th and 10th percentile 8-hour daily maximum ozone concentrations are nearly identical to corresponding maps for the 1-hour daily maxima. Similarly, 8-hour and 1-hour maps of the difference between 90th and 10th percentiles are also very similar.

 

Limitations: Limitations noted for Husar’s analysis of 1-hour daily maxima also apply to this study with the exception that an appropriately more stringent data completeness criterion was applied to the analysis of 8-hour exceedances, thus reducing the potential for distortions resulting from missing data in these results. This study was done prior to EPA’s announcement of plans to replace the current 1-hour ozone NAAQS of 0.12 ppm with an 8-hour, 0.08 ppm standard and therefore did not directly compare areas of violation of the current and proposed standards, although exceedance maps for these two concentration levels are provided. Such area of violation maps have, however, been produced by EPA and others.

 

Scientific Implications: One-hour and eight-hour daily maximum concentrations are highly correlated both temporally and spatially. The spatial patterns of threshold exceedances and percentiles for 1-hour and 8-hour daily maxima are very similar to one another. Comparisons of the magnitude of same-day 1-hour and 8-hour peaks discussed above indicate that, under ozone episode conditions, concentrations at high ozone urban sites tend to exhibit relatively sharp 1-hour peaks. As a result, on a same day basis daily maximum 8-hour averages at such sites are relatively small compared to 1-hour maxima. In contrast, rural remote sites are characterized by low amplitude diurnal cycles in 1-hour values and therefore maximum 8-hour values tend to be only slightly less than corresponding 1-hour maxima.

 

Maps presented in this study show that multiple exceedances per year of an 8-hour, 80 ppb level covered a much broader area between 1991-1995 as compared to multiple exceedances per year of a 1-hour, 120 ppb level: the latter are confined to the immediate vicinity of a few large urban areas while the former cover much of the OTAG domain with the exception of the extreme northwest, northern Main, and the deep South indicating the broad spatial extent of such "mid-level" ozone concentrations. The threshold levels for which the frequency of 8-hour exceedances is comparable to the frequency of 1-hour exceedances of 120 ppb for the OTAG domain as a whole is approximately 102 ppb.

 

Policy Implications: Results presented in this study indicate that daily maximum 1-hour and daily maximum 8-hour concentrations are very closely related both spatially and temporally. Differences in the areas over which 1-hour vs. 8-hour standard violations occur depend on the level and form of the 8-hour standard and were not explicitly addressed here. However, the above results do indicate that at locations experiencing the highest 1-hour concentrations in the OTAG domain (e.g., downwind of major urban areas such as in southern Connecticut and along the eastern shore of Lake Michigan), the 1-hour peak exceeds the 8-hour peak by a greater amount than in other areas. Thus, all else being equal, an 8-hour standard may be relatively less stringent in these locations.

Weekly Pattern of Ozone over the OTAG Region

 

 

Participants: Rudy Husar, CAPITA, Washington University, St. Louis, MO

 

Reference: Weekly Pattern of Ozone Over the OTAG Region, Rudy Husar 16 September 1996 (http://capita.wustl.edu/otag/Reports/otagweek/otagweek.html).

 

Purpose: Analyze the influence of weekly cycles in anthropogenic emissions on daily maximum ozone concentrations.

 

Method: A five year (1991-1995) data set of June-August daily maximum 1-hour ozone concentrations for the eastern U.S. was obtained by combining data from AIRS, CASTNET, and several other monitoring networks. All sites with at least 25 percent complete data were included in the analysis (thus, some of the sites used may only have data for less than a single season) . The final data consisted of a total of 715 monitoring sites. Six of these sites were discarded as a result of quality control procedures. After disagregation by day of week, average concentrations and various percentiles of the concentration distribution were calculated for each monitoring site and contour plots prepared for each summary statistic. Specific emphasis was placed on the spatial distribution of the mean, 10, 50, and 90th percentiles and on exceedances of 120 ppb.

 

Results: For the OTAG region as a whole, seasonal peaks (90 and 95th percentiles) of daily maximum ozone concentrations were found to be about seven percent lower on Sundays than during the week. After correcting for a constant 40 ppb background, the weekday to Sunday difference is seen to represent about 33 percent of the OTAG region ozone excess above background. This "Sunday effect" is most pronounced in the 90th percentiles around the major urban areas where peak concentrations are highest but is notably absent (actually reversed with slightly higher values on Sundays than on Fridays) along a broad band stretching northeastward from western Tennessee through Kentucky, West Virginia near the Ohio border, Pittsburgh, and then northwestward to Cleveland. With the exception of Pittsburgh, this band overlaps the region of relatively high 10th percentile ozone concentrations noted by Husar (1996a). This "negative" Sunday effect is also evident at a few other isolated locations. No significant weekly cycle is evident in average or 50th percentile concentrations.

 

Weekly patterns in the frequency of exceedance of 120 ppb are similar to those for the 95th percentile of daily maximum concentrations with one third as many exceedances on Sundays as on Fridays. The number of exceedances increases nearly linearly from Monday through Friday. In most cases, the largest positive weekday - weekend differences in exceedances (i.e., more exceedances on Fridays than on Sundays) are over the urban areas with smaller differences elsewhere. In some urban areas, however, exceedances are actually somewhat more common on Sundays than on Fridays. Most of these urban areas fall within the geographic region noted above in which 90th percentile concentrations on Sundays are slightly higher than on Fridays. Several other isolated areas also exhibit a weak "negative" Sunday effect. The weekly cycle noted at the 120 ppb exceedance threshold is progressively damped at lower thresholds (100, 80, 60 ppb). Almost no weekend effect is evident at 60 ppb.

 

Scientific Implications: Results of this study indicate that the changes in average emissions between weekdays and weekends are sufficient to significantly reduce peak ozone concentrations on weekends while mean concentrations are largely unaffected. Unfortunately, since the nature (magnitude of emission reductions, relative source mix, diurnal pattern) of the differences in emissions on weekends as compared to weekdays have not been quantified, the mechanism resulting in lower peak ozone levels on weekends in urban areas is not yet fully understood. The gradual increase in upper percentiles of the concentration distribution and in the numbers of exceedances of high concentration thresholds (e.g., 120 ppb) during the course of the work week is indicative of the influence of carry over of ozone and precursors from one day to the next. This carry over keeps Saturday concentrations higher than they would otherwise be but by Sunday the full influence of the weekend emission changes are evident. The weekend effect is most pronounced at the upper end of the concentration distribution and therefore changes in urban areas, where high concentrations occur most frequently, tend to be larger than in rural areas. In some urban areas, however, peak concentrations on Sundays are actually slightly higher than on Fridays (e.g., Pittsburgh, PA). The reasons for this anomalous behavior are not known but may be related to geographic variations in combinations of several factors (mix of source types, activity patterns, VOC/NO_x ratios). Geographically, the overlap between areas of relatively high 10th percentile concentrations and areas of negative weekend effect in the central portion of the OTAG domain may be related to the impacts of elevated NO_x sources associated with base-load utility boilers characterized by relatively small weekly variations in emissions. However, the temporal pattern of emissions in this region has not been examined and the extent of any weekday/weekend differences are unknown.

 

Limitations: These results are highly dependent on the spatial distributions of monitors, the spatial interpolation methods used, and the accuracy, precision, and completeness of the data. Quantitative estimates of spatial interpolation errors applicable to the AIRS/CASTNet data subset of this study are discussed by Falke (1996) but the impact of these errors are not discussed in connection with the principal findings of this study. A complete spatial analysis of interpolation errors for the integrated data set used in the spatial analysis would be needed to confirm some the findings noted above. In particular, since urban, near-urban, and rural sites were weighted equally in the interpolation procedure, it is possible that the interpolation from urban or near urban sites (which make up most of the network) produced a bias in the results in some locations. Also, since extreme concentrations (e.g., 90th percentiles) exhibit larger spatial gradients than mean or median values, the interpolation procedure may give a less accurate picture of the spatial pattern of extreme events as compared to average events. It should also be noted that the author uses a 25 percent data completeness criteria for including monitors in the analysis rather than the more commonly used 75 percent criteria. No analysis of the effect of the less stringent completeness criteria on uncertainties in computed summary statistics is provided. This is particularly of concern for the more extreme (i.e., 10 and 90) percentiles and the exceedance statistics. Application of the completeness criteria to the integrated five year data set rather than year-by-year is also a potential concern since missing values are unlikely to be randomly distributed over the five-year period and individual years are known to differ markedly in mean and peak ozone concentrations due to variations in meteorological conditions. Thus, blocks of missing values at key groups of monitoring sites could be biasing the results. Finally, reasons for the "negative" weekend effect found in some areas are not well understood and require further study. In particular, no analysis is presented here of weekday/weekend differences in precursor emissions.

 

 

Policy Implications: Results of this analysis imply that a control scenario mimicking weekday to weekend emission reductions would result in decreases in peak ozone concentrations in most urban areas. For the OTAG region as a whole, exceedances of the current NAAQS (120 ppb) would be reduced by roughly a factor of three. Unfortunately, the exact nature of weekday/weekend emission differences is not currently known. Not only the magnitude of the emission reductions on weekends, but also changes in the relative mix of VOC vs. NO_x emissions and of low-level vs. elevated NO_x sources are likely to be important. Weekday/weekend differences in the diurnal pattern of emissions are also likely to be important: differences in the timing of mobile source emissions may be as important in reducing ozone concentrations as differences in the total amount of emissions.

 

 

 

 

Spectral Decomposition of Ozone Time Series

 

 

 

Participants: P.S. Porter, University of Idaho, Idaho Falls, ID 83405; S.T. Rao, I. Zurbenko, and E. Zalewsky, State University of New York, Albany, NY 12222; R.F. Henry and J.Y. Ku, NY State Department of Environmental Conservation, Albany, NY 12222.

 

References:

Statistical Characteristics of Spectrally-Decomposed Ambient Ozone Time Series Data, P.S. Porter, S.T. Rao, I. Zurbenko, E. Zalewsky, R.F. Henry, and J.Y. Ku, Final Report, August, 1996.

 

Purpose: Estimate the spatial representativeness of data collected at routine ozone monitoring sites and the typical size of airsheds (zones of influence). Identify principal features of the spatial and temporal characteristics of daily maximum 1-hour and 8-hour ozone concentrations within three distinct frequency domains: long-term (i.e., annual), seasonal, and short-term (i.e., synoptic variations on time scales of less than about 30 days).

 

Methodology: Using multiple pass moving average filters (Kolmorgorov-Zurbenko, or KZ filters) described by Rao and Zurbenko (1994), the authors develop a spectral decomposition of the time series of natural logarithms of daily maximum 1-hour and 8-hour average ozone concentrations into long-term (annual), seasonal, and short-term (synoptic scale) components. This is done at all routine ozone monitoring sites in the U.S. reporting to EPA’s AIRS database for which few values are missing for the period 1983 - 1994. Monitors are segregated into those that operate year-round and those that operate only during the summer ozone season. Characteristics of each frequency domain are investigated, including temporal variance and autocorrelation, spatial patterns in means and variances, spatial correlations, and the influence of meteorological conditions.

 

Findings: Using two KZ filters, one a 15-day average, 5-pass filter and the other a 365-day, 3-pass filter, the authors find that the time series of log-transformed daily maximum concentrations can be cleanly separated into (1) a long-term component representing the influence of trends in emissions and (possibly) short-term climatic fluctuations, (2) a seasonal component representing the influence of the earth’s rotation about the sun, and (3) a short-term component representing the influence of fluctuating synoptic meteorological conditions and random processes (noise). An analysis of variance showed that covariances between the different components were generally small (less than 2% of the total variance). Long-term component variances were less than 10% of the total at more than ninety percent of all monitoring sites, with median values of 3.6% for summer season only monitors. For monitors with year-round data, the seasonal component accounts for up to 73% of the total variance (median value of 51%) while the median contribution of the short-term component is 51%. For summer season only monitors, the median seasonal component variance contribution was 12%, while the median short-term component was 77%. Seasonal components are higher in the Northeast than in the South. Contributions of short-term variance to the total are highest in coastal areas and lowest in the Midwest. In absolute terms, the highest short-term variances are in the Northeast and along the Gulf Coast.

 

An evaluation of the statistical properties of the short-term component, W(t) revealed it to be significantly negatively skewed and therefore not normally distributed at nearly all monitoring sites, while exp(W) was more nearly normally distributed in most locations. Autocorrelations in W(t) at one day lag had a median value of 0.35, indicating some day-to-day dependence resulting from persistence in meteorological conditions. Nevertheless, modeling the short-term component as a random variable normally distributed with zero mean, the authors found that it is possible to reproduce the observed median concentrations at nearly all sites (to within the 95% confidence interval of the predicted value); 95th percentiles of the daily maximum values were less accurately reproduced. Exceedance counts were similarly evaluated, but it is not clear how to interpret these results since most monitors will have had very few or no exceedances over the 12-year period examined. Rao et al. (1996) provide comparisons of observed and modeled exceedance counts and 95th percentiles at five locations which experienced multiple exceedances per year (at least prior to 1989) which show generally close agreement with some exceptions.

 

A trends analysis for the period 1985 - 1995 was also presented by the authors. Trends were computed for the ozone baseline (long-term plus seasonal component) both before and after adjusting for variations in temperature. Raw (unadjusted) baseline trends are mixed, with generally negative trends in the southern tier states, Illinois, and the Philadelphia - New York urban corridor but increases at most sites in Indiana, Ohio, West Virginia, Western Pennsylvania, and the Washington D.C. area. After adjusting for temperature fluctuations, negative trends were found at a much larger number of sites.

 

Spatial correlations in baseline and short-term components of both ozone and surface temperature time series were examined. For the Washington D.C. area, temperature baseline correlations remained constant and near unity over the maximum distance examined (1,280 km or 800 miles), while correlation in the short-term component decreases linearly with distance over this radius, dropping to 0.3 at 1,120 km (700 miles). Ozone baseline spatial correlations drop off more rapidly but also respond approximately linearly with distance, at least out to the maximum distance considered (800 km or 500 miles), at which point the correlation coefficient is approximately 0.8. Short-term ozone component spatial correlations decrease exponentially with distance over this range, dropping to a value of 0.3 at about 720 km (450 miles) along the "direction of transport". A map of spatial correlation isopleths for the short-term component is presented by the authors for five cities in the OTAG domain (Atlanta, Chicago, Cincinnati, Greenbelt MD, Pittsburgh). In all cases, correlations drop to 0.4 within 560 to 640 km (350 to 400 miles) of the selected monitors. It is suggested that these are the spatial domains which should be considered in designing control strategies.

Comparisons of time series of 1-hour average daily maximum ozone to 8-hour average daily maximums indicated that variances are higher for the 8-hour averages, both in an absolute sense, relative to the mean, and for the short-term component only. Looking at daily maximum n-hour averages at one site (Cliffside Park, NJ), the authors found that variances of the short-term components peak at n=10 hours (? = 0.38 as compared to ? = 0.27 at n=1 and ? = 0.35 at n=24). The statistical significance of these differences are not discussed.

 

Finally, using the technique described above of modeling the short-term component, W(t) as white noise, the authors compute the percent reduction needed in the ozone baseline component to achieve attainment of the ozone NAAQS for the 1987-1989 and 1991-1993 attainment periods at each monitoring site. These required reductions range from 0% (for monitors already in attainment), to 50% at monitors in high ozone locations in the Northeast. Generally lower reductions are required during the latter period due to the progress towards attainment already achieved.

 

Limitations: From a regulatory perspective, much of the value in these analyses is based on the viewpoint that the baseline component of the ozone time series (i.e., the component remaining after removal of short-term fluctuations using the KZ_15,5 filter) is "deterministic" in the sense that it is controlled entirely by seasonal meteorological fluctuations and year-to-year trends in emissions. The remaining, short-term component, on the other hand, resemble random fluctuations. Thus, management of the ozone problem should focus on the effects of control strategies on the baseline rather than being concerned with a highly fluctuating peak statistic (e.g., 4th highest concentration in three years) in which much of the variability is associated with "noisy" processes that cannot be accurately modeled. However, in order to make use of this approach, one must be able to accurately estimate the impact of future control strategies on the baseline, and such methods have yet to be successfully demonstrated (i.e., it is not clear how to adapt episodic photochemical models to models of baseline ozone). In addition, one must be able to estimate the impact (or convincingly demonstrate that there is no impact) of control scenarios on the statistical characteristics of the short-term component. To date, neither of these conditions have been met. Of course, it should also be noted that the ability of episodic photochemical models to accurately predict the impact of future-year control strategies on ozone levels has also yet to be successfully demonstrated.

 

Although the trends analysis results presented by the authors are in general agreement with results of other studies (see review prepared by Morris, 1996), the fact that nearby sites often exhibit opposing trends is indicative of a complex spatial structure in trend statistics which may not be fully resolvable with the current monitoring network. Due to the large number of sites examined, the trend maps presented by the authors are difficult to read in urban areas with many closely spaced monitoring sites due to the overlapping of green disks (denoting negative trends) on top of red ones (denoting positive trends); green disks are always on top, thus obscuring an unknown number of sites with positive trends. Tables of trend values for each site are available from the authors for review and Rao et al. (1995) present a table of trends for sites in the eastern U.S. for 1983 - 1992. Similar trend maps for the northeastern U.S., but showing spatially smoothed trends (where the smoothing was done with spatial KZ filters) are presented by some of the same authors (Zurbenko, et al., 1995) and these maps do not suffer from this difficulty. Unfortunately, the spatial smoothing may obscure some important small scale trend differences. Also, interpretation of trend statistics would be enhanced by further characterization of trends by site type (i.e., rural, remote, suburban, downwind urban peak, urban center, etc.).

 

Published demonstrations of the independence of short-term components of daily maximum ozone and temperature (Rao and Zurbenko, 1994; Flaum, Rao, and Zurbenko, 1996) are not conclusive because seasonal influences on the correlation were not resolved. Therefore, it has not been demonstrated that the short-term ozone fluctuations during the summer season are completely independent of meteorological effects. Thus, spatial correlations in this component may be partially attributable to spatial correlations in meteorological conditions in addition to emissions forcing and defining a spatial domain for control strategy development on the basis of zero lag time spatial correlations in the short-term components may not properly address the area of source influence. Furthermore, correlations at non-zero lag times (e.g., one or two day lags) also need to be considered. Spatial scales of correlations in the short-term temperature component appear to be reasonable from a synoptic scale meteorological perspective. The fact that these scales are greater than those for the ozone component may indicate that much of the remaining variance in the ozone component (i.e., that portion not related to temperature) is associated with other meteorological factors that operate on smaller spatial scales.

 

Implications: Perhaps the most important implication of the above findings is that, when only the summer high ozone season is considered, short-term fluctuations are the dominant source of variability in daily maximum concentrations (either 1-hour or 8-hour average). Thus, identifying the impact of control strategies on ozone levels is extremely difficult due to the low signal to noise ratio. Furthermore, areas close to the attainment /nonattainment threshold are likely to jump in and out of attainment (where attainment/nonattainment is determined on the basis of one to three years of monitoring data), thus complicating management efforts. This study shows that the problem appears to be particularly severe in the Northeast - precisely where accurate estimates of control strategy impacts are most needed.

 

When seasonal and long-term trends are removed from the ozone time series, the above results show the spatial scale of the remaining short-term fluctuations to be 560 - 640 km (350 - 400 miles), suggesting a scale for the coherence of same-day fluctuations in peak ozone which may be useful for air quality management efforts. However, the degree to which this spatial scale is determined by spatial correlations in meteorological conditions as compared to same-day transport of ozone and precursors is unknown.

 

Additional References:

Morris, 1996. Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States. ENVIRON International Corp., 21 February 1996.

 

Rao and Zurbenko, 1994. Detecting and Tracking Changes in Ozone Air Quality. J. Air and Waste Management Assoc., Vol. 44: 1089 - 1092, 1994.

 

Rao, S.T., Zurbenko, I.G., Porter, P.S., Ku, J.Y. and R.F. Henry, 1996. Dealing with the Ozone Non-Attainment Problem in the Eastern United States. Environmental Manager, January, 1996, pp. 17-31.

 

Flaum, J.B., Rao, S.T., and I.G. Zurbenko, 1996. Moderating the influence of meteorology on ambient ozone concentrations. J. Air & Waste Manage. Assoc., 46: 35-46, 1996.

 

 

Zurbenko I.G., Rao, S.T. and R.F. Henry, 1995. Mapping Ozone in the Eastern United States. Environmental Manager, Vol. 1 pp.24-30, January, 1996.

 
Ozone Exceedances Data Analysis: Representativeness of 1995
 

 

Participants: Lyle Chinkin, Richard Reiss, Douglas Eisenger, Timothy Dye, and Christopher Jones, Sonoma Technology, Inc., Santa Rosa, CA

 

Reference: Ozone Exceedances Data Analysis: Representativeness of 1995. L. Chinkin et al., Sonoma Technology, Inc., 1996.

 

Purpose: Assess the representativeness of the 1995 ozone season in the Northeast in the context of the past 15 ozone seasons. The motivation for this study was the realization that the summer of 1995 was one of the hottest on record for the Northeastern United States, and that the 1995 summer season was therefore a strong candidate to experience high ozone concentrations.

 

Methodology: The authors undertook an investigation of whether the above normal temperatures during the summer of 1995 were associated with above normal ozone concentrations. The authors utilized air quality and meteorological data from seven metropolitan areas within the Northeast’s Ozone Transport Region (OTR), as well as the OTR overall. The metropolitan areas studied include Baltimore, Boston, Hartford, New York, Philadelphia, Providence, and Washington, D.C. Numerous regional and subregional trends in 1-hour average ozone concentrations were evaluated for the 1980s and 1990s to determine to what extent 1995 was consistent with these trends. Year-to-year fluctuations in the frequency and severity of meteorological conditions conducive to ozone formation were accounted for using several different methods including: 1) An examination of year-to-year differences in the relationship between the frequency or severity of high temperature days (e.g., the number of days above 90°F) and the frequency or severity of ozone episodes (e.g., number of exceedance days), 2) the Poison regression model developed by Cox and Chu (1995), and 3) The synoptic typing scheme of Comrie and Yarnal (1992).

 

Findings: It was found that despite near record breaking temperatures, the 1995 ozone season was no worse than the average ozone season experienced during the 1990s. In fact, 1995 is consistent with overall trends showing declining ozone concentrations in the Northeast. The major findings cited by the authors are as follows:

 

• The number of days per year above the national ozone air quality standard has declined by about 70 percent from 1981-1995. The summer of 1995, despite being one of the hottest on record, is consistent with that trend.

 

• The severity of individual ozone events is declining and 1995 is consistent with that trend. For example, on days above the national standard, the average concentration by which the standard was exceeded has declined by about 70 percent from 1981-1995.

 

• The extent of the geographic area experiencing days above the ozone standard is declining.

 

• During the 1990s ozone concentrations above the national standard occurred at higher temperatures than in previous years: the most plausible explanation is that ozone precursor emissions have decreased.

 

• Between 1985 and 1994, VOC emissions in the Northeast (EPA Regions I, II, and III) decreased 13 percent, while NO_x emissions decreased only 5 percent.

 

 

Limitations: As with most other trends studies, the emphasis here is on peak concentrations in urban areas in the Northeast. However, the study did include a comparison of trends at sites designated as urban or suburban in the AIRS database with sites determined to be in rural locations based on a review of their map coordinates. This analysis indicated no discernable difference between urban and rural trends. It should also be noted that uncertainties in the emission inventory trends (especially in the NO_x trends) are not evaluated. This is particularly important given the lack of attention to ambient NO_x trends which could be used to corroborate the inventory trend (assuming sufficient high quality NO_x data are available). Uncertainties in the correction for meteorological conditions must also be taken into consideration. In particular, the Cox and Chu Poison regression method assumes underlying (i.e., emission-related) trends are linear in time - this is a rough approximation at best. In addition, year-to-year changes in the ratio of the number of days in a given season above a temperature threshold to the number of ozone exceedance days are very sensitive to the temperature and concentration thresholds used and are subject to numerous factors not related to emissions. These ratios thus provide only a very rough indication of actual precursor emission trends.

 

Scientific Implications: Results of this study are generally consistent with those of other recent trends analyses which show concurrent declines in peak ozone levels and VOC emissions. Data on ambient precursor concentration trends are needed to verify estimated trends in emissions.

 

Policy Implications: Although not entirely definitive for the reasons noted above, this study supports other independent analyses of northeastern ozone and emission trends, strongly suggesting that recent VOC emission reductions resulted in decreases in peak 1-hour ozone levels over the routine monitoring network. Concurrent but relatively small NO_x emission reductions may have contributed to the ozone reductions. Of course, these studies do not provide any direct information on the relative effectiveness of future VOC controls in the region.

 

Additional References:

Comrie, A.C. and B. Yarnal, 1992. Relationships between synoptic-scale atmospheric circulation and ozone concentrations in metropolitan Pittsburgh, Pennsylvania. Atmos. Environ., 26B, 301-312.

 

Cox, W.M. and S-H Chu, 1996. Assessment of interannual ozone variations in urban areas from a climatological perspective. Atmospheric Environment, 30(14): 2615-.

 

 
Ozone Measurement Trend Studies in the Northeast
 

 

Participants: Ralph Morris, ENVIRON, 101 Rowland Way, Novato, CA (reviewing work performed by USEPA (1994 Air Quality Trends Report), S. T. Rao et al., New York State Department of Environmental Conservation, George Wolff and Patricia Korsog, General Motors Corporation (GMC), William Cox and Shao-Hang Chu, OAQPS, USEPA, Kay Jones, Zephyr Consulting

 

Reference: Review of Recent Ozone Measurement and Modeling Studies in the Eastern United States. R.E. Morris, ENVIRON International Corp., March, 1996 and references therein.

 

Purpose: Review and prepare an integrated summary of recent major northeastern U.S. air quality and emission trend analyses.

 

Methodology: Several researchers have performed trends analysis of ozone in the eastern U.S. in recent years. These analyses dif