Preview of 1994 Ozone Precursor Concentrations in the Norteastern U.S.
7.1 Influence from Stationary NOx Sources
Distinguishing among, and developing effective control strategies for, the source types and source locations that contribute to the accumulation of high ozone concentrations is a complex business:
- different sources (which require different controls) exhibit similar chemical composition;
- similar sources (with similar chemical profiles) are prevalent in multiple, widespread locations;
- the most reactive VOC and nitrogen compounds (which contribute most to ozone formation) are rapidly depleted by photochemical processes, substantially altering the original emissions profiles;
- ozone itself is highly reactive and is continually destroyed by physical reactions at the earth's surface, and chemical reactions with some of the same chemicals which have led to its formation.
This complexity is further confounded by highly variable and inter-correlated meteorological factors:
- solar radiation and associated temperature influence both the emissions from natural and anthropogenic sources, and the rates at which these compounds react to form (or destroy) ozone;
- solar radiation and temperature are also strong statistical surrogates for other meteorological factors which influence ozone formation (high pressure, low mixing height, stagnation, southerly flows), such that a model which includes temperature may accurately predict ozone despite inaccuracies in emissions, chemistry and meteorological input data;
- formation, destruction and transport of ozone and precursors occurs at local, meso-scale, synoptic and global spatial scales through different vertical layers of the atmosphere;
- transport of ozone and precursors over water is particularly important in the Northeast, as physical and chemical ozone destruction mechanisms are impeded over water, and as southwesterly transport winds align major urban source regions along the Northeast coastal corridor. These conditions often occur concurrently with high pressure, and elevated levels of solar radiation, and temperature. Without offshore measurements, its difficult to separate effects of land and sea breeze recirculation from transport along the coastal corridor.
Complex photochemical models will always be required to synthesize the concurrent influences of changing emissions, chemistry, meteorology and transport over broad spatial scales, and especially to predict effects of future emissions controls (or increases). Ambient measurements from PAMS and related programs can supplement these model results in a number of useful ways:
- PAMS meteorological measurements will substantially enhance the models' meteorological data base, and/or provide a basis for developing more realistic meteorological model subroutines;
- PAMS chemical measurements will provide a basis for developing improved emission inventories, and for evaluating and refining chemical model subroutines;
- PAMS chemical and meteorological data provide an alternative to photochemical grid models for empirical assessment of influences of different sources, chemical interactions and transport.
Transport is one of the most difficult aspects of the ozone phenomena to evaluate from a surface measurement campaign like PAMS. It will ultimately be necessary to distinguish among the specific sources of ozone formed from local precursor emissions, ozone formed from precursors transported from upwind sources, and ozone formed during transport from upwind sources. The simple questions of "what was upwind, where and when?" don't often lend themselves to simple answers. Ozone molecules in a single, well-mixed, afternoon air parcel during the midst of a multi-day, regional episode have likely "originated" from a wide variety of local, regional and distant sources.
Routine ozone measurements alone can often provide strong empirical evidence of transport - for example along the coast of Maine during the July 20-22, 1994 episode. Peak hourly concentrations and hour of occurrence at sites along the north Atlantic coast on 7/21/94 are displayed in Figure 7.1.
Figure 7.1 Maximum Ozone and Hour of Occurrence on 7/21/94
Concentrations in excess of the 80 ppb Maine State Standard were recorded at all of these coastal sites, four of which also recorded exceedances of the federal standard. With persistent southwesterly winds throughout the day, the hour of maximum concentration occurs progressively later in the day as one moves to the northeast, ranging from 12 noon at Lynn, MA to as late as 9 pm at Jonesport, ME.
The ozone levels from these North Atlantic Coastal sites are also displayed as 8-hour running averages in Figure 7.2, and provide a clear impression of ozone transport along the coast.
Figure 7.2 8-Hour Moving Average Ozone Copncentrations on 7/21/94
In addition to the smooth, northeasterly time progression and broadening of the plume over time, the 8-hour averages also show the chronic nature of the transported ozone exposures. An 8-hour standard of 0.09 ppm would have been exceeded at 5 of these sites; 0.08 ppm would have been exceeded at 7 sites; and 0.07 ppm exceeded at all of these coastal sites.
On the morning of 7/21/94, surface winds (from AIRS sites) were predominantly from the West or West-Southwest (Figure 7.3).
Figure 7.3 Moving Winds Around Boston on 7/21/94
Morning wind speeds were moderate (4-5 mph) at coastal sites, and increased to 8 to 12 mph by late afternoon (Figure 7.4). By 3 PM (hour of maximum ozone concentration of 148 ppb at Cape Elizabeth, ME), the surface winds throughout northern New England are persistently from the Southwest, aligning coastal Maine directly with higher emission areas around Boston (and further South).
Figure 7.4 NE Coast Surface wind speeds on 7/21/94
The measured afternoon surface wind directions (figure 7.5) are quite consistent with the apparent ozone plume movement (Figures 7.1 and 7.2). However, there's a noteworthy discrepancy between the measured afternoon surface wind speeds at coastal sites (8 to 10 mph) and the implied speed of the coastal ozone plume from Figures 7.1 and 7.2 (20 to 25 mph). The ozone plume is apparently moving at 2 to 3 times the speed of the surface winds.
Figure 7.5 Surface Winds at 3 PM on 7/21/94
This suggests that bulk of the "ozone transport" during this episode may have occurred well above the surface (and/or over water) - at higher wind speeds, and perhaps at different directions. Conceivably, this kind of approach to PAMS data analysis could provide useful information for refinement of meteorological models. The discrepancy between ozone speed and (land-based) surface wind speeds suggests that meteorological models which rely too heavily on land surface data, or which fail to adequately reproduce sea breeze effects, may underestimate the relevant distances and/or mis-identify the directional influences of transport sources under certain conditions - particularly along the Northeast coastal corridor.
Certainly, this also helps illustrate the importance of PAMS upper air meteorological measurements (required, but not yet in place at this time).
As indicated in Section 3 (Figure 3.6) both studied episode periods were characterized by atypically high regional morning NOx concentrations.
As displayed in Figure 7.6, NOx levels at the Lynn, MA PAMS site were lower than average throughout the day on 7/21/94, while NOx levels at Cape Elizabeth, ME were substantially higher than average during the sunrise to noon period. The peak NOx concentration at Cape Elizabeth occurred at the unlikely hour of 11 AM EST (noon, local time). This was the highest NOx level recorded at Cape Elizabeth during July, 1994, and the only time that month when concentrations there were higher than at Lynn.
Figure 7.6 Diurnal Nox Levels at Lynn, MA and Cape Elizabeth, ME Monthly Averages for July, 1994, and Specifies for July 21, 1994
Another unusual feature of the Cape Elizabeth PAMS data on 7/21 was the simultaneous arrival at 11 AM of both NOx and elevated VOC concentrations of a relatively fresh, unreacted nature. Recall from Sections 6.2 and 6.3 that a benzene to toluene (B/T) ratio of about 0.4 is predicted (from automotive-related sources), and consistently observed at Northeastern urban sites (with the exception of Chicopee, MA where a local toluene source substantially reduces the B/T ratio). This "fresh" B/T ratio increases, particularly during daylight hours, at downwind sites, as the more reactive toluene is differentially depleted during transport. Cape Elizabeth typically experienced an average B/T ratio of 0.8 (nearly double the anticipated fresh emission ratio) during the studied episode periods (Figure 6.8), suggesting relatively minor influence from "fresh", local, mobile source emissions.
In Figure 7.7, the Y scale is for benzene in ppbc, while the toluene values are multiplied by 0.4, such that, where the plotted values for benzene and toluene are similar, the B/T ratio is 0.4. This shows that the VOC mix at Cape Elizabeth is relatively unreacted when peak NOx and toluene levels are recorded at 11 AM. At this point, the B/T ratio declines as benzene continues to climb, while toluene and NOx levels are falling.
Figure 7.7 Nox, Benzene and Toluene at Cape Elizabeth, ME on 7/21/94
Surface wind directions on 7/21/94 for the Cape Elizabeth, ME and Lynn, MA PAMS sites are displayed in Figure 7.8. At Lynn, winds were persistent throughout the day, ranging from West to Southwest. Cape Elizabeth experienced similar wind directions in the early morning and late afternoon/evening, but showed a distinct shift to Southerly winds (173 degrees) at mid-day (11 AM EST). The elevated NOx and unreacted VOCs were blowing in from the ocean.
Figure 7.8 Cape Elizabeth and Lynn Surface Winds Directions on 7/21/94
These observations are quite consistent with a scenario whereby emissions from the greater Boston area are blown out over the water during the early morning (and preceding night), and then delivered at Cape Elizabeth by a temporary offshore wind shift (sea breeze) at 11 AM. For this particular episode, there is strong evidence of transport of both ozone and unreacted precursors along the Coast of Maine. Peak ozone levels at Cape Elizabeth occurred 4 hours later (3 PM) at higher wind speeds and a wind direction of about 200 degrees. In this case, it would appear that transport of ozone is more important than transport of precursors for Cape Elizabeth. The transported, mid-day precursors at Cape Elizabeth are likely to have contributed to enhanced ozone levels further up the coast.
One perplexing question is: how is it possible for a large dose of unreacted NOx and VOC precursors to arrive at a site relatively distant from major emissions sources at such a late hour on a hot, sunny day? Possibly the transport over relatively cold water impeded the reaction rate; or possibly a localized cloud bank reduced the available solar radiation? Another possibility is that the VOC and NOx were transported through different, relatively stable layers of the atmosphere. The mid-day sea breeze, combined with enhanced vertical turbulence at mid-day would tend to mix the separate layers together and bring them down toward the surface on arrival at the Maine coast. This scenario might occur if the VOC concentrations originated primarily from ground level sources (the observed B/T ratios are indicative of mobil sources), and if the NOx concentrations were primarily from stationary sources (offset spatially or vertically from the mobile source emissions).
Identifying separate influences from elevated, stationary NOx sources poses an extreme challenge for PAMS (or any other surface-based measurement program). During daylight hours, with good vertical mixing, NO emissions from an elevated point source would be depleted rapidly and their influence at a surface receptor site would be experienced as a variety of photochemical reaction products, including ozone. At night, under stable atmospheric conditions, elevated NO emissions would not be mixed down to the surface, and/or would be rapidly depleted through reactions with ozone in the case of nocturnal ozone transport aloft. The inverse correlation between NO and Ozone is one of the strongest statistical relationships observable at PAMS sites - as illustrated in the Figure 7.9 scatter plot of ozone vs. NO from all available PAMS sites for Summer, 1993.
Figure 7.9 Ozone vs. NO at All Available PAMS Sites in AIRS (at Right) for Summer, 1993
Unfortunately, few PAMS sites measure SO2, fine particle composition (S, V, As, Se, Ni, etc.) or other species which could provide an indication of stationary source influence. Although CO is not required at PAMS Sites, it is fortunately often measured at many of them. As indicated in figure 7.10, there was a moderately strong, positive correlation between CO and NO at these sites.
Figure 7.10 Ozone vs. NO at All Available PAMS Sites in AIRS (at Right) for Summer, 1993
Although NO is highly reactive and CO is not, this positive correlation is not surprising, given common sources and common meteorological influences on both pollutants.
The ratio of NOx (predominately NO) to CO emissions in fresh automotive emissions is approximately 0.1, while the ratio in most stationary combustion source emissions (except woodburning) is generally greater than 1.0. Consequently, the ratio of NO or NOx to CO might provide some indirect indication of the relative influence from these different source types. Because NOx is rapidly depleted under most conditions, the ambient air ratio of NOx to CO is likely to be much lower than the ratio in fresh emissions. Note that in figure 7.10, few of the plotted data points for any of the 1993 PAMS sites exhibited NO/CO ratios greater than 0.1. A ratio of 0.05 is more typical, and many points fall below the 0.025 line. This does not indicate that there was no influence from any stationary source at any PAMS site in 1993. Nor does a NO/CO ratio of greater than 0.1 necessarily provide clear evidence of stationary source influence. But as a general rule, the higher the ratio, the more probable the influence from stationary source NOx emissions.
As indicated in Section 3, both studied episode periods experienced atypically high regional average morning NOx concentrations (Figure 3.6). Regional average CO levels were relatively high during the 7/6-8/94 episode, and low during the 7/20-22/94 episode (Figure 3.7).
Figure 7.11 displays the regional average NOx to CO ratios for the month of July, 1994. Some caution is warranted here, as there were relatively few (16 - 18) New England sites measuring CO and NOx during this period, and the 2 pollutants were measured concurrently at only about half of these sites. It is, however, of interest that the highest regional average NOx to CO ratios for the month were experienced at 5 AM on the mornings preceding these 2 episode periods.
Figure 7.11 Average NE Regional Nox to CO Ratios for 7/94
Figure 7.12 displays the NO/CO ratios for sites in Boston, MA and Worcester, MA - where CO and NO measurements were co-located during July, 1994. On the morning of 7/7/94, when (light, Easterly) winds were generally in the direction of Boston toward Worcester (and when high afternoon ozone levels were recorded from central MA and South), the CO/NO ratio at Worcester was (>0.2) substantially higher than could be accounted for by mobile sources alone. On the morning of 7/21/94, when (stronger, Westerly) winds were generally in the direction of Worcester toward Boston (and when high afternoon ozone levels were recorded North of Boston), the NO/CO ratio at this Boston site (0.15) was again substantially higher than could be accounted for by mobile sources.
Figure 7.12 7/94 NO to CO Ratios for Boston and Worcester, MA
While there are other possible explanations for these patterns, there is at least some indirect indication that both episode periods were initiated by atypically high NOx influence from stationary sources. For the 7/21/94 episode in particular, there is evidence of transport of both ozone and unreacted precursor pollutants. For the transported precursors, unreacted VOCs, exhibited a composition consistent with mobile source emissions, and NOx concentrations were preceded by NO/CO ratios suggestive of substantial influence from stationary sources within the region. The fact that regional NOx and NOx/CO ratios were high in advance of both episodes, also suggests the possibility of regional scale stationary source influence from sources upwind of the New England domain.
