This summary was prepared as an informational
document for use by the OTAG Air Quality Analysis Workgroup.
Two complementary assessments were performed and are summarized
here. (1) Transport and mixing phenomena related to ozone exceedances
in the northeast U.S. were examined using data from 1995 NARSTO-Northeast
measurements. (2) Results of the OTAG modeling for the northeast
U.S. were evaluated using available air quality data for 1988
and 1991 and using air quality and meteorological data from NARSTO-Northeast
for 1995. Stand-alone reports were prepared for each of these
This summary is a working draft document
and has not been peer reviewed or reviewed by the NARSTO-Northeast
participants. All conclusions are those of the authors and not
of NARSTO-Northeast or the sponsors.
The authors appreciate the cooperation
and input of the NARSTO-Northeast participants. This summary
and the underlying reports draw heavily on the NARSTO-Northeast
database and on information presented at the First NARSTO-Northeast
Data Analysis Symposium and Workshop at Norfolk VA in December
1996. We are especially grateful for data, analyses, and technical
input from Bob Gaza and Doc Taylor of NYDEC, Bill Ryan of the
University of Maryland, Jian Zhang of the State University of
New York at Albany, and Paul Roberts, Jerry Anderson, and
Tami Haste of Sonoma Technology, Inc. (STI).
This effort was initiated and coordinated
by Jeffrey West of General Public Utilities (GPU) and was made
possible by funding from GPU, New England Electric System (NEES),
and the Electric Power Research Institute (EPRI).
The authors thank the U.S. EPA and the
OTAG modeling centers for providing the UAM-V model results and
routine observational data, and thank the NARSTO-Northeast sponsors
for making the special study data available.
The OTAG model evaluation report is partially
based on a separate report titled ìAssessment of UAM-V
Model Performance in the Northeastî (Londergan and Moore,
1997) prepared by Earth Tech for a consortium of Northeast utilities.
The authors appreciate the hard work and
long hours of Carolee DeWitt, Sandy Barger, and Martina Shultz
of the STI publications staff who made it possible to complete
these reports on a very short time schedule.
In the summer of 1995, NARSTO-Northeast
measurements were performed as part of the North American Research
Strategy for Tropospheric Ozone (NARSTO) to enhance the understanding
of factors governing ozone formation and transport in the Northeast
United States. NARSTO-Northeast provided an extensive database
on the three-dimensional distribution and transport of ozone and
precursors in the northeast US. This database has made possible
(1) an assessment of the transport and mixing phenomena affecting
ozone episodes in the Northeast Ozone Transport Region (OTR) and
(2) a detailed evaluation of modeling performed in mid-1996 by
the Ozone Transport Assessment Group (OTAG) for subregions of
Analyses of NARSTO-Northeast data showed
that transport of ozone and precursors is affected by flow regimes
that differ with altitude. Near-surface flows typically were
from the south to southwest, flows from 200-800 m msl were from
the southwest to west, and flows from 800-2000 m msl were from
the southwest to northwest. The aloft flows resulted in transport
of ozone and precursors over distances of 300-800 km (200-500
miles) in 24 hours, often with the greatest velocities at night.
The near-surface speeds were lighter than those aloft and near
calm during late night hours, resulting in accumulation of emissions
in the night and morning hours and subsequent transport distances
of half or less of those aloft.
During the episodes studied, 30-50 ppb
of excess ozone that was transported aloft overnight, in combination
with an assumed non-anthropogenic background of about 40 ppb,
resulted in measured regional background concentrations of 70-90
ppb or more over much of the OTR. If such conditions typically
occur more than two days/year, this transport contribution might
be sufficient to cause exceedances of the proposed new
eight-hour 80 ppb federal ozone standard in some regions.
The above background amount is about half
of the 150-180 ppb peak one-hour concentrations seen downwind
of urban areas. If the chemistry were linear, simple superposition
would suggest that clean air and transported excess ozone each
might account for about 25 percent of the one-hour peak ozone
downwind of urban areas, with the other 50 percent coming from
same-day urban emissions. Simple box-model calculations,
however, demonstrate that the chemistry is not linear and suggest
that reducing the excess ozone aloft may not reduce the
peak surface concentrations downwind of urban areas in proportion.
Emissions from urban areas were seen to
cause exceedances of the current federal standard 50-250 km downwind
and could have done so even without a contribution from excess
ozone aloft. In particular, ozone and precursors transported
offshore near the surface from Massachusetts resulted in ozone
concentrations of over 150 ppb at shoreline sites in Maine.
For the OTAG modeling of the July 14-15,
1995 ozone episode in the OTR, the speeds, directions, and transport
distances of the simulated regional flows above 500 m agl were
qualitatively similar to the measured flows, however details of
mesoscale flows such as the night jet were not resolved. The
near-surface flows were less accurately resolved, resulting in
errors in the location of impact of urban emissions, especially
near the coast. These results are not unexpected due to the
coarse resolution of the OTAG models. The OTAG modeling was intended
to capture the larger regional-scale features of ozone episodes
and to characterize the relative contribution of regional versus
local emission sources. It was not designed to simulate accurately
the detailed concentration distributions near individual urban
For the OTAG modeling for 1988, 1991, and
1995, the magnitude of the highest one-hour simulated concentration
in a subregion of the Northeast was often comparable to the highest
observed at any station in the subregion. However, the exact
time and location of the simulated one-hour daily maximum concentrations
were rarely accurate. This mismatch in space and time is partially
due to inaccuracies in the simulated wind fields and is typical
of air quality model simulations.
For most days, the comparisons of observed
and simulated ozone concentrations in the Northeast Corridor showed
little prediction bias for peak and mean concentrations in ìurban
plumes.î The model error was comparable to typical urban-scale
modeling results for many subregions and days. However, there
were some days for which the modelís bias and error did
not meet EPAís urban-scale photochemical modeling acceptance
criteria, suggesting caution about the reliability of the modeling
For each episode evaluated, the mean bias
(throughout the model domain) in ozone showed a systematic trend
with time during the episode (known as bias creep). The model
consistently underestimated the observed concentrations for the
first few days of the episodes and overestimated the observed
concentrations for the last few days of the episodes. This systematic
trend may indicate a problem with the model formulation or model
inputs that may influence the accuracy of its simulated response
to emissions controls.
Boundary layer ozone flowing into the urban
corridor was consistently underestimated in the simulations.
The daily maximum concentrations in the upwind areas typically
were underestimated by 10-20 ppb. The model commonly underestimated
the morning ozone aloft and, therefore, the carryover of ozone
from the previous day. Comparisons of simulated and observed
vertically integrated ozone concentrations show the model simulations
typically underestimated ozone aloft by 10 and 5 ppb in the morning
and afternoon, respectively. Therefore, the simulated and observed
afternoon vertical profiles often seemed to show good agreement
and little bias.
Comparisons of simulated and observed vertically integrated NOy,, RHC, and CO concentrations show the simulations mostly overestimated NOy concentrations and mostly underestimated RHC and CO concentrations. Based on all of the aircraft samples collected during the July 1995 episode, the model estimated 27 ppbC of RHC aloft when 90 ppbC was observed; however, the number of data points for this comparison was small.
PREFACE AND ACKNOWLEDGMENTS ii
1. INTRODUCTION S-1TRANSPORT AND MIXING PHENOMENA THAT AFFECT OZONECONCENTRATIONS IN THE OTR S-3EXTENT THAT OTAG MODEL RESULTS REFLECT IMPORTANT TRANSPORTPHENOMENA S-7DIFFERENCES BETWEEN MEASURED AND MODELED OZONECONCENTRATIONS S-105. REFERENCES S-13
The Ozone Transport Assessment Group (OTAG)
was initiated by the Environmental Council of the States (ECOS)
in 1995 to develop a consensus on how to deal with the transport
of ozone and precursors in the eastern United States. Among the
issues confronting OTAG is the scope of regional transport of
ozone and its precursors: how to assess the influence of transport
on ozone concentrations and to design emissions management strategies
to account for it.
OTAG performed ozone modeling for the eastern
United States to characterize regional transport and assess the
effects of emission reductions (OTAG, 1996a). The modeling was
performed for a large region extending from Florida to northern
Michigan and from Texas to Maine. The scientific credibility
of the OTAG modeling is derived from the formulation of the model,
the quality of the input data, and the performance of the model
in these specific applications. To evaluate model performance,
OTAG provided a comparison of measurements and model simulations
for the entire domain and for four large portions of the domain,
including the Northeast (OTAG, 1996b). OTAG did not evaluate
the model for subregions within the Northeast.
During the summer of 1995, a public-private
consortium sponsored expanded measurements as part of the North
American Research Strategy for Tropospheric Ozone (NARSTO) to
enhance the understanding of factors governing ozone formation
and transport in the Northeast United States. Dubbed "NARSTO-Northeast,"
this study provided an extensive database on the three-dimensional
distribution and transport of ozone and precursors. Extensive
surface and upper-air air quality and meteorological data were
obtained for several ozone episodes in 1995 and 1996, and limited
data were obtained in 1994. The 1995 observational campaigns are
described by Roberts et al. (1995), and the database and data
access procedures are described by Korc et al. (1996).
The NARSTO-Northeast database has made
possible (1) an assessment of the transport and mixing phenomena
influencing ozone episodes in the Northeast Ozone Transport Region
(OTR) and (2) a comparison of the OTAG model simulations with
measurements for subregions of the Northeast portion of the modeling
domain. Complementary reports addressing these two topics have
been prepared by Blumenthal et al. (1997) and Lurmann et al. (1997),
and their results are summarized here. These assessments were
intended to address the following objectives.
A series of questions was posed for each
objective. The first objective was addressed by Blumenthal et
al. (1997) and the third by Lurmann et al. (1997). Time was not
available to perform specific analyses to address the second objective,
but the questions for the second objective were partially addressed
through examination of the results reported in the above two reports.
The following sections summarize the answers
to the questions posed for each objective.
This section summarizes important transport
and mixing phenomena identified during NARSTO-Northeast that were
related to exceedances of the federal ozone standard during the
1995 measurement period. The results described are based on the
ozone episodes of July 13-15 and July 31-August 1, 1995.
Transport of ozone and precursors is influenced
by flow regimes that differ with altitude. These regimes can
be divided into three general categories plus a subset of the
1) Boundary Layer Synoptic Transport roughly
800-2000 m msl
- Transports ozone and precursors across the Appalachians - from western OTR and Midwest
- Flows are from west to northwest during most episodes
- Results in transport distance of 300-800 km (200-500 miles) in 24 hours
- Does not have strong diurnal component
2) Channeled Flows Below the Ridge Heights
roughly 200-800 m msl
- Includes nighttime low-level jets (occurred on 6 of 9 1995 regional O3 episodes)
- Includes "lee trough" flow caused by the interaction of the synoptic flow and the Appalachians
- Often transports substances from southwest along urban corridor, but other directions occur, including flow from the west through gaps in the Appalachians
- Results in transport distances of 200-400
km (125-250 miles) overnight or similar to synoptic flow over
3) Near Surface Flows roughly 0-200 m msl
- Light winds in night and morning allow accumulation
- Fresh emissions and urban plumes move downwind and react during daytime
- O3 aloft and aged precursors are entrained as the mixing layer deepens
- Transport is typically to the north through
east along the urban corridor for 50-250 km (30-150 miles) by
evening. Transport distances over 24 hours are typically less
than half of those for the above flow regimes.
3a) Offshore Flows (subset of 3 above) roughly
0-200 m msl
- Light winds in night and morning allow accumulation onshore
- Accumulated urban emissions can transport offshore and react during daytime
- Transport is typically to the northeast through east: Boston to NH, ME; Philadelphia and NJ to Long Island; Baltimore/Washington across bay to DE
- Transport can be 200 km (125 miles) or more during daytime and evening
- Layer stays stable and thin over water, with minimal mixing and no added emissions
- Can cause high concentrations at shoreline;
dilutes with mixing as transported inland
During the episode periods studied, the
near-surface flows typically were from the south to southwest,
the channeled flows were from the southwest to west, and the synoptic
flows were from the southwest to northwest. The channeled flow
directions varied during the nights, with more southerly flow
earlier in the night and more westerly flows later at night.
The surface speeds were lighter than those aloft, resulting in
transport distances of half or less than those aloft. Typically,
the surface speeds were lightest at night, while the channeled
flow speeds were highest at night.
During the episode nights studied, the
first two flow regimes above became decoupled from the surface
flow, and the channeled-flow regime accelerated. During the day,
winds throughout the boundary layer become coupled, and wind speeds
and directions can become similar through all three regimes.
During the day, frictional effects can slow the winds aloft below
the nighttime speeds. On most episode nights studied, a jet occurred
in a layer just above the surface, where the speeds were higher
than those above or below. On some nights, the winds aloft did
not form a jet (i.e., they were not faster than the winds both
above and below). They were, however, faster than the winds below.
This is a normal occurrence on most summer nights. When the
frictional effects are decoupled, the winds aloft will be faster
than the surface winds. When the aloft winds are decoupled from
the surface, ozone and precursors aloft can be transported without
deposition to or input from the surface layer.
The wind speeds aloft also varied from
south to north, with lighter winds and shorter transport distances
in the southern portion of the OTR. Average boundary layer synoptic
wind speeds during episodes are 5-10 m/s in the New York City
area and about 4 m/s in the Washington, DC area.
Trajectory analyses and aloft ozone measurements
show ozone can be transported 300-800 km (200-500 miles) in 24
hours across the Appalachians at altitudes of 800-2000 m msl and
200-400 km (125-250 miles) overnight along the corridor or through
gaps in the mountains below the ridge heights at altitudes of
200-800 m msl. Over 24 hours, the transport distances for the
200-800 m msl channeled flows are similar to those for the synoptic
flow above, but more of the transport occurs at night in the channeled
The aloft transport can result in the widespread
occurrence of overnight ozone carryover aloft at average boundary
layer concentrations of 70-90 ppb and with layers of over 100
ppb. The resulting background ozone is a mixture of ozone transported
from different locations at different altitudes. When mixed to
the surface, this transport can lead to surface ozone concentrations
of 70-90 ppb. This surface background contribution is roughly
30-50 ppb above clean air, assuming a clean-air value of about
40 ppb. This background carryover ozone can be caused by emissions
or ambient ozone coming from 200-800 km (125-500 miles) away since
the prior day.
From early morning ozone, NOy,
and VOC measurements aloft, long-range transport seems to contribute
mainly ozone and aged precursors, but fresh primary NOx
emissions also can be transported overnight in these flows.
Same-day plumes (or pulses) of ozone downwind
of urban areas are embedded in a regional background. The peak
urban plume concentrations were up to 80-100 ppb higher than the
nearby regional upwind or crosswind concentrations. Offshore
transport of near-surface emissions was seen to cause downwind
offshore and shoreline concentrations of over 150 ppb, which were
80-100 ppb higher than the concentrations higher aloft and
at downwind onshore areas not impacted by the transported plume.
This increased near-surface ozone was confined near the surface
and did not mix with the lower concentrations aloft until it was
Daytime transport distances (0500-1900
EST) of these near-surface emissions were in the range of 50-250
km (30-150 miles). During the night, surface winds were generally
light and transport distances were on the order of 25-75 km.
Under these conditions, even with clean
air as a background, urban plumes resulting from same-day emissions
could cause ozone concentrations to exceed the one-hour federal
standards 50-250 km (30-150 miles) downwind.
Mixing heights increased more slowly on
widespread episode days. The lower morning mixing heights can
result in accumulation of higher precursor concentrations in the
surface layer in the morning. They will also result in higher
precursor concentrations in layers transported offshore in the
Ozone aloft can influence surface concentrations
through addition of aloft ozone and precursors to the surface
concentrations as the mixing layer deepens and entrains them.
During episode conditions in the OTR, ozone
transported aloft combined with the clean air background of 40
ppb can provide regional background concentrations of 70-90 ppb
or more. This amount by itself may be adequate to cause exceedances
of the proposed new federal standard; however it is only about
half of the peak concentrations seen downwind of urban areas.
This would suggest that if the chemistry were linear, clean air
and transported ozone each might account for about 25 percent
of the peak ozone downwind of urban areas, with the rest coming
from same-day urban emissions.
To account for the chemistry of the interaction
between ozone transported at various levels aloft and fresh surface
emissions, simple two-layer box-model simulations were performed
for conditions similar to August 1, 1995 and with many simplifying
assumptions. Assumed urban emissions were varied to assess sensitivity
to emissions, and two different chemical mechanisms were used.
These simulations indicated that ozone carryover aloft might
increase the maximum surface concentrations downwind of major
urban areas by less than one third to over 100 percent of the
aloft excess over 40 ppb (clean air) for the range of emissions
used. Most of the simulations were in the bottom half of this
range. On August 1, 1995, ozone carryover aloft was roughly 70-80
ppb. Given this range for the initial concentration in the upper
box of the model, the maximum ozone values simulated for the lower
box of the model increased by 13 to 45 ppb from what they would
have been with only 40 ppb of ozone aloft.
For the range of conditions simulated with
the box model, roughly 60-75 percent of the maximum ozone seen
in the lower layer was due to same-day emissions in the lower
layer plus an assumed 40 ppb natural background from aloft; while
roughly 25-40 percent was due to carryover and formation in the
upper box. However, reducing ozone concentrations in the upper
box generally did not decrease the concentrations in the lower
box in proportion.
These results imply that reductions in transported ozone might lower the regional background concentrations upwind of OTR urban areas by an amount similar to the reduction in transported ozone, but that these same reductions in transported ozone might not result in proportional decreases in the peak concentrations downwind of the urban areas.
The primary objectives of the OTAG modeling
were to characterize the contributions to ozone nonattainment
in the eastern United States due to regional transport, and to
assess the effects of regional emission reductions on peak ozone
in identified ìproblem areas.î The OTAG modeling
was intended to capture the larger regional-scale features of
ozone episodes and to characterize the relative contribution of
regional versus local emission sources. It was not intended to
accurately simulate the detailed concentration distributions near
individual urban areas. For these reasons, the OTAG air quality
and meteorological models were applied using a 12 km grid spacing
for the Northeast. The meteorological models also had limited
vertical resolution. The grid spacing was not adequate to resolve
the local variations in emission density or in topography and
surface characteristics that are important for estimating the
time and location of maximum ozone concentrations near a large
urban area, nor was it adequate to resolve fine-scale features
of the flow fields.
Due to the nature of the modeling objectives,
it was not expected that the OTAG model would accurately simulate
many of the fine-scale transport and mixing processes occurring
in the Northeast. Detailed analyses to assess how well the model
results reflect the transport phenomena described in Section 2
were not performed, but insights can be obtained by examining
the formulation of the model as well as data and analyses related
to the regional model evaluation tasks for the July 1995 episode.
For this episode, the UAM-V air quality model was used with an
input wind field generated by the RAMS prognostic meteorological
Some specific questions are addressed below.
Comparisons of NARSTO-Northeast wind profiler
measurements for the 1995 episode with OTAG model wind fields
show similar transport characteristics. In most cases, model
wind speeds are comparable to measured values at the surface and
aloft, recognizing that the model vertical layer structure smoothes
out some features of the wind profiles. (See more specific discussions
below concerning night jet and coastal effects.) During periods
with very light winds, the model surface level wind speeds are
often lower than observed. Comparisons between back trajectories
derived from the OTAG model and from interpolation of the actual
upper-air data (using CALMET) for the July, 1995 episode show
similar transport distances over a 24-hour period, for both near-surface
and 500 m level winds. However, simulated near-surface wind directions
sometimes differ from measured directions by 60 degrees or more.
These differences are usually greater for light wind conditions,
and at locations where local influences (topography, coastal)
are present. For the 500 m elevation, comparisons generally
showed qualitatively close agreement between observed and simulated
wind directions for July 13 and 14, but differed by up to 45 degrees
on the 15th. Such errors in wind direction would lead to errors
in the superposition of emissions from different source areas
and errors in the mixing of surface emissions with ozone transported
The modeled winds for the 1995 episode
show no evidence of a ìnight jetî (e.g., a wind speed
maximum a few hundred meters above the surface). The OTAG modeling
approach (12 km grid, limited vertical resolution, use of RAMS
model) is not able to predict such phenomena. Even if the RAMS
were to predict a night jet aloft, use of layer-average winds
with layers at 250-500 m and 500-1500 m in the air quality model
would wash out much of the effect. However, the model does resolve
the decoupling of the aloft flow from the surface, showing light
winds at the surface and continued higher-speed flow aloft at
night. It also resolves the general clockwise turning of the
wind direction with altitude.
It is difficult to isolate the impact of
transport associated with the ìnight jetî on air
quality. Basically, we know that simulated transport is often
incorrect, in terms of where and when a particular air parcel
aloft originated, but we do not know how much different the pollutant
loading of that air parcel would be if the transport were correctly
simulated, or what the effect on peak simulated ozone concentrations
might be. In general, however, the model reflects that nighttime
ozone transport aloft occurs, even if it does not get the details
of the jet correct.
We did not perform a specific evaluation of the temperature profile/mixing height inputs for OTAG modeling. At elevations below 500 m agl, the RAMS and UAM-V models have adequate resolution to simulate the slow rise of the mixing height on episode days; but no evaluation was performed to see if the mixing height was simulated properly. Above 500 m, the UAM-V vertical layer structure can only account very roughly for mixing height changes since there is only one layer from 500 m to 1500 m. This large layer depth makes it difficult for the model to resolve the effects on ozone transport of the mixing height increasing through the channeled flow regime into the synoptic regime.
Model input wind fields generally do not reflect the mesoscale influence of the land-sea interface. Review of wind measurements at coastal locations (e.g., Connecticut and Long Island) during the 1995 OTAG episode indicates several days when regional-scale transport was dominant and sea-breeze circulation produced only modest local effects. On July 15, however, failure to account for sea-breeze circulation led to erroneous transport predictions and poor model performance by UAM-V in the Northeast. Measured surface-level wind directions at Brookhaven, NY (on Long Island) and Millstone Point (on the Connecticut shoreline) reflected an onshore WSW sea breeze flow on July 15, while UAM-V model inputs were WNW. This difference was enough for the model to miss the impact of the New York City area plume on the Connecticut shoreline.
The coastal flow regime and local transport around the Chesapeake Bay on July 15 are more complex, and we have less meteorological measurements for that region, but it is also clear that OTAG model results did not correctly predict transport in the B-W subregion on that day.
For July 14, trajectories based on near-surface
and 500 m level winds show good agreement between the OTAG model
and CALMET diagnostic wind fields. RAMS (OTAG) surface-level
trajectories were about 25 percent longer (higher wind speeds),
but transport directions were similar, ranging from S to WSW.
Much closer agreement was seen aloft, with transport from WSW
For July 15, RAMS back trajectories starting
at 1500 showed surface-level transport from W to WNW across the
entire NE Corridor, with transport at 500 m from WNW to NW. CALMET
(observed) back trajectories at inland locations (e.g., Ware,
MA and Rutgers) were very similar to RAMS results, both near-surface
and aloft. Back trajectories for Millstone Pt., CT, Brookhaven,
NY, and Lums Pond, DE, however, all indicated surface-level transport
from WSW and 500 m level transport from W for the first 6 hours
(back to 0900). (See discussion of coastal influence above.)
From the comparisons of the trajectories
and wind fields, it appears that the speeds, directions, and transport
distances of the simulated regional flows are qualitatively correct.
However, as expected from the coarse model resolution, the near-surface
flows are less accurately resolved, resulting in errors in the
location of impact of urban emissions.
Lurmann et al. (1997) examined model performance
in four subregions within the Northeast Corridor (Baltimore-Washington,
Philadelphia, New York, and Boston nonattainment areas) and in
the boundary areas immediately upwind of the Northeast Corridor.
The evaluation included (1) a review of
observed and simulated ozone concentrations in the Northeast Corridor;
(2) comparisons of observed and simulated ozone at ìboundary
sitesî located along the western boundary of the Northeast
Corridor; and (3) comparison of simulated and observed ozone aloft
and transport for the July 1995 ozone episode.
The OTAG air quality and meteorological
models were applied using a 12 km grid spacing for the Northeast.
This grid spacing is not adequate to resolve the local variations
in emission density or in topography and surface characteristics
that are important for estimating the time and location of maximum
ozone concentrations near a large urban area. The performance
analysis was therefore focused on issues pertaining to regional-scale
performance, recognizing the practical compromises that were necessary
to achieve OTAGís mission with the available data and computational
The following findings emerged from the
assessment of the OTAG simulations:
Comparisons of observed and simulated ozone
concentrations for subregions corresponding to the major metropolitan
areas along the Northeast Corridor show more scatter than the
regionwide performance statistics reported in the OTAG evaluation.
Results for individual subregions and days in the Northeast Corridor
include some cases with excellent agreement between the model
and the measurements and other cases of poor agreement. The excellent
agreement cases have peak ozone levels simulated within 15 ppb
of the observed values and within 50 km of the observed peak locations,
as well as mean normalized bias and error statistics that meet
EPAís acceptance criteria. In contrast, the poor agreement
cases have either bias in peak ozone levels of more than ±40 ppb
or mean normalized bias and error statistics that fail to meet
EPAís acceptance criteria (or both).
The daily maximum concentrations unpaired
in space and time compare more favorably with the observed maxima
than the simulated maximum concentrations paired in space or paired
in space and time. That is, the magnitude of the highest simulated
concentration in a subregion is often comparable to the highest
observed at any station in the subregion. However, the exact
time and location of the simulated daily maximum concentrations
are rarely accurate. This mismatch in space and time is partially
due to inaccuracies in the modeled wind fields and is typical
of air quality model simulations.
On most days, the comparisons of observed
and simulated ozone concentrations in the Northeast Corridor show
little prediction bias for peak and mean concentrations in ìurban
plumes.î The model error is comparable to typical urban-scale
modeling results for many subregions and days. However, there
are a modest number of days in the 1988 episode and a small number
of days in the 1991 and 1995 episodes where the modelís
bias and error do not meet EPAís urban-scale photochemical
modeling acceptance criteria, suggesting caution about the reliability
of the modeling effort. These poor performance days occur in
the middle of episodes, as well as at the beginning and end of
episodes (see below).
The model performance is less biased for
all hourly concentrations above 40 ppb and the daily 90th percentile
concentration than for the daily maximum concentrations. The
coarse spatial resolution selected for the OTAG simulations (12
km) and differences between the modeled and measured transport
winds probably compromise the accuracy of the maximum concentration
For each episode, the mean normalized bias
for ozone shows a systematic trend with time during the episode
from underprediction to overprediction (known as bias creep).
The model consistently underestimated the observed concentrations
for the first few days of the episodes and overestimated the observed
concentrations for the last few days of the episodes. This trend
clearly demonstrates a regional-scale buildup of simulated ozone
that is not evident in the observations. This systematic trend
may indicate a problem with the model formulation or model inputs
that may influence the accuracy of its simulated response to emissions
Analysis of concentrations along the western
boundary of the urban corridor indicates the boundary layer ozone
flowing into the corridor was systematically underestimated in
the simulations. This bias is evident on most days in all of
the episodes. The regional transport contribution to ozone at
the boundary of the urban corridor was 5-10 ppb lower than
observed on average. The daily maximum concentrations in the
upwind area were underestimated by 10-20 ppb on average.
Comparison of vertical profiles of ozone
concentrations (using NARSTO-Northeast aircraft data for 1995)
were mixed, with some showing good agreement and others showing
poor agreement. The model commonly underestimated the morning
ozone aloft and, therefore, the carryover of ozone from the previous
day. The model sometimes overestimated the extent of vertical
mixing in the morning. The simulated and observed afternoon vertical
profiles often showed good agreement and little bias for the Northeast.
The coarse vertical resolution in the model simulations limits
the modelís ability to resolve the vertical structure,
especially in the morning and over the ocean.
Comparisons of simulated and observed vertically
integrated ozone concentrations show the UAM-V model simulations
underestimate ozone aloft by 10 and 5 ppb on average in the morning
and afternoon, respectively. The morning bias in ozone aloft
may be important, but the afternoon value may not be significant.
Comparisons of simulated and observed vertically
integrated NOy,, RHC, and CO concentrations show the
simulations overestimate NOy concentrations and underestimate
RHC and CO concentrations on average. The vertically integrated
concentrations of these species and the biases between model and
observations are comparable in the morning and afternoon. The
magnitude of RHC underestimation is large. On average, the model
estimates 27 ppbC of RHC aloft when 90 ppbC was observed. These
results must be interpreted cautiously because the number of data
points for this comparison is small. Nevertheless, there is concern
because if this RHC bias were widespread, then it would suggest
that the VOC emissions are underrepresented in the modeling system
and that the model may be getting the right ozone for the wrong
Overall, the 1991 and 1995 episodes were
simulated more accurately than the 1988 episode; this may be largely
due to the differences in the meteorological models used in the
These findings indicate that the estimated ozone transport contributions into the Northeast Corridor and between subregions within the corridor are subject to greater bias and uncertainty than the simulated contributions from nearby urban-scale emission sources (although the location of the urban plumes was not necessarily simulated accurately).
Blumenthal D.L., Lurmann F.W., Kumar N.,
Ray S.E., Korc M.E., Londergan R., and Moore G. (1997) Transport
and mixing phenomena related to ozone exceedances in the Northeast.
Prepared for Ozone Transport Assessment Group Air Quality Analysis
Workgroup by Sonoma Technology, Inc., Santa Rosa, CA and Earth
Tech, Concord, MA, STI-996133-1710-FR, March.
Korc M.E., Roberts P.T., and Blumenthal
D.L. (1996) NARSTO-Northeast data management plan. Version 3.1.
Report prepared for Electric Power Research Institute, Palo Alto,
CA by Sonoma Technology, Inc., Santa Rosa, CA, STI-95141-1537,
Research Project EPRI WO9108-01, May.
Lurmann F.W., Kumar N., Londergan R., and
Moore G. (1997) Evaluation of the UAM-V model performance in the
northeast region for OTAG episodes. Prepared for the Ozone Transport
Assessment Group, Air Quality Analysis Workgroup by Sonoma Technology,
Inc., Santa Rosa, CA, STI-996133-1716-FR, March.
OTAG (1996a) Modeling protocol. Report
prepared by Ozone Transport Assessment Group, U.S. Environmental
Protection Agency, Research Triangle Park, NC.
OTAG (1996b) Evaluation of the UAM-V model
performance in OTAG simulations: summary of performance against
surface observations. Draft report prepared for Ozone Transport
Assessment Group, U.S. Environmental Protection Agency, Research
Triangle Park, NC, October.
Roberts P.T., Korc M.E., Blumenthal D.L.,
and Mueller P.K. (1995) Description of the NARSTO-NE 1995 Summer
Ozone Study. Version 1.1. Report prepared by Sonoma Technology,
Inc., Santa Rosa, CA and Electric Power Research Institute, Palo
Alto, CA, November.
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