EVALUATION OF THE UAM-V MODEL PERFORMANCE IN THE NORTHEAST REGION FOR OTAG EPISODES

WORKING DRAFT NO. 2.1
STI-996133-1716-WD2.1

By:
Frederick W. Lurmann
Naresh Kumar
Sonoma Technology, Inc.
5510 Skylane Boulevard, Suite 101
Santa Rosa, CA 95403
Richard Londergan
Gary Moore
Earth Tech
196 Baker Avenue
Concord, MA 01742

Prepared for submission to:
Ozone Transport Assessment Group
Air Quality Analysis Workgroup

March 14, 1997

Abstract

This report is in DRAFT form. It is the second part of an assessment of regional transport issues in the Northeast and how well the transport and its effects are represented by models applied to the area.


Contents


PREFACE AND ACKNOWLEDGMENTS

This report has been prepared as an informational document for use by the OTAG Air Quality Analysis Workgroup. It includes analyses of NARSTO-Northeast data. It is a working draft document and has not been peer reviewed by the NARSTO-Northeast participants. All conclusions are those of the authors and not of NARSTO-Northeast or the sponsors. It is being circulated for review and comment and will be revised in response to comments received. Suggestions for improvements are welcome.

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. 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). This report is partially based on a separate report titled "Assessment of UAM-V Model Performance in the Northeast" (Londergan and Moore 1997) prepared by Earth Tech for a consortium of Northeast utilities.


1.0 INTRODUCTION

The Ozone Transport Assessment Group (OTAG) performed ozone modeling in the eastern United States in order 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. This report focuses on the performance of the UAM-V photochemical model in the baseline simulations in the Northeast region. OTAG provided an evaluation of model performance for the entire domain and for four large subregions (OTAG, 1996b), including the Northeast (as a single region). This report examines 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.

1.1 MODEL EVALUATION OBJECTIVES

This analysis addresses several questions that were not addressed in the OTAG model performance report. These questions are:

  1. How well do model simulations agree in time and space to observed surface and aloft ozone and precursor concentrations in the Northeast?
  2. How do model simulations compare with the observed vertical distributions of ozone and precursors? Are the layers aloft and carryover of ozone resolved by the model?
  3. Can concentration differences be partially explained by differences between measured and modeled transport phenomena?
Note, initially this report was also intended to address the question:
  1. If the modeled transport phenomena and the measurements agreed better, what would be the directional implications for simulated ozone concentrations? How would individual transport phenomena affect simulated downwind concentrations?

However, Question 4 was not addressed in this study because answering it required reapplying the UAM-V model with more accurate windfields which was beyond the scope of the study.

Our analysis includes (1) a review of observed and predicted ozone concentrations in the Northeast Corridor; (2) comparisons of observed and predicted ozone at "boundary sites" located along the western boundary of the Northeast Corridor; and (3) comparison of predicted and observed ozone aloft, based on NARSTO-Northeast aircraft measurements for the 1995 episode. The findings from these analyses are described in subsequent sections, with illustrative examples of each type of analysis.

1.2 Modeling Approach

The primary objective of OTAG modeling was 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 "problem areas." Our analysis is focused on model performance within the Northeast Ozone Transport Region (OTR), specifically on predictions for the serious/severe nonattainment areas along the Northeast Corridor from Baltimore-Washington to Maine, and on the transport of ozone and precursors into that region and between different urban subregions within the OTR.

For OTAG, the UAM-V air quality model and the RAMS and SAIMM meteorological models were applied using a 12 km grid spacing in 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 goal of OTAG modeling was to capture the larger regional-scale features of ozone episodes and to characterize the relative contribution of regional versus local emission sources (OTAG, 1996a). Our performance analysis has therefore focused on issues pertaining to regional-scale performance, recognizing the practical compromises that were necessary to achieve OTAG's mission with the available data and computational resources. In order to assess model performance in the Northeast, however, it is important to distinguish the impacts associated with each of the major metropolitan areas that represent the dominant source regions contributing to ozone in the Northeast.


2. MODEL PERFORMANCE FOR OZONE IN THE NORTHEAST CORRIDOR

Model performance for each of the four OTAG episodes was evaluated using subregions that reflect the locations of major metropolitan areas and the predominant southwest-to-northeast transport commonly associated with ozone episodes in the Northeast. Geographic subdivision provides a clearer picture of model performance by providing comparisons of observed and predicted ozone concentrations associated with the "urban plumes" for the Boston, New York City, Philadelphia, and Baltimore-Washington subregions. Monitoring stations in the Northeast were assigned to five categories (the four urban subregions plus "outside of Corridor" sites), as depicted in Figures 2-1, 2-2, 2-3, 2-4, and 2-5. The set of ozone monitors varied by episode; these figures represent a composite based on the 1988, 1991, and 1995 episodes. The total number of ozone monitors in the Northeast is roughly 190, of which about 125 are located in the corridor. Model evaluation was performed for the stations in the Northeast Corridor and stations in the western boundary region (see Section 3).

For the subregions in the corridor, the monitoring network provides relatively good spatial coverage of areas with high observed ozone concentrations. This was confirmed by inspection of the daily maximum 1-hour ozone concentrations by subregion; the top two or three concentration values generally agreed within 10 ppb. (Exceptions are noted below in the discussion of results for individual episode days.)

Three of the four OTAG episodes were included in the analysis: July 4-15, 1988, July 16-21, 1991, and July 10-18, 1995. The initial model "spin up" days and some of the later days were excluded from our analysis because of low peak concentrations. The July 22­29, 1993 OTAG episode was excluded from the analysis because the peak concentrations in the Northeast were significantly lower than in the other three episodes. Ozone exceedances occurred on only three of the eight episode days in the Northeast in 1993 and the maximum observed values were 137, 142, and 163 ppb on these days. In addition, the model simulated peak concentrations were consistently low in the Northeast during the 1993 episode.

2.1 APPROACH

A number of statistical measures were used to evaluate the model performance. Experience has indicated that no one measure is adequate to characterize performance; multiple measures are needed. The specific measures are:

  1. The daily maximum 1-hr observed concentration in a subregion to which three predicted concentrations are compared:
  2. The predicted and observed 90th percentile concentrations for all hours. The 90th percentile concentration is statistically more robust than the maximum 1-hr concentration which behaves more like an extreme value statistic. The 90th percentile concentrations provide a relatively stable measure of the upper tail of the predicted and observed concentration distributions. This measure was not used in the OTAG model performance evaluation (OTAG, 1996b).
  3. The mean bias (ppb) and mean normalized bias (%) in the predicted hourly concentrations for all stations and hours where the observed concentrations exceed 40 ppb. These metrics measure bias at all hours of the day and night when concentrations exceed the clean air background concentration of ozone. They are important when assessing transport because transport occurs at all hours. Model bias in the base case simulations is also important because it can be amplified in the nonlinear chemistry in emission control simulations. For example, if consistent underprediction of ozone occurs in a base case simulation because of underestimation of VOC emissions, it is likely that the model will overestimate the benefits of reducing VOC emissions and underestimate the benefits of reducing NOx emissions. EPA guidelines for the urban scale photochemical model use a 15 percent mean normalized bias as the cutoff for acceptability (EPA, 1991).
  4. The mean error (ppb) and mean normalized error (%) in the predicted hourly concentrations for all stations and hours where the observed concentrations exceed 40 ppb. Unlike the bias statistic where overpredicted and underpredicted values cancel one another, the error statistic is the absolute value of the difference. A modest amount of imprecision or error is expected in regional ozone modeling. EPA guidelines for the urban scale photochemical model use a 35 percent mean normalized error as the cutoff for acceptability (EPA, 1991).

These measures are calculated for each day using all of the available ozone data for the region. The gridded model outputs were spatially interpolated to the location of the stations to derive the model estimates at the stations.

Key features of observed and predicted ozone concentrations in the Northeast for each episode are discussed below. Peak concentrations in urban-scale subregions were compared to assess whether the model predictions reproduce the basic features of individual episode days, including regional transport into the corridor and between subregions within the corridor.

2.2 MODEL PERFORMANCE FOR THE JULY 1988 EPISODE

The UAM-V model was applied to the July 4-15, 1988 episode. The highest observed and predicted impacts in the Northeast occurred in the six-day period from July 6 through July 11. The predicted and observed maximum concentrations, 90th percentile concentrations, and mean concentrations as well as model bias and model error for July 6-12 are summarized in Tables 2-1, 2-2, and 2-3. This episode was particularly notable in the Northeast for its high ozone concentrations and long duration. For six consecutive days, maximum observed 1-hour ozone concentrations in the Northeast exceeded 180 ppb, and levels above 120 ppb were observed along the Northeast Corridor from Virginia to Maine. The episode was particularly extreme in the Philadelphia subregion, which experienced peak observed concentrations comparable to those in the New York City and Baltimore-Washington subregions.

On Wednesday, July 6, the highest observed ozone occurred at isolated locations around Baltimore, Philadelphia, and New York City. On July 7, high concentrations (>160 ppb) were more widespread in Baltimore-Washington and Philadelphia subregions, with an isolated peak of 158 ppb in New York City. On Friday, July 8, concentrations above 160 ppb were observed in a broad area stretching from Maryland to Massachusetts, with a maximum of 187 ppb in the Philadelphia subregion. On July 9, observed maximum concentrations were above 160 ppb only in Baltimore-Washington, while on Sunday, July 10, areas with high observed ozone (>160 ppb) included Philadelphia, Baltimore, and the Connecticut shore. On July 11, ozone concentrations above 180 ppb were observed along the Northeast Corridor from Maryland to Massachusetts. On July 12, the maximum observed concentrations in the Corridor dropped to 90-126 ppb.

The UAM-V model simulations reproduced important features of the observed ozone concentrations in the Northeast for this episode. While the magnitude of maximum predictions unpaired in space were 14 ppb lower than observed on average, the model predicted five consecutive days with peak impacts above 180 ppb. The model's maximum ozone estimates paired in space and paired in space and time were substantially lower than the observed maximum in most cases, which indicates the model does not predict the precise location and timing of the peak well. For example, at the Danbury, CT station where 222 ppb was observed at 1600 on July 11 the model predicted 121 ppb at the hour of the observed maximum and 142 ppb two hours after the observed maximum, while 75 km away at Madison, CT the model predicted 200 ppb at 1600 hrs. The model tracked the multiday buildup of ozone to its peak on July 11 fairly well. For example, in the New York City subregion the model predicted a build up from 142 ppb daily maximum on July 9 to 195 and 200 ppb daily maxima for July 10 and 11 when the observed buildup was from 138 ppb on July 9 to 203 ppb on July 10 and 222 ppb on July 11. In the Baltimore-Washington and New York City subregions, the simulations also tracked the day-to-day changes in maximum observations relatively well. For example, the predicted daily maximum concentration were 179, 148, 179, 172, and 204 ppb for July 7-11 in the Baltimore-Washington subregion when 210, 169, 190, 180, and 218 were observed, respectively. In contrast, the results for the Philadelphia subregion did not track the day-to-day changes in ozone levels as well and showed substantial underprediction (35 to 59 ppb unpaired) throughout the worst episode days. For the Boston subregion, there was good agreement (maximum unpaired ozone within 10 ppb) for July 8, when the highest daily ozone was observed, but overprediction by 20-30 ppb for July 7, 9 and 10. For July 12, the estimated maximum ozone concentrations decreased by 40-70 ppb, which was less than the observed decrease. The model did not simulate the "cleaning out" of the Northeast Corridor well.

The predicted and observed 90th percentile ozone concentration agreed within 10 ppb on only 8 of the 28 subregion-days in the 1988 episode which is fewer days than expected. The upper tail of the predicted ozone concentrations did not agree with observations well in the 1988 episode simulations as in the 1991 and 1995 episode simulations.

When all station-hours with concentrations above the 40 ppb clean air background concentrations were considered, there was negative bias (underprediction) on July 6 and positive bias (overprediction) on July 12 in all subregions. In the Boston subregion, the hourly concentrations were overpredicted on average for July 7-11 (daily mean biases ranging from +5 to +56 percent). The model bias for July 7-10 in Boston exceeded EPA's acceptance criteria for urban-scale modeling. In the New York City subregion, the hourly concentrations were significantly overpredicted for July 7-8 (+25 percent bias on average) and reasonably unbiased for July 9-11 (0 to +10 percent bias on average). In the Philadelphia subregion, the hourly concentrations were underpredicted on average for July 7-11 (daily mean biases ranging from 0 to -17 percent), which was consistent with the bias in peak ozone in this subregion. The predictions in the Baltimore-Washington area were relatively unbiased for July 7-10 (­13 to +4 percent) and positively biased on July 11 (+29 percent) on average. For the entire episode (July 6-12) and the entire Northeast Corridor, the model overpredicted hourly ozone concentrations by 3 ppb (84 vs 81 ppb), which is reasonably good performance. Performance on specific days in specific subregions, especially Boston, was problematic.

The model error for all station-hours varied significantly between subregions and days. Model predictions for July 6 had the lowest error, while those for July 12 had the highest error in most subregions. The mean normalized error in the Boston subregion varies from 24 to 56 percent for July 6-12, with the worst model error occurring on July 9 when the model overpredicted at all stations and hours. The mean normalized error in the New York City subregion varied from 25 to 46 percent for July 6-12, with the worst errors occurring on July 7. In the Philadelphia subregion, the mean normalized error varied from 24 to 47 percent for July 6-12 and was generally smaller than in the other subregions. The model error for the Baltimore-Washington subregion was modest (ranging from 23 to 32 percent) through July 10 and significantly higher on the last two days of the episode (38 and 59 percent). Overall, the UAM-V model predictions had less than 35 percent error on about half of the subregion-days. The errors for this simulation were larger than those for the 1991 and 1995 episodes.

For the 1988 episode, model performance was generally better for the Baltimore-Washington and New York City subregions, which had the greatest concentration of local emission sources, than for Philadelphia, where regional transport (both between urban subregions and into the corridor from the west) was expected to be a greater factor. Both observed and predicted concentrations were generally lower in the Boston subregion than in the rest of the corridor for the 1988 episode (except on July 8) and the model performance in the Boston region did not meet EPA's acceptance criteria on most days.

2.3 MODEL PERFORMANCE FOR THE JULY 1991 EPISODE

The UAM-V model was applied to the July 16-21, 1991 episode. The predicted and observed maximum concentrations, 90th percentile concentrations, and mean concentrations as well as model bias and model error for July 18-21 are summarized in Tables 2-4, 2-5, and 2-6. During this episode, ozone concentrations above 160 ppb were observed in the Northeast on three consecutive days, July 18-20. On Wednesday, July 17 and Thursday, July 18, high concentrations occurred primarily in the New York City and Boston subregions, with maximum observed concentrations of 155 and 174 ppb, respectively, at the Connecticut monitors. On July 19, the area with ozone concentrations above the Federal standard extended from Philadelphia to Maine, with a maximum of 161 ppb observed in Connecticut. The episode-maximum observed concentrations occurred on Saturday, July 20, with 178 ppb measured in Virginia and 175 ppb measured on Long Island, NY. On July 21, the observed peak concentration dropped to 135 ppb in Connecticut.

The accuracy of the UAM-V model's predictions for maximum ozone levels in the Northeast Corridor is quite variable for the 1991 episode. The model estimates of peak ozone concentrations (unpaired in space or time) are within 20 ppb for about half of the subregion-days. The results for the other half of the subregion-days show a mixture of under- and overestimation. The model's predictions paired in space or space and time tend to be equal or lower than the observed maximum. On July 18, the peak unpaired predictions show good agreement with observations for the New York City, Philadelphia, and Baltimore-Washington subregions and poor agreement (underprediction) for the Boston subregion. Boston's highest ozone in the 1991 episode occurred on July 18 and the model predicted 109 ppb when 169 ppb was observed. For July 19, the model predicts the New York City and Boston peaks accurately, but underestimates the peak values in the Philadelphia and Baltimore-Washington subregions. On July 20, the unpaired peak concentrations are in good agreement in the Philadelphia subregion where 151 ppb was observed. However, in the New York City and Baltimore-Washington subregions where 175 and 178 ppb were observed, respectively, the model underestimates the peak concentrations by 17-33 ppb. The 1-hr maximum concentration in Boston was also underestimated (124 versus 157 ppb observed) on July 20. For July 21, maximum values were estimated accurately in the Boston subregion and significantly overestimated in the New York City, Philadelphia, and Baltimore-Washington subregions. The UAM-V did not simulate the observed decrease in peak concentrations on the final day of the episode as was seen in the 1988 simulation.

The 90th percentile concentrations often show good agreement between the model and observed values. The predicted and observed 90th percentile concentrations were within 10 ppb for about two thirds of the subregion-days in 1991. Furthermore, the 90th percentile concentrations often show good agreement in cases where the peak values were significantly underpredicted. For example, the predicted and observed 90th percentile concentrations were 117 and 121 ppb on July 20 in the Baltimore-Washington subregion where the 178 ppb observed peak was underestimated by 32 ppb. Similarly, the predicted and observed 90th percentile concentrations were 103 and 105 ppb on July 20 in the New York City subregion where the 175 ppb observed peak was underestimated by 17 ppb. For the entire region and episode, the predicted and observed 90th percentile concentrations were 108 and 105 ppb, which is good agreement.

The mean bias and error for all station-hours with ozone above 40 ppb were significantly smaller for the 1991 episode than for 1988. The mean normalized bias was within 15 percent on 14 of the 16 subregion-days. The mean normalized error was within 35 percent on 15 of the 16 subregion-days. On the majority of subregion-days, the mean normalized bias was within 8 percent and the mean normalized error was within 22 percent, which is considered good performance for photochemical modeling. Overall, for the entire episode and region, the mean bias was 1 ppb or 1 percent, and the mean error was 17 ppb or 24 percent.

The smaller bias and error in this episode simulation compared to those for 1988 indicate the model did a better job of simulating the relevant atmospheric processes, including simulation of ozone transport. This may in part be due to differences in the meteorological models used (the RAMS model for 1991 and the SAIMM for 1988). Overall, the UAM-V model simulations of the July 1991 episode exhibited good performance in the Northeast corridor with the exception of underprediction of some of the maximum concentrations.


2.4 MODEL PERFORMANCE FOR THE JULY 1995 EPISODE

2.4.1 Model Performance Statistics

The UAM-V model was applied to simulate the relatively severe episode that occurred July 10-18, 1995. It was one of the most severe episodes in the Northeast since the July 1988 episode. Ozone concentrations above the Federal standard occurred on July 12-15 in the Northeast. The predicted and observed maximum concentrations, 90th percentile concentrations, and mean concentrations as well as model bias and model error for July 12-15 are summarized in Tables 2-7, 2-8, and 2-9. The subregion with the highest ozone shifts from Baltimore-Washington on Wednesday, July 12 to New York City on July 13, New York City and Philadelphia on July 14, then back to Baltimore-Washington on Saturday, July 15. The highest concentrations were observed on July 14 and 15.

The model's estimates of the maximum concentrations were of varying accuracy in the 1995 episode simulation. The 1-hr maximum values were predicted within 20 ppb on 10 of the 16 subregion-days (unpaired in space or time). However, the higher observed values were frequently underestimated, and comparisons of predicted maxima paired in space or space and time were all lower than the observed maxima. The model provided reasonably accurate estimates of the maximum ozone on July 12 when 136 ppb was observed in the Baltimore-Washington area and the model predicted 150 ppb. The agreement was closer in the other subregions on July 12. (e.g., 127 ppb predicted versus 125 observed in the Philadelphia subregion). On July 13, the model predicted a 145 ppb peak concentration in the New York City subregion when a maximum of 157 ppb was observed. The maximum ozone (unpaired) on July 13 was estimated within 12 ppb in the Boston, New York City, and Baltimore-Washington subregions, and overestimated by 28 ppb in in the Philadelphia subregion. On July 14, the maximum ozone (unpaired) was accurately estimated in the New York City (175 ppb predicted versus 176 observed), and underestimated in the other three subregions. The largest underprediction on July 14 was in the Philadelphia subregion where the model predicted a 135 ppb maximum when 170 ppb was observed. On July 15, the model underestimated the maximum concentrations by 33 to 46 ppb in the subregions with exceedances. For example, the model predicted 132 and 138 ppb in the New York City and Baltimore-Washington subregions when a maximum of 165 ppb was observed in both subregions. The only region in the Northeast with predictions above 140 ppb on July 15 was over the Atlantic Ocean south of Long Island, which indicates the transport was poorly simulated on this day. Clearly, the model did not capture the peak values well in this episode except in New York on the July 13.

The 90th percentile ozone concentrations agree within 5 ppb on about half of the subregion-days and agree within 10 ppb on 14 of the 16 episode days. During the July 12-15 period, the model slightly underestimated the 90th percentile ozone in the Boston subregion and slightly overestimated the 90th percentile ozone in the New York City, Philadelphia, and Baltimore-Washington subregions. The bias in the 90th percentile ozone was usually directionally consistent with the bias in peak ozone (unpaired) in most cases. For example, the 90th percentile concentrations were underestimated by 6 to 8 ppb on July 15 when the peak values were underestimated by 33 to 46 ppb in the New York City, Philadelphia, and Baltimore-Washington subregions. There was little bias in the 90th percentile or maximum ozone on July 14 in the New York subregion; the predicted and observed 90th percentile ozone were 114 and 112 ppb, respectively.

The mean bias and error statistics showed good model performance on this episode. The bias in hourly ozone concentrations above 40 ppb was within 15 percent on 15 of the 16 subregion-days. The model error was less than 27 percent in all of the subregion-days. On average for July 12-15 in the Northeast Corridor, the model bias was +2 percent and the model error 15 ppb or 21 percent. These performance statistics are better than those for the 1988 simulations, and almost as good as those for the 1991 simulations.

2.4.2 Spatial Patterns and Transport in the 1995 Episode

The July 1995 OTAG episode provides a valuable opportunity for examining model performance. The information available from the 1995 NARSTO-Northeast measurements program makes it possible to assess model performance in greater detail, in the context provided by a conceptual model of regional transport processes. In particular, the detailed meteorological information collected for the NARSTO-Northeast program provide a basis for evaluation of the observed and model-input wind information, in addition to observed and predicted spatial patterns of concentrations. Such comparisons provide a more complete picture of the strengths and limitations of the OTAG modeling approach for estimating regional and local contributions during ozone episodes. Model performance by subregion in the Northeast is discussed below for July 13, 14, and 15, 1995, the three "key days" with the highest observed and predicted ozone concentrations during this episode in the Northeast, with particular attention to predicted and observed spatial patterns and to comparisons of two sets of back trajectories, the first based on observed winds (constructed using the diagnostic CALMET meteorological model), and the second based on the RAMS prognostic meteorological model winds which were used in the UAM-V model simulations.

The spatial ozone pattern predicted for 2:00 p.m. (EST) on July 13 is shown in Figure 2-6. The spatial pattern of observed ozone concentrations showed distinct differences in transport patterns between subregions. The highest observed concentrations occurred in the New York City subregion, with a maximum of 157 ppb in Danbury, CT, about 50 km northeast of New York City. An area with measured peak concentrations above 125 ppb extended from Danbury into south-central Massachusetts, about 150 km from New York City. Peak predicted concentrations in the New York City subregion for July 13 were comparable in magnitude (145 ppb), but the region with predicted exceedances was smaller than observed (only extending about 80 km downwind of New York City) and was farther south, along the Connecticut shore. As shown in Figure 2-7, surface level (20 m) back trajectories starting from Ware, MA and East Hartford, CT derived from observed winds tracked back to Long Island, about 20 km east of the New York City metropolitan area, while the 500 m level CALMET back trajectories for these sites (see Figure 2-8) passed north of New York City and originated in north-central Pennsylvania on the morning of July 13.

In the Baltimore-Washington subregion, the area with observed exceedances (maximum 137 ppb) on July 13 was south of Baltimore and in between the two major metropolitan areas. In contrast, the model's peak ozone (136 ppb maximum) was in Delaware and eastern Maryland, roughly 60 km east-northeast of Baltimore. In the Philadelphia subregion, observed concentrations of 110-120 ppb occurred over a broad region north and northeast of the city with no localized "hot spot", while the predicted pattern showed a more well-defined "urban plume" extending into northern New Jersey, about 75 km northeast of Philadelphia. CALMET back trajectories starting at 1500 from Rutgers (northeast of Philadelphia) indicated transport from the west at 20 m and 500 m, while trajectories for northern Delaware and near Washington, DC indicated transport from the south. The measured winds in the Baltimore-Washington and Philadelphia subregions were much lighter and less organized than those in the New York City subregion.

Analysis of the spatial ozone patterns for July 13 indicate the observed New York City urban plume was transported in the northeasterly direction and the simulated urban plume was transported in an east-northeasterly direction. For the Baltimore-Washington and Philadelphia subregions, transport from southwest to northeast was predicted, but the observed pattern showed little organized transport.

On July 14, the highest ozone concentrations again occurred in the New York City subregion, with the highest values observed along the Connecticut shoreline and in Rhode Island. The spatial ozone pattern predicted for 2 p.m. (EST) on July 14 is shown in Figure 2-9. For this day, the predicted magnitude and spatial pattern of peak ozone concentrations were in excellent agreement with observed values (a maximum observed of 175 ppb in Madison, CT and a maximum predicted of 176 ppb in Groton, CT). The area of high observed ozone extended from east-northeast of New York City along the Connecticut shoreline and inland into eastern Connecticut and Rhode Island. The predicted area of high ozone was slightly displaced to the south from the observed, extending along the Connecticut shoreline and Long Island.

In the Philadelphia subregion, peak ozone concentrations of 170 and 163 ppb were observed at two sites in central New Jersey that were located 50 and 90 km east northeast of the city, respectively. While the peak predicted concentration was only 135 ppb, the location of the peak matched the observations. Both observed and predicted peak concentrations on July 14 were lower for the Baltimore-Washington subregion (144 ppb observed maximum, 121 ppb predicted), with spatial patterns indicating transport from west to east. Back trajectories of surface and 500 m level winds for sites in Connecticut, New Jersey, and Delaware, shown in Figure 2-10, showed persistent transport from the west-southwest on July 14. Excellent agreement is also seen between trajectories based on observed (CALMET) and model-input (RAMS) 500 m level winds (see Figure 2-11), which helps to explain the good air quality model performance for this day.

On July 15, the highest observed ozone concentrations in the northeast occurred in the Baltimore-Washington subregion, with a maximum of 184 ppb in northern Delaware, and observed concentrations exceeding 155 ppb at seven stations, four in an area within 40 km north and east of Washington, DC, the remaining three within 60 km east northeast of Baltimore. The spatial ozone pattern predicted for 1:00 p.m. (EST) is shown in Figure 2-12. Peak predictions in the Baltimore-Washington subregion for July 15 were substantially lower, with a maximum of 138 ppb predicted 30 km east of Washington, DC and 120 ppb predicted downwind of Baltimore. Predicted transport is to the east and east-southeast.

In the New York City subregion, the highest observed ozone concentrations on July 15 occurred east of New York City along the Connecticut shoreline (165 ppb at Madison, CT) and on Long Island. The area with the highest ozone predictions was displaced farther south (to southern Long Island and over the Atlantic Ocean), with the predicted maximum at a monitor of 132 ppb. Back trajectories based on observed winds for sites with peak observed ozone concentrations in Delaware and Connecticut (see Figure 2-13) indicated transport from the west and southwest at the surface (i.e., from Baltimore and New York City, respectively) , while trajectories based on RAMS model winds (see Figures 2-14) showed a west-northwesterly surface layer transport. The RAMS winds appeared to transport the ozone offshore rather than along the shoreline on July 15. The observed 500 m winds to the high ozone areas in Delaware and Conneticut were northwesterly earlier in the day and westerly in the afternoon while the RAMS wind were mostly northwesterly (see Figure 2-15).

Clearly, modest differences between the observed and model transport winds can have substantial impact on the source-receptor relationships and the predicted concentrations at specific monitors. Modest differences in transport can easily cause displacement of the location of maximum concentrations by 50 to 100 km. The differences in the transport winds probably contribute more to errors in maximum values than mean values.

2.5 SUMMARY

Each of the OTAG episodes represents a different pattern of regional transport, including both transport into the OTR and interactions between subregions within the OTR. The statistical measures of model performance for the Northeast Corridor were within EPA's acceptance crieria on most simulated days with high ozone. The simulations reproduced the magnitude (within 30 ppb) and approximate locations (within 100 km) of high ozone occurrences in the Northeast for most episode days. The performance was generally better in the 1991 and 1995 episodes than the 1988 episode. The results also pointed out several important shortcomings, including the following:

Table 2-1. Maximum observed and predicted ozone for the July 6-12, 1988 episode.
Maximum
Maximum Predicted (ppb)

Region

Date
Observed (ppb)
At Any Station-

Any Hour
At Peak Station-Any Hour
At Peak Station and Peak Hour
Boston
July 6, 1988
146
124
98
92
New York City
July 6, 1988
178
150
120
115
Philadelphia
July 6, 1988
177
142
119
117
Balt­Washington
July 6, 1988
194
142
126
126
NE Corridor
July 6, 1988
194
150
126
126
Boston
July 7, 1988
146
165
149
149
New York City
July 7, 1988
158
156
156
135
Philadelphia
July 7, 1988
210
151
132
132
Balt­Washington
July 7, 1988
210
179
106
99
NE Corridor
July 7, 1988
210
179
132
132
Boston
July 8, 1988
184
174
164
163
New York City
July 8, 1988
179
181
181
133
Philadelphia
July 8, 1988
187
149
119
119
Balt­Washington
July 8, 1988
169
148
104
104
NE Corridor
July 8, 1988
187
181
119
119
Boston
July 9, 1988
119
154
154
151
New York City
July 9, 1988
138
142
85
73
Philadelphia
July 9, 1988
149
101
95
95
Balt­Washington
July 9, 1988
190
179
145
116
NE Corridor
July 9, 1988
190
179
145
116
Boston
July 10, 1988
120
146
127
124
New York City
July 10, 1988
203
195
162
151
Philadelphia
July 10, 1988
205
149
149
143
Balt­Washington
July 10, 1988
180
172
116
87
NE Corridor
July 10, 1988
205
195
149
143
Boston
July 11, 1988
153
145
127
110
New York City
July 11, 1988
222
200
142
121
Philadelphia
July 11, 1988
204
154
132
132
Balt­Washington
July 11, 1988
218
204
186
174
NE Corridor
July 11, 1988
222
204
142
121
Boston
July 12, 1988
126
107
96
79
New York City
July 12, 1988
119
154
94
87
Philadelphia
July 12, 1988
90
126
88
87
Balt­Washington
July 12, 1988
102
135
80
63
NE Corridor
July 12, 1988
126
154
96
79
Boston
Average
142
145
131
124
New York City
Average
171
168
134
116
Philadelphia
Average
175
139
119
118
Balt­Washington
Average
180
165
123
110
NE Corridor
Average
191
177
130
119

Table 2-2. Comparison of predicted and observed 90th percentile ozone concentrations in the July 6-12, 1988 episode.

Subregion

Date
Observed

90th Percentile
Predicted

90th Percentile
Boston
July 6, 1988
112
96
New York City
July 6, 1988
109
98
Philadelphia
July 6, 1988
150
121
Balt­Washington
July 6, 1988
124
114
NE Corridor
July 6, 1988
124
106
Boston
July 7, 1988
106
129
New York City
July 7, 1988
106
121
Philadelphia
July 7, 1988
140
122
Balt­Washington
July 7, 1988
136
134
NE Corridor
July 7, 1988
126
126
Boston
July 8, 1988
128
148
New York City
July 8, 1988
129
142
Philadelphia
July 8, 1988
131
114
Balt­Washington
July 8, 1988
121
105
NE Corridor
July 8, 1988
125
126
Boston
July 9 1988
94
128
New York City
July 9 1988
93
96
Philadelphia
July 9 1988
100
84
Balt­Washington
July 9 1988
107
121
NE Corridor
July 9 1988
98
113
Boston
July 10, 1988
94
114
New York City
July 10, 1988
126
137
Philadelphia
July 10, 1988
131
114
Balt­Washington
July 10, 1988
112
124
NE Corridor
July 10, 1988
116
123
Boston
July 11, 1988
122
115
New York City
July 11, 1988
147
145
Philadelphia
July 11, 1988
133
139
Balt­Washington
July 11, 1988
122
155
NE Corridor
July 11, 1988
131
141
Boston
July 12, 1988
91
90
New York City
July 12, 1988
76
100
Philadelphia
July 12, 1988
75
104
Balt­Washington
July 12, 1988
65
107
NE Corridor
July 12, 1988
75
101
Boston
Average
107
117
New York City
Average
112
120
Philadelphia
Average
123
114
Balt­Washington
Average
112
123
NE Corridor
Average
114
119

Table 2-3. Mean observed and predicted ozone concentrations, mean bias and normalized bias, and mean error and normalized error for the July 6-12, 1988 episode.a


Region


Date
Mean

Observed

(ppb)
Mean

Predicted

(ppb)
Mean

Bias

(ppb)
Mean

Normalized

Bias (%)
Mean

Error

(ppb)
Mean

Normalized

Error (%)
Boston
July 6, 1988
84
73
­11
­10
19
24
New York City
July 6, 1988
78
71
­8
­5
20
25
Philadelphia
July 6, 1988
106
86
­20
­17
24
24
Balt­Washington
July 6, 1988
90
75
­16
­17
23
26
NE Corridor
July 6, 1988
88
75
­13
­12
22
25
Boston
July 7, 1988
78
98
20
32
29
42
New York City
July 7, 1988
75
88
12
25
30
46
Philadelphia
July 7, 1988
96
86
­9
­5
20
23
Balt­Washington
July 7, 1988
99
95
­4
­1
21
23
NE Corridor
July 7, 1988
88
92
4
11
25
33
Boston
July 8, 1988
90
116
25
36
27
38
New York City
July 8, 1988
82
96
14
26
29
40
Philadelphia
July 8, 1988
89
78
­11
­10
21
26
Balt­Washington
July 8, 1988
84
70
­13
­13
23
28
NE Corridor
July 8, 1988
85
86
1
7
25
33
Boston
July 9, 1988
67
101
34
56
34
56
New York City
July 9, 1988
69
71
2
10
24
36
Philadelphia
July 9, 1988
71
58
­14
­17
22
32
Balt­Washington
July 9, 1988
75
77
2
4
21
32
NE Corridor
July 9, 1988
71
77
6
12
25
38
Boston
July 10, 1988
67
86
19
33
23
38
New York City
July 10, 1988
87
84
­3
0
26
32
Philadelphia
July 10, 1988
96
85
­12
­9
18
19
Balt­Washington
July 10, 1988
86
86
0
2
20
25
NE Corridor
July 10, 1988
85
85
0
5
22
29
Boston
July 11, 1988
86
86
­1
5
18
24
New York City
July 11, 1988
95
93
­3
2
28
31
Philadelphia
July 11, 1988
92
91
­1
0
19
24
Balt­Washington
July 11, 1988
88
108
21
29
28
38
NE Corridor
July 11, 1988
91
96
5
10
25
31
Boston
July 12, 1988
65
69
4
12
17
28
New York City
July 12, 1988
62
69
7
13
21
35
Philadelphia
July 12, 1988
59
83
24
44
26
47
Balt­Washington
July 12, 1988
58
89
30
56
32
59
NE Corridor
July 12, 1988
61
76
15
28
24
41
Boston
Average
77
90
13
23
24
36
New York City
Average
78
82
3
10
25
35
Philadelphia
Average
87
81
­6
­2
21
28
Balt­Washington
Average
83
86
3
9
24
33
NE Corridor
Average
81
84
3
9
24
33

a For all station-hours with observed concentrations above 40 ppb.

Table 2-4. Maximum observed and predicted ozone for the July 18-21, 1991 episode.
Maximum
Maximum Predicted (ppb)

Region

Date
Observed (ppb)
At Any Station-

Any Hour
At Peak Station-Any Hour
At Peak Station and Peak Hour
Boston
July 18, 1991
169
109
77
76
New York City
July 18, 1991
174
168
157
147
Philadelphia
July 18, 1991
134
124
108
101
Balt­Washington
July 18, 1991
127
122
104
103
NE Corridor
July 18, 1991
174
168
157
147
Boston
July 19, 1991
156
162
121
118
New York City
July 19, 1991
161
164
146
144
Philadelphia
July 19, 1991
153
125
122
122
Balt­Washington
July 19, 1991
137
111
75
74
NE Corridor
July 19, 1991
161
164
146
144
Boston
July 20, 1991
157
124
86
85
New York City
July 20, 1991
175
158
158
127
Philadelphia
July 20, 1991
151
152
127
127
Balt­Washington
July 20, 1991
178
146
117
110
NE Corridor
July 20, 1991
178
158
117
110
Boston
July 21, 1991
101
102
81
80
New York City
July 21, 1991
135
157
135
124
Philadelphia
July 21, 1991
124
160
126
124
Balt­Washington
July 21, 1991
121
138
106
104
NE Corridor
July 21, 1991
135
160
135
124
Boston
Average
146
124
91
90
New York City
Average
161
162
149
135
Philadelphia
Average
141
140
121
119
Balt­Washington
Average
141
129
100
98
NE Corridor
Average
162
162
139
131

Table 2-5. Comparison of predicted and observed 90th percentile ozone concentrations in the July 18-21, 1991 episode.

Subregion

Date
Observed

90th Percentile
Predicted

90th Percentile
Boston
July 18, 1991
107
86
New York City
July 18, 1991
116
117
Philadelphia
July 18, 1991
115
104
Balt­Washington
July 18, 1991
98
95
NE Corridor
July 18, 1991
108
102
Boston
July 19, 1991
114
130
New York City
July 19, 1991
105
124
Philadelphia
July 19, 1991
99
107
Balt­Washington
July 19, 1991
88
80
NE Corridor
July 19, 1991
101
111
Boston
July 20, 1991
102
98
New York City
July 20, 1991
105
103
Philadelphia
July 20, 1991
117
119
Balt­Washington
July 20, 1991
121
117
NE Corridor
July 20, 1991
113
113
Boston
July 21, 1991
85
74
New York City
July 21, 1991
91
101
Philadelphia
July 21, 1991
102
105
Balt­Washington
July 21, 1991
102
113
NE Corridor
July 21, 1991
97
106
Boston
Average
102
97
New York City
Average
104
111
Philadelphia
Average
108
109
Balt­Washington
Average
102
101
NE Corridor
Average
105
108

Table 2-6. Mean observed and predicted ozone concentrations, mean bias and normalized bias, and mean error and normalized error for the July 18-21, 1991 episode.a



Region


Date
Mean

Observed

(ppb)
Mean

Predicted

(ppb)
Mean

Bias

(ppb)
Mean

Normalized

Bias (%)
Mean

Error

(ppb)
Mean

Normalized

Error (%)
Boston
July 18, 1991
76
60
­16
­19
19
25
New York City
July 18, 1991
81
72
­8
­10
20
26
Philadelphia
July 18, 1991
86
80
­6
­6
14
19
Balt­Washington
July 18, 1991
72
69
­3
­2
14
21
NE Corridor
July 18, 1991
79
70
­8
­9
17
23
Boston
July 19, 1991
84
92
8
14
20
26
New York City
July 19, 1991
75
85
9
20
25
38
Philadelphia
July 19, 1991
76
83
7
14
17
25
Balt­Washington
July 19, 1991
72
65
­7
­6
17
24
NE Corridor
July 19, 1991
76
81
4
11
20
29
Boston
July 20, 1991
74
73
­1
2
15
21
New York City
July 20, 1991
83
73
­10
­8
23
29
Philadelphia
July 20, 1991
90
92
3
4
13
16
Balt­Washington
July 20, 1991
91
91
1
4
15
19
NE Corridor
July 20, 1991
85
82
­3
0
17
22
Boston
July 21, 1991
66
57
­9
­12
13
19
New York City
July 21, 1991
71
71
0
1
15
22
Philadelphia
July 21, 1991
75
80
4
8
13
19
Balt­Washington
July 21, 1991
80
86
6
9
13
18
NE Corridor
July 21, 1991
74
75
1
2
14
20
Boston
Average
75
71
­4
­4
17
23
New York City
Average
78
75
­2
1
21
29
Philadelphia
Average
82
84
2
5
15
20
Balt­Washington
Average
79
78
­1
1
15
21
NE Corridor
Average
78
77
­1
1
17
24

a For all station-hours with observed concentrations above 40 ppb.

Table 2-7. Maximum observed and predicted ozone for the July 12-15, 1995 episode.
Maximum
Maximum Predicted (ppb)

Region

Date
Observed (ppb)
At Any Station-

Any Hour
At Peak Station-Any Hour
At Peak Station and Peak Hour
Boston
July 12, 1995
91
96
77
75
New York City
July 12, 1995
90
100
65
52
Philadelphia
July 12, 1995
125
127
88
81
Balt­Washington
July 12, 1995
136
150
142
131
NE Corridor
July 12, 1995
136
150
142
131
Boston
July 13, 1995
120
125
110
110
New York City
July 13, 1995
157
145
128
123
Philadelphia
July 13, 1995
121
149
112
109
Balt­Washington
July 13, 1995
137
136
119
119
NE Corridor
July 13, 1995
157
149
128
123
Boston
July 14, 1995
143
122
108
65
New York City
July 14, 1995
175
176
162
158
Philadelphia
July 14, 1995
170
135
135
133
Balt­Washington
July 14, 1995
144
121
94
94
NE Corridor
July 14, 1995
175
176
162
158
Boston
July 15, 1995
90
102
72
70
New York City
July 15, 1995
165
132
90
63
Philadelphia
July 15, 1995
162
128
102
99
Balt­Washington
July 15, 1995
184
138
97
93
NE Corridor
July 15, 1995
184
138
97
93
Boston
Average
111
111
92
80
New York City
Average
147
138
111
99
Philadelphia
Average
145
135
109
105
Balt­Washington
Average
150
137
113
109
NE Corridor
Average
163
153
132
126

Table 2-8. Comparison of predicted and observed 90th percentile ozone concentrations in the July 12-15, 1995 episode.

Subregion

Date
Observed

90th Percentile
Predicted

90th Percentile
Boston
July 12, 1995
70
74
New York City
July 12, 1995
67
70
Philadelphia
July 12, 1995
91
100
Balt­Washington
July 12, 1995
97
115
NE Corridor
July 12, 1995
85
94
Boston
July 13, 1995
96
93
New York City
July 13, 1995
98
104
Philadelphia
July 13, 1995
108
122
Balt­Washington
July 13, 1995
98
98
NE Corridor
July 13, 1995
99
105
Boston
July 14, 1995
93
82
New York City
July 14, 1995
112
114
Philadelphia
July 14, 1995
114
114
Balt­Washington
July 14, 1995
106
104
NE Corridor
July 14, 1995
108
107
Boston
July 15, 1995
59
57
New York City
July 15, 1995
82
76
Philadelphia
July 15, 1995
113
106
Balt­Washington
July 15, 1995
117
109
NE Corridor
July 15, 1995
100
99
Boston
Average
80
77
New York City
Average
90
91
Philadelphia
Average
107
111
Balt­Washington
Average
105
107
NE Corridor
Average
98
101

Table 2-9. Mean observed and predicted ozone concentrations, mean bias and normalized bias, and mean error and normalized error for the July 12-15, 1995 episode.a



Region


Date
Mean

Observed

(ppb)
Mean

Predicted

(ppb)
Mean

Bias

(ppb)
Mean

Normalized

Bias (%)
Mean

Error

(ppb)
Mean

Normalized

Error (%)
Boston
July 12, 1995
58
55
­3
­3
12
21
New York City
July 12, 1995
57
56
­1
1
11
19
Philadelphia
July 12, 1995
75
83
8
13
14
20
Balt­Washington
July 12, 1995
75
87
12
17
17
25
NE Corridor
July 12, 1995
66
71
4
8
14
22
Boston
July 13, 1995
73
72
­1
1
12
18
New York City
July 13, 1995
74
73
­1
1
19
27
Philadelphia
July 13, 1995
82
92
11
15
17
23
Balt­Washington
July 13, 1995
73
78
5
11
14
22
NE Corridor
July 13, 1995
75
78
3
7
16
23
Boston
July 14, 1995
71
63
­7
­8
15
21
New York City
July 14, 1995
78
75
­3
­3
17
23
Philadelphia
July 14, 1995
83
85
2
4
13
17
Balt­Washington
July 14, 1995
80
79
­1
2
13
18
NE Corridor
July 14, 1995
78
75
­2
­1
15
20
Boston
July 15, 1995
53
50
­3
­5
9
16
New York City
July 15, 1995
63
57
­7
­9
15
23
Philadelphia
July 15, 1995
80
76
­4
­4
14
19
Balt­Washington
July 15, 1995
85
84
­1
3
16
20
NE Corridor
July 15, 1995
72
68
­4
­4
14
20
Boston
Average
64
60
­4
­4
12
19
New York City
Average
68
65
­3
­2
15
23
Philadelphia
Average
80
84
4
7
14
20
Balt­Washington
Average
78
82
4
8
15
21
NE Corridor
Average
73
73
0
2
15
21

a For all station-hours with observed concentrations above 40 ppb.


3. MODEL PERFORMANCE FOR WESTERN BOUNDARY OF THE URBAN CORRIDOR


Ozone levels within the Northeast Corridor reflect the combined effect of contributions from "within-corridor" emission sources plus regional transport of ozone and ozone precursors. Since near-surface transport within the corridor during ozone episodes generally has a west-to-east component (between south-southwest and northwest), ozone concentrations at the monitoring sites immediately west of the corridor serve to characterize the inflow concentrations and regional transport component. A comparison of observed and predicted ozone concentrations at monitoring locations in Virginia, Pennsylvania, and New York was carried out to assess possible model bias in the inflow region. The locations of the monitoring stations used in this analysis are shown in Figure 3-1. Note, this "western boundary" analysis relies upon a rather simplistic transport scenario which has obvious limitations. Transport from south to north (without a significant westerly component) can occasionally produce impacts from in-corridor sources at a number of these "boundary" sites. Over the course of each episode, however, air quality at these monitors is generally dominated by boundary layer transport from the west (northwest through southwest).

Table 3-1 summarizes the mean predicted and observed concentrations and the mean bias and error in the model at these western boundary stations. When the data for hours with observed concentrations above 40 ppb are considered, the results showed that the model consistently underestimated the observed concentrations in the western boundary region. On average, the model underestimated concentrations by 9 ppb in 1988 and 1991, and 5 ppb in 1995. The magnitude of the bias was small on most days, although, it was very consistent. The mean ozone was underpredicted by more than 10 ppb on 5 of the 14 days considered in the analysis. The model error was 25 percent in 1988, 25 percent in 1991, and 23 percent in 1995 on average, which is comparable to elsewhere in the Northeast.

The daily maximum observed and predicted ozone concentrations (unpaired in time) at the western boundary stations are summarized in Table 3-2 for the 1988 episode. The stations are listed from north to south. The average daily maximum ozone concentration over all stations showed a systematic underprediction of 7 to 24 ppb over the critical episode days from July 7 through 11. The bias was particularly large on July 7, when the observed and predicted 12-station average daily maximums were 131 and 107 ppb, and on July 10 when the observed and predicted 12-station average daily maximums were 105 and 89 ppb. Early in the episode, a north-south concentration gradient was evident in both observed and predicted values. The gradient in predicted values persisted through July 11, but was less evident in observed values. In addition, the time of the observed and predicted maxima (not shown) often differed by two to four hours.

The daily maximum observed and predicted concentrations (unpaired in time) at western boundary sites for the 1991 episode are listed in Table 3-3. Average differences for the critical days of July 18-20 were quite similar to those from the 1988 episode, and again showed a systematic underprediction bias. The 14-station average daily maxima were underpredicted by 14 to 21 ppb on these three days. Both observed and predicted concentrations were lower for 1991 than for 1988. For the 1991 episode, however, peak concentrations were more variable across the network of monitors (than in 1988), indicating perhaps a less organized flow field and some impacts from local emission sources in this region.

The maximum concentrations (unpaired in time) for the 1995 episode are listed in Table 3-4. For the key episode days of July 12-15, peak observed and predicted concentrations for the "northern tier" stations (north of the Centre/State College site) showed systematic underprediction bias of 10-15 ppb, with higher predictions and minimal bias at southern boundary sites. On average, the model underestimated peak concentrations in this upwind region by 8 ppb on July 12, 19 ppb on July 13, 1 ppb on July 13, and 4 ppb on July 15 at this group of 14 stations.

These comparisons indicate that regional transport of ozone into the Northeast Urban Corridor from the west was systematically underpredicted by 5 to 10 ppb on average in the OTAG simulations. Relative to a nominal "clean background" ozone concentration of 40 ppb, the difference between a "regional background" of 70-90 ppb, as predicted, versus 80-100 ppb, as observed, represents a 15 to 25 percent underestimate of regional transport.

Table 3-1. Mean observed and predicted hourly ozone surface concentrations, mean model bias, and error at the western boundary sites.

Date
Mean Observed

(ppb)a
Mean

Predicted

(ppb)a
Mean

Bias

(ppb)a
Mean

Normalized Bias (%)a
Mean

Error

(ppb)a
Mean

Normalized

Error (%)a
July 7, 1988
99
82
­16
­8
29
31
July 8, 1988
85
82
­3
3
19
25
July 9, 1988
80
74
­6
­1
20
27
July 10, 1988
84
69
­15
­15
20
24
July 11, 1988
79
75
­4
­2
15
20
July 12, 1988
68
59
­9
­10
16
22
Average 1988
82
74
­9
­6
20
25
July 18, 1991
76
64
­12
­11
20
27
July 19, 1991
77
70
­7
­5
20
28
July 20, 1991
80
69
­11
­12
20
26
July 21, 1991
74
69
­5
­4
13
18
Average 1991
77
68
­9
­8
18
25
July 12, 1995
67
59
­7
­10
13
20
July 13, 1995
77
63
­14
­15
22
30
July 14, 1995
71
73
2
5
15
22
July 15, 1995
69
67
­2
0
12
19
Average 1995
71
66
­5
­5
16
23

a For all station-hours with observed hourly ozone concentrations greater than 40 ppb.


Table 3-2. Comparison of daily maximum observed and predicted ozone concentration at western boundary sites for 1988 episodes.
1988 Episode
July 7
July 8
July 9
July 10
July 11
July 12
Station ID
LAT
LON
obs
pred
obs
pred
obs
pred
obs
pred
obs
pred
obs
pred
361098001 TOMPKIN42.40 -76.65
120
97.8
121
80.1
110
82.8
105
74.2
87
72.7
80
56.2
420690100 CARBOND41.47 -75.57
113
87.4
94
107.4
97
92.9
97
77.5
118
75.9
95
55.7
420692006 SCRANTO41.44 -75.62
126
100
101
117.1
113
104.6
100
88.1
125
82
94
61.2
420791101 WILKES41.26 -75.84
109
103.2
95
123.9
105
112.8
90
91.6
114
92.4
95
63.3
420791100 NANTICO41.20 -76.00
128
97.8
129
119.9
110
108.2
89
83.8
121
89.7
97
63.1
CNPSU106_ CENTRE40.72 -77.92
121
121.5
111
92.3
114
79.9
134
86.9
125
109.4
95
72.3
420990301 PERRY C40.45 -77.16
128
97.6
112
104.8
110
97.3
127
80.5
115
114.2
102
79.8
420431100 HERSHEY40.27 -76.68
143
114.3
124
114.5
106
123.4
103
95.9
77
112.8
77
82.6
420430401 HARRISB40.24 -76.84
137
130.6
111
111.9
114
123.9
93
100.7
103
107.2
63
77.8
421330008 YORK39.96 -76.69
155
124.7
122
114.3
115
116.7
98
95.2
99
115.9
65
80.6
CNARE128_ADAM39.91 -77.30
132
116
124
113.4
108
89.7
110
90.6
120
117.7
84
84.5
CNSHN118_MADISON38.51 -78.43
160
92.5
151
103.3
111
79.1
114
102.9
Average - All Sites
131
107
116
109
109
101
105
89
109
99
86
71

Table 3-3. Comparison of daily maximum observed and predicted ozone concentration at western boundary sites for 1991 episodes.
1991 Episode
July 18
July 19
July 20
July 21
Station ID
LAT
LON
obs
pred
obs
pred
obs
pred
obs
pred
CNCTH110_ TOMPKIN42.40 -76.65
94
59.9
88
64.5
89
68.1
81
67.1
360270007 DUTCHESRUR 42.29-74.21
121
54
130
62.8
115
56.7
93
65
361111005 ULSTER42.13 -74.51
100
59
85
63.2
99
57.8
87
64.2
420690100 CARBOND41.47 -75.57
102
74.3
106
91
106
68
80
66.5
420692006 SCRANTO41.44 -75.62
106
78.5
109
100.7
95
67.2
88
66.3
420791101 WILKES41.26 -75.84
111
80.4
104
108.5
81
73.8
72
65.3
420791100 NANTICO41.20 -76.00
108
95.8
95
110.2
76
75
75
64.8
CNPSU106_ CENTRE40.72 -77.92
117
117.9
110
130.5
99
93.8
89
105.5
420990301 PERRY C40.45 -77.16
101
100.1
111
104.2
102
115.2
103
100.2
420431100 HERSHEY40.27 -76.68
105
103.4
109
93.6
101
119.8
93
106.3
420430401 HARRISB40.24 -76.84
102
99.3
116
95.1
110
120
107
110.7
421330008 YORK39.96 -76.69
96
80.3
90
74.4
116
113.3
111
112.7
CNARE128_ ADAMS39.91 -77.30
111
75.2
122
84.5
109
99.8
116
104
CNSHN118_ MADISON38.51 -78.43
58
51.8
74
46.4
89
67.2
95
88.9
Average - All Sites
102
81
104
88
99
85
92
85

Table 3-4. Comparison of maximum ozone concentrations at western boundary sites for 1995 episodes.

1995 Episode
July 12
July 13
July 14
July 15
Station ID
LAT
LON
obs
pred
obs
pred
obs
pred
obs
pred
CNCTH110_ TOMPKIN42.40 -76.65
72
62.9
88
52.2
79
73.8
90
78.6
360270007 DUTCHESRU 42.29-74.21
84
48.3
100
47.3
86
61.8
73
59.7
361111005 ULSTER42.13 -74.51
56
46.7
86
47.5
89
63.2
75
60.2
420692006 SCRANTO41.44 -75.62
83
67.1
113
70.1
89
79.5
76
78.6
420791101 WILKES41.26 -75.84
79
64.3
101
79.6
99
85.2
91
77.3
420791100 NANTIC41.20 -76.00
68
64.3
96
80.2
87
85.4
63
74.1
CNPSU106_ CENTRE40.72 -77.92
76
53.9
106
101.7
112
99.8
88
83.7
420990301 PERRY C40.45 -77.16
82
64.6
91
107.9
80
93.7
74
87.3
420431100 HERSHEY40.27 -76.68
95
90.8
113
99.3
90
110.8
100
98.6
420430401 HARRISB40.24 -76.84
87
82.1
99
96.2
92
107
97
98.6
421330008 YORK39.96 -76.69
86
89.6
92
90.8
85
115
92
93.1
CNARE128_ ADAMS39.91 -77.30
89
100.7
98
97.2
99
112.1
98
91
510690010 FREDERI39.28 -78.08
90
104.3
106
60.4
87
96.3
97
106.9
CNSHN118_ MADISON38.51 -78.43
77
69.8
80
93
83.3
107
91.6
Average - All Sites
80
72
98
79
91
90
87
84

4. MODEL PERFORMANCE ALOFT IN THE NORTHEAST

For four days during the 1995 OTAG episode, aircraft measurements were made in the Northeast as part of the NARSTO-Northeast measurements program. Comparisons of observed and predicted concentrations aloft provide additional insights concerning model performance. In this analysis, the model performance for ozone, NOy, reactive hydrocarbons (RHC), and CO are evaluated against data collected by aircraft. The general pattern of aircraft flights during the July 12-15, 1995 episode is illustrated in Figure 4-1. On each morning, a flight was made along the "western boundary of the urban corridor" starting in Poughkeepsie, NY and ending at Shenandoah, VA, with spirals to measure vertical profiles (from 100 m above ground level (agl) to 1500 m above sea level (msl)) at Poughkeepsie, Kunkletown, PA, Gettysburg, PA and Shenandoah. On the tranverses between spiral locations, the plane flew at 250 to 650 m agl. The same plane flew an afternoon "in corridor" route starting at Manassas, VA and ending at Poughkeepsie. On July 14, a second plane flew from Poughkeepsie to Brookhaven, NY and along the "northern corridor" up to Portland, ME in the morning and returned in the afternoon.

Comparison of observed and predicted concentrations aloft is somewhat compromised by the coarse vertical resolution used in the OTAG simulations. Model predictions represent "layer average" concentrations for 50-100 m agl (layer 2), 100-250 m agl (layer 3),

250-500 m agl (layer 4), 500-1500 m agl (layer 5), and 1500-2500 m agl (layer 6).

4.1 COMPARISON WITH OZONE ON SELECTED TRAVERSES AND SPIRALS

Comparison of observed and predicted ozone concentrations aloft for the morning "western boundary" flights showed consistent underprediction bias on all four episode days. Figures 4-2, 4-3, 4-4, 4-5, 4-6, 4-7, 4-8, and 4-9 display results for July 14 and 15. On July 14, the spiral at Poughkeepsie showed a layer with 90 to 125 ppb of ozone between 600 and 1100 m agl, which was substantially higher than the model prediction of 75 ppb for layer 5. At Kunkletown, PA (Figure 4-3), the observed ozone concentrations were 90-100 ppb below 1000 m agl, compared to layer 5 predictions of about 50 ppb, while the layer 6 predicted concentration was 100 ppb, nearly double the observed. Thus, the predicted vertical ozone concentration profile was inverted and the opposite of the observed profile. At Gettysburg, PA, observed ozone was

30-40 ppb higher than predicted from 200 to 1000 m agl. The traverse between Kunkletown and Gettysburg (Figure 4-5) showed a consistent difference of about 30 ppb between observed and predicted values at 600 m agl. Results for July 15 showed similar bias, except for the spiral at Kunkletown (Figure 4-7) where observed and predicted ozone concentrations were similar above 500 m agl.

These results suggest that the model is underpredicting regional transport into the OTR from the west. The measured ozone concentrations also provide additional measurements at several "western boundary" sites which were discussed in Section 3. The peak ground-level concentrations observed at numerous western boundary sites (e.g., Gettysburg, PA) for

July 14 and 15 (see Table 3-4) were comparable to (or less than) the peak concentrations measured aloft near these sites in the morning. This suggests that the observed surface ozone concentrations at the western boundary of the urban corridor may be mostly due to transport rather than local emissions and local ozone formation.

Measurements from "in corridor" afternoon flights showed little prediction bias. Figure 4-10, 4-11, 4-12, 4-13, and 4-14 displays results from the July 15 afternoon flight. The spiral at Manassas, VA (Figure 4-10) showed a well-mixed layer of observed ozone of 90-100 ppb up to 1000 m agl, about 15 ppb higher than predicted. The traverse from Manassas to Lums Pond, DE (Figure 4-11) showed increasing observed and predicted concentrations, with an observed peak concentration above 150 ppb over Chesapeake Bay. The spiral above Lums Pond (where the peak ground-level concentration a few hours later reached 184 ppb) showed observed concentrations of about 110 ppb up to 600 m agl, then a sharp drop, while predicted concentrations were well mixed up to 1500 m agl. The spiral at Kunkletown, west of the corridor, showed comparable observed and predicted ozone up to 1200 m agl; predicted concentrations remained constant into layer 6, while observed concentrations decreased. Over the Atlantic Ocean south of Long Island, observed ozone concentrations (see Figure 4-14) showed a complicated vertical structure, including a shallow layer with high ozone concentrations below 200 m agl. The predicted vertical profile showed higher concentrations aloft.

Data from three additional flights with interesting features are shown in Figures 4-15, 4-16, and 4-17. The early morning traverse on July 14 from Poughkeepsie to Brookhaven showed a "spike" in observed ozone near Poughkeepsie and nearly constant ozone of 80 ppb as the flight path crossed the New York City urban plume. The predicted ozone at 300 to 500 m agl dropped from 65 to 20 ppb, and then increased to 70 ppb as the flight path cross the New York City urban plume. The model most likely overestimated the vertical extent of mixing since its predictions show ozone scavenging in the urban plume and the data do not show this effect. On the afternoon of July 14, (Figure 4-16), the flight from Ware, MA to New Haven, CT encountered the New York City urban plume, with a peak in both observed and predicted concentrations (130-150 ppb) as the flight approached New Haven. The afternoon spiral on July 14 (Figure 4-17) over the Atlantic Ocean south of Long Island showed observed ozone reaching 150 ppb between 300 and 800 m agl, then dropping to 70 ppb at 1500 m agl, while the predicted "layer average" concentration was about 115 ppb. Again, predicted vertical gradients in ozone were smaller in magnitude and in the opposite direction than the observed vertical gradient.

In summary, this comparison with detailed aircraft data indicates that the OTAG simulations predict afternoon ozone concentrations in the corridor for the 1995 episode without much bias, despite systematic underprediction of morning ozone concentrations aloft (between 200 and 1500 agl) by 20-30 ppb. Observed ozone profiles of ozone concentrations offshore over the ocean also show a pronounced, complex vertical structure, which the model is unable to simulate, given the limited horizontal and vertical resolution of the OTAG grid.
4.2 COMPARISON OF VERTICALLY INTEGRATED OZONE, NOy, RHC, AND CO DATA

The aircraft collected numerous vertically integrated hydrocarbon and CO samples on vertical spirals on the July 13-15, 1995 flights. The high time resolution ozone and NOy, data have been averaged to correspond to the hydrocarbon data in order to concurrently evaluate the model predictions aloft for these species. Figures 4-18, 4-19, 4-20, and 4-21 show scatter plots for the predicted and observed vertically integrated ozone, NOy, RHC, and CO concentrations. The individual data are shown in Tables 4-1 and 4-2. The data are mostly integrated over 50 to 500 m agl, with some samples collected at higher elevations. For purpose of comparing the modeled and observed hydrocarbons, only the reactive hydrocarbons (RHC) are considered. The measured reactive hydrocarbon concentrations were estimated from the nonmethane hydrocarbon, ethane, propane, and benzene concentrations as follows:

RHC = NMHC - 0.8*[Ethane] - 0.5*[Propane] - 0.8*[Benzene]

The models' reactive hydrocarbons included paraffin bonds (PAR), ethene (ETH), olefins (OLE), toluene (TOL), and xylenes (XYL)..

The vertically integrated results show some bias for ozone aloft. For the morning samples, the average predicted and observed vertically integrated ozone concentrations were 60 and 70 ppb, respectively (or -13 percent bias). Comparison of the afternoon samples indicate better agreement, with the model predicting 87 ppb when 91 ppb was observed on average (-5 percent bias). Overall, the average predicted vertically integrated ozone was 73 ppb when 81 ppb was observed. The scatter plots show cases with over- and underpredictions. The predicted and observed ozone are reasonably well correlated

(R2 = 0.66, even with two outliers included). These results are consistent with the point-by-point comparison presented above and the analysis of all of the 1995 aircraft data reported by Adamski (1997), in that they show more ozone underestimation bias in the morning than in the afternoon.

The scatter plot for NOy shows significantly more scatter than the corresponding ozone plot, nevertheless, the predicted and observed NOy are correlated (R2 =0.46). The average vertically integrated NOy concentration is 8.3 ppb, which is slightly above the 6.6 ppb observed. The NOy concentrations and bias are similar in the morning and afternoon samples. Note, however, that the model's estimate of NOy should be biased low (by 1 to 10 percent) because not all of the NOy species were archived in the simulations. The bias in the NOy predictions aloft is similar to that found in the surface data in the Northeast (OTAG, 1996b).

The scatter plot for RHC shows the model seriously underestimates the RHC concentrations aloft in this region. On average, the model estimates 27 ppbC RHC compared to 90 ppbC observed in the aircraft spirals. The RHC concentrations and bias are similar in the morning and afternoon samples. The discrepancy for RHC aloft is somewhat larger than the discrepancy found in evaluation of surface RHC data at rural NARSTO-Northeast stations (OTAG, 1996b), although it is directionally similar. In addition, there is no correlation (R2 =0.01) between predicted and observed RHC aloft. This comparison is based on a relatively small number of observations. If the underestimation of RHC is widespread, it would suggest the VOC emissions are underrepresented in the modeling system and that the model may be getting the right ozone for the wrong reasons.

The scatter plot for CO shows greater variability in the observed CO aloft than the predicted CO. The dynamic range of the predicted CO is narrower (125 to 312 ppb) than the observed range (144 to 497 ppb). The average vertically integrated CO concentration aloft is 209 ppb, which is substantially lower than the 338 ppb observed aloft. The bias in CO predictions aloft is smaller than the bias for RHC, and probably not of great concern for the modeling.

Table 4-1. Vertically integrated observed and predicted ozone and NOy concentrations (ppb) above selected measurement sites.


Site

Date

Start Time
Observed Ozone
Predicted Ozone
Observed NOy
Predicted NOy
Boat
7/13/95
13:05
85.13
98.29
3.8
10.8
Boat
7/14/95
15:05
130.9
129.5
7.8
16.8
Boat
7/15/95
15:11
99.6
119.7
5.4
11.0
Brookhaven
7/14/95
15:13
116.0
115.6
11.7
13.5
Brookhaven
7/14/95
4:33
67.9
66.7
11.6
6.6
Gettysburg
7/12/95
6:09
52.8
58.3
9.4
4.6
Gettysburg
7/13/95
6:16
63.1
57.9
10.8
7.9
Gettysburg
7/14/95
6:06
66.7
50.5
7.6
7.0
Gettysburg
7/15/95
6:13
93.2
71.2
7.0
3.8
Kunkletown
7/12/95
13:43
69.8
61.0
7.5
5.5
Kunkletown
7/12/95
13:56
62.1
43.7
1.8
2.6
Kunkletown
7/13/95
14:20
75.0
48.4
0.7
4.5
Kunkletown
7/13/95
5:14
66.7
43.8
6.5
2.6
Kunkletown
7/13/95
5:06
91.8
86.6
9.3
12.7
Kunkletown
7/13/95
14:11
107.7
100.4
7.8
21.1
Kunkletown
7/14/95
14:09
71.3
55.7
2.0
1.3
Kunkletown
7/14/95
13:59
86.5
87.7
5.0
8.6
Kunkletown
7/14/95
4:56
75.5
48.2
7.9
9.7
Kunkletown
7/14/95
5:06
53.9
62.6
1.3
2.8
Kunkletown
7/15/95
14:17
57.1
58.8
2.1
1.8
Kunkletown
7/15/95
5:18
74.2
58.7
1.6
3.2
Kunkletown
7/15/95
5:08
91.6
73.3
4.7
10.4
Kunkletown
7/15/95
14:08
80.8
78.2
5.4
4.2
Near Kunkletown
7/12/95
5:16
66.4
52.8
1.3
3.0
Luray Caverns
7/12/95
7:03
56.9
51.5
6.3
11.7
Luray Caverns
7/13/95
7:16
60.8
45.9
8.2
6.0
Luray Caverns
7/14/95
7:01
72.5
51.3
6.0
9.1
Luray Caverns
7/15/95
7:04
75.8
70.3
7.4
12.9
Manassas
7/12/95
12:03
82.9
68.2
4.5
13.6
Manassas
7/12/95
7:39
76.7
64.6
4.8
9.3
Manassas
7/13/95
7:52
69.1
61.7
6.0
5.6
Manassas
7/14/95
7:38
74.0
59.7
4.6
5.6
Manassas
7/14/95
12:10
92.0
59.4
7.0
4.5
Manassas
7/15/95
12:17
97.3
82.9
6.6
8.0
New Haven
7/14/95
5:07
58.4
85.3
2.8
5.3
New Haven
7/14/95
14:52
54.2
92.1
2.3
1.8
New Haven
7/14/95
4:55
58.8
50.6
21.5
44.0
New Haven
7/14/95
14:41
146.6
129.7
19.1
14.4


Table 4-1. Vertically integrated observed and predicted ozone and NOy concentrations (ppb) above selected measurement sites.


Site

Date

Start Time
Observed Ozone
Predicted Ozone
Observed NOy
Predicted NOy
Offshore
7/14/95
6:58
76.0
54.9
9.6
8.7
Offshore
7/14/95
12:29
95.0
90.4
8.4
7.0
Summit
7/12/95
13:00
88.8
87.4
6.9
4.3
Summit
7/13/95
15:09
98.7
110.2
6.3
12.2
Summit
7/14/95
13:10
118.5
118.3
8.7
17.6
Summit
7/15/95
13:20
110.8
92.0
8.3
3.4
Ware
7/14/95
5:51
76.8
55.9
9.0
4.6
Ware
7/14/95
13:45
77.9
66.2
5.4
3.3
Morning Average
70
60
6.9
8.3
Afternoon Average
92
87
6.3
8.3
Average
81
73
6.6
8.3

Table 4-2. Vertically integrated observed and predicted CO and RHC concentrations (ppbC) above selected measurement sites.


Site

Date

Start Time
Observed CO
Predicted CO
Observed RHC
Predicted RHC
Boat
7/13/95
13:05
333
237
65
35
Boat
7/14/95
15:05
433
281
143
21
Boat
7/15/95
15:11
386
285
78
17
Brookhaven
7/14/95
15:13
88
47
Brookhaven
7/14/95
4:33
89
30
Gettysburg
7/12/95
6:09
311
215
92
23
Gettysburg
7/13/95
6:16
73
30
Gettysburg
7/14/95
6:06
433
224
108
43
Gettysburg
7/15/95
6:13
427
215
152
8
Kunkletown
7/12/95
13:43
330
184
74
30
Kunkletown
7/12/95
13:56
172
125
33
9
Kunkletown
7/13/95
14:20
209
135
54
13
Kunkletown
7/13/95
5:14
59
11
Kunkletown
7/13/95
5:06
61
43
Kunkletown
7/13/95
14:11
410
288
96
49
Kunkletown
7/14/95
14:09
181
143
55
12
Kunkletown
7/14/95
13:59
343
202
132
37
Kunkletown
7/14/95
4:56
473
212
131
12
Kunkletown
7/14/95
5:06
145
162
103
17
Kunkletown
7/15/95
14:17
151
174
62
20
Kunkletown
7/15/95
5:18
174
160
70
19
Kunkletown
7/15/95
5:08
441
233
125
13
Kunkletown
7/15/95
14:08
430
219
114
6
Near Kunkletown
7/12/95
5:16
165
151
80
19
Luray Caverns
7/12/95
7:03
321
171
87
12
Luray Caverns
7/13/95
7:16
63
31
Luray Caverns
7/14/95
7:01
383
232
84
51
Luray Caverns
7/15/95
7:04
396
182
100
36

Table 4-2. Vertically integrated observed and predicted CO and RHC concentrations (ppbC) above selected measurement sites.


Site

Date

Start Time
Observed CO
Predicted CO
Observed RHC
Predicted RHC
Manassas
7/12/95
12:03
309
208
85
31
Manassas
7/12/95
7:39
272
174
62
30
Manassas
7/13/95
7:52
58
40
Manassas
7/14/95
7:38
355
233
146
11
Manassas
7/14/95
12:10
447
186
132
31
Manassas
7/15/95
12:17
490
194
104
12
New Haven
7/14/95
5:07
67
28
New Haven
7/14/95
14:52
44
26
New Haven
7/14/95
4:55
134
98
New Haven
7/14/95
14:41
77
57
Offshore
7/14/95
6:58
67
11
Offshore
7/14/95
12:29
86
10
Summit
7/12/95
13:00
395
272
89
27
Summit
7/13/95
15:09
380
271
79
43
Summit
7/14/95
13:10
498
313
145
56
Summit
7/15/95
13:20
469
240
150
10
Ware
7/14/95
5:51
110
15
Ware
7/14/95
13:45
49
19
Morning Average
319
194
92
27
Afternoon Average
354
220
88
27
Average
338
209
90
27

5. CONCLUSIONS

The UAM-V model performance for ozone concentrations and regional transport contributions to ozone in the Northeast region was assessed, through review of the OTAG model performance report and related analysis products, plus a series of independent analyses. The following findings emerged from our assessment of the UAM-V model performance in the OTAG simulations:

These findings indicate that the estimated transport contributions into the Northeast Corridor and between subregions within the corridor are subject to greater bias and uncertainty than the predicted contributions from nearby urban-scale emission sources.


6. REFERENCES

Adamski W. (1997) An analysis of measured and predicted concentrations aloft of ozone and total reactive nitrogen in the eastern U.S. during July 1995. Report prepared by the Wisconsin Department of Natural Resources, Madison, WI.

EPA (1991) Guideline for regulatory application of the urban airshed model. Report prepared by U.S. Environmental Protection Agency, Research Triangle Park, NC, EPA-450/4-91-013.

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.


Figure List

2-1. Ozone monitoring stations in the Northeast region used in the OTAG model performance evaluation

2-2. Ozone monitoring stations in the Boston subregion used in the OTAG model performance evaluation

2-3. Ozone monitoring stations in the New York City subregion used in the OTAG model performance evaluation

2-4. Ozone monitoring stations in the Philadelphia subregion used in the OTAG model performance evaluation

2-5. Ozone monitoring stations in the Baltimore-Washington subregion used in the OTAG model performance evaluation

2-6. Spatial distribution of predicted ozone concentrations at 2 pm (EST) on July 13, 1995

2-7. Surface (20 m) air parcel back trajectories based on diagnostic model winds for July 13, 1995

2-8. 500 m air parcel back trajectories based on diagnostic model winds for July 13, 1995

2-9. Spatial distribution of predicted ozone concentrations at 2 pm (EST) on July 14, 1995

2-10. Surface (10 m) and 500 m air parcel back trajectories based on diagnostic model winds for July 14, 1995

2-11. 500 m air parcel back trajectories based on RAMS prognostic model winds for July 14, 1995

2-12. Spatial distribution of predicted ozone concentrations at 1 pm (EST) on July 15, 1995

2-13. Surface (10 m) and 500 m air parcel back trajectories based on diagnostic model winds for July 15, 1995

2-14. Surface (10 m) air parcel back trajectories based on RAMS prognostic model winds for July 15, 1995

2-15. 500 m air parcel back trajectories based on RAMS prognostic model winds for July 15, 1995

3-1. Ozone monitoring stations in the western boundary subregion used in the OTAG model performance evaluation

4-1. NARSTO-Northeast aircraft flight paths for summer 1995

4-2. Predicted and observed vertical ozone profile on July 14, 1995

4-3. Predicted and observed vertical ozone profile on July 14, 1995

4-4. Predicted and observed vertical ozone profile on July 14, 1995

4-5 Predicted and observed ozone aloft along a traverse on July 14, 1995 at various elevations

4-6. Predicted and observed vertical ozone profile on July 15, 1995

4-7. Predicted and observed vertical ozone profile on July 15, 1995

4-8. Predicted and observed vertical ozone profile on July 15, 1995.

4-9. Predicted and observed vertical ozone profile on July 15, 1995

4-10. Predicted and observed vertical ozone profile on July 15, 1995

4-11. Predicted and observed ozone aloft along a traverse on July 15, 1995 at various elevations

4-12. Predicted and observed vertical ozone profile on July 15, 1995

4-13. Predicted and observed vertical ozone profile on July 15, 1995

4-14. Predicted and observed vertical ozone profile on July 15, 1995

4-15. Predicted and observed ozone aloft along a traverse on July 14, 1995 at various elevations 4-19

4-16. Predicted and observed ozone aloft along a traverse on July 14, 1995 at various elevations

4-17. Predicted and observed vertical ozone profile on July 14, 1995

4-18. Predicted and observed vertically integrated ozone concentrations on July 13-15,1995 in the Northeast

4-19. Predicted and observed vertically integrated NOy concentrations on July 13-15, 1995 in the Northeast

4-20. Predicted and observed vertically integrated RHC concentrations on July 13-15, 1995 in the Northeast

4-21. Predicted and observed vertically integrated CO concentrations on July 13-15, 1995 in the Northeast

LIST OF TABLES

Table

2-1. Maximum observed and predicted ozone for the July 6-12, 1988 episode

2-2. Comparison of predicted and observed 90th percentile ozone concentrations in the July 6-12, 1988 episode

2-3. Mean observed and predicted ozone concentrations, mean bias and normalized bias, and mean error and normalized error for the July 6-12, 1988 episode

2-4. Maximum observed and predicted ozone for the July 18-21, 1991 episode

2-5. Comparison of predicted and observed 90th percentile ozone concentrations in the July 18-21, 1991 episode

2-6. Mean observed and predicted ozone concentrations, mean bias and normalized bias, and mean error and normalized error for the July 18-21, 1991 episode

2-7. Maximum observed and predicted ozone for the July 12-15, 1995 episode

2-8. Comparison of predicted and observed 90th percentile ozone concentrations in the July 12-15, 1995 episode

2-9. Mean observed and predicted ozone concentrations, mean bias and normalized bias, and mean error and normalized error for the July 12-15, 1995 episode

3-1. Mean observed and predicted hourly ozone surface concentrations, mean model bias, and error at the western boundary sites

3-2. Comparison of daily maximum observed and predicted ozone concentration at western boundary sites for 1988 episodes

3-3. Comparison of daily maximum observed and predicted ozone concentration at western boundary sites for 1991 episodes

3-4. Comparison of maximum ozone concentrations at western boundary sites for 1995 episodes

4-1. Vertically integrated observed and predicted ozone and NOy concentrations (ppb) above selected measurement sites

4-2. Vertically integrated observed and predicted CO and RHC concentrations (ppbC) above selected measurement sites


Submit your comments, feedback, questions, and ideas pertaining this page. Your input will be automatically added to the existing annotations. In order to add a new comment, you must be registered with the OTAG/AQA Peoples Page.