Preview of 1994 Ozone Precursor Concentrations in the Norteastern U.S.
This report focuses primarily on two 3-day ozone episode periods during the Summer of 1994: July 6, 7, 8 and July 20, 21, 22. These episode periods had notable similarities and differences. Both occurred during the same days of the week (Wed. - Fri.), were characterized by high ambient temperatures (in excess of 90 degrees F at the Bradley International Airport, Windsor Locks, CT - central to the NESCAUM region), and resulted in multi-site ozone exceedances. The spatial patterns of these episodes were quite different. High afternoon ozone levels during the first episode were primarily limited to the southern half of the region (Figure 3.1)
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Figure 3.1 Afternoon (3 pm) Ozoner on 7/7/94
with morning surface winds at most sites primarily from the east southeast (Figure 3.3).
Figure 3.3 Morning (10 pm) Surface winds on 7/7/94
High ozone levels during the second episode were primarily in the northeast of the region (figure 3.2)
Figure 3.2 Afternoon (3 pm) Ozoner on 7/21/94
with surface winds from the southwest (Figure 3.4)
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Figure 3.4 Morning (10 pm) Surface winds on 7/21/94
Figure 3.5 shows regional average ozone levels for the month of July, 1994 (averaged over sites displayed in map at left). These regional averages, displayed as both 1 hour and as 24 hour averages, indicate that relatively high ozone values occurred throughout the region during the selected episode periods. The 2 selected episodes accounted for 1/5 of the 1994 ozone exceedances in the NESCAUM region, but were not atypical of elevated regional values which occur periodically throughout the region during Summer.
Figure 3.5 Regional Average 1 Hour and 24 Hour Ozone Levels (for sites displayed at left) during July, 1994
Average regional hourly and 24 hour NOx levels - (averaged over a much smaller number of available sites than ozone) for July, 1994 are displayed in figure 3.6. An examination of data from the (18) individual sites indicates that the high hourly average values on the mornings of 7/7/94 and 7/20/94 are not driven by extreme values at a few sites. High morning NOx levels were recorded concurrently at many sites throughout the New England States in the midst of these 2 episode periods.
Figure 3.6 Regional Average 1 Hour and 24 Hour NOx Levels (for sites displayed at right) during July, 1994
Carbon Monoxide (CO) data are available for a relatively small number of sites (16) in the New England region during July, 1994. During summer, CO emissions at most Northeastern sites are likely to be predominantly related to motor vehicle exhaust. As CO is relatively non-reactive, regional CO concentrations are likely to vary as a combined function of diurnal variations in motor vehicle traffic and meteorological conditions. With the possible exception of weekend days, traffic patterns are not anticipated to fluctuate widely on a daily basis. Consequently, site-specific and regional CO levels can provide a useful index of automotive emissions as influenced by meteorology. As displayed in figure 3.7, average regional CO levels were relatively high during the July 6-8 (Wednesday - Friday) episode period - indicating relatively poor meteorological dispersion for ground-level automotive emissions. By contrast, regional average CO levels were relatively low during the July 20-22 (Wednesday - Friday) episode period - indicating conditions favorable to dispersion (and transport) of ground-level automotive emissions.
Figure 3.7 Regional Average 1 Hour and 24 Hour CO Levels (for sites displayed at left) during July, 1994
This CO-based observation is consistent with surface wind speed data for the 2 episode periods. Wind speeds at sites throughout the region (particularly at night and during the early morning hours) were generally much higher during the 7/20-22 period than during the early July episode.
