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4 The Effects of Meteorology on Tropospheric Ozone
Pages 93-108

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From page 93...
... This chapter examines the effects of weather on tropospheric ozone formation, accumulation, and transport and discusses aspects of those processes that are important for predicting ozone concentrations through the use of mathematical models. The chapter also includes a discussion of rural ozone data for the United States, with a focus on the effects of meteorology.
From page 94...
... As the slow-moving air in the shallow boundary layer passes over major metropolitan areas, pollutant concentrations nse, and as the air slowly flows around the high-pressure system, photochemical production of ozone occurs at peak rates. Major high-pressure systems at the earth's surface are associated pith ridges of high-pressure surfaces in the middle and upper troposphere.
From page 95...
... He found that the net ozone flux in the cloud layer was a linear function of the difference in ozone concentration between the boundary layer and the cloud layer, and that cloud turbulence contributed about 30% of the total cloud flux. Ozone fluxes in the regions between clouds were usually smaller than the cloud fluxes, but their contribution to the net transport of ozone was important because they occur over a larger area.
From page 96...
... They described a case when a massive complex of thunderstorms swept through the eastern U.S., which was under the influence of a stagnant high pressure system. The MCSs removed over half a million square kilometers of polluted boundary layer air and replaced it with cleaner middle tropospheric air, leading to significant decreases in ozone and sulfate concentrations and increases in visibility.
From page 97...
... The strength and position of a stagnant high-pressure system in the lower atmosphere are related to the presence of a ridge of high pressure in the middle and upper troposphere, which is reasonably well predicted by current global forecasting models for several days (van den Dool and Saha, 1990~. Perhaps the greatest uncertainty in the prediction of ozone concentrations when there are strong stagnant highpressure systems is whether a large mesoscale convective system will form on the periphery of the stagnant high and invade the interior of the system, sweeping large volumes of boundary layer ozone and other species into the middle and upper troposphere.
From page 98...
... Despite the fact that higher average temperatures are associated with higher rates of ozone production, if the large-scale pressure pattern shifted such that the East Coast were preferentially under a trough in the early summer, that region would likely experience reduced concentrations of ozone. Unfortunately, although general circulation models predict a shift in the largescale ridge-trough pattern associated with global warming, no two models predict the same shift in patterns, and none of the predictions of pattern shifts should be viewed with confidence.
From page 99...
... Studies that use surface and aircraft data have shown that the high concentrations are superimposed on elevated background concentrations during high-ozone episodes. The higher background concentrations are presumably due to enhanced photochemical production of natural and anthropogenic ozone in the warm, cloud-free conditions that characterize such episodes.
From page 100...
... The annual cycle of monthly mean and monthly maximum values of ozone and the diurnal cycle in July are shown in Figure =1 for typical rural sites in the eastern and western United States. Cumulative probability distributions (providing percentile rank scores)
From page 101...
... The upper panels show the seasonal distribution of daily average values; the middle panels show monthly averages of the daily maximum values. The lower panels show the diurnal behavior of ozone in July.
From page 102...
... Whiteface Mountain. Sites are identified by state; results are plotted on probability paper; normally distributed data define a straight line.
From page 103...
... ~11 ! IT r A M J J A S _ I T ~ FIGURE ~3 Time series of daily maximum ozone concentrations at rural sites in the northeastern United States in 1979.
From page 104...
... argued that the plume generated 57 ppb ozone; the ozone maximum at the rural site was 73 ppb, 31 ppb below the plume's maximum value. On the day with the highest maximum ozone, 180 ppb, ozone concentrations were about 90 ppb at rural sites.
From page 105...
... compared maximum ozone concentrations from stations upwind and downwind of Atlanta and at a rural site 125 km away. On days when urban ozone concentrations exceeded 100 ppb, the ozone concentration was 80-85 ppb at the upwind station and 110-125 ppb at the downwind station, suggesting that the city contributed 30-40 ppb above the immediate background.
From page 106...
... Maximum values of non-urban ozone commonly exceed 90 ppb during these episodes, compared with average daily maximum values of 60 ppb in summer. An urban area need contribute an increment of only 30 ppb over the regional background during a high ozone episode to cause a violation of the National Ambient Air Quality Standard (NAAQS)
From page 107...
... need was recognized by EPA and led to the development of the Regional Oxidant Model, discussed in Chapters 10 and 11. Regional models for ozone require a meteorological component that realistically describes the atmospheric wind field and its turbulence and mixing characteristics.


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