Relationship between Surface and Free Tropospheric Ozone in the Western U.S.

摘要

Ozone is an important air pollutant that affects lung function. In the U.S., the EPA has reduced the allowable O_3 concentrations several times over the last few decades. This puts greater emphasis on understanding the interannual variability and the contributions to surface O_3 from all sources. We have examined O_3 data from 11 rural CASTNET sites in the western US for the period 1995-2009. The 11 surface sites show a similar seasonal cycle and generally a good correlation in the deseasonalized monthly means, indicating that there are large scale influences on O_3 that operate across the entire western US. These sites also show a good correlation between site elevation and annual mean O_3, indicating a significant contribution from the free troposphere. We examined the number of exceedance days for each site, defined as a day when the Maximum Daily 8-h Average (MDA8) exceeds a threshold value. Over this time period, more than half of these sites exceeded an MDA8 threshold of 70 ppbv at least 4 times per year, and all sites exceeded a threshold value of 65 ppbv at least 4 times per year. The transition to lower threshold values increases substantially the number of exceedance days, especially during spring, reflecting the fact that background 63 peaks during spring. We next examined the correlation between surface O_3 and free tropospheric O_3 in the same region, as measured by routine balloon launches from Boulder, CO. Using ozone measured by the balloon sensor in the range of 3-6 km above sea level we find statistically significant correlations between surface and free tropospheric O_3 in spring and summer months using both monthly means, daily MDA8 values, and the number of surface exceedance days. We suggest that during spring this correlation reflects variations in the flux of O_3 transport from the free troposphere to the surface. In summer, free tropospheric and surface concentrations of O_3 and the number of exceedance days are all significantly correlated with emissions from biomass burning in the western US. This indicates that wildfires significantly increase the number of exceedance days across the western U.S.