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

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 O-3 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.