Influence of baseline ozone on surface air quality in the western U.S

As the ozone air quality standard is made more stringent, cities, counties and states will need a better understanding of the sources of ozone. While Eulerian models can provide important indications of the sources, they are often challenged by episodic emissions (eg wildfires), interannual variability and/or coarse model resolution. These challenges are especially problematic in the western United States. Thus we need improved tools to directly use available observations to better understand and identify ozone sources. In this study we examine the relationship between measured free tropospheric/baseline ozone and surface ozone at several receptor sites in the western U.S. Our focus is on air quality sites in the Pacific Northwest and Rocky Mtn region that exceed current or new air quality standards that have been proposed. Elevated sites in the Rocky Mtns are exposed to high ozone from the free troposphere. Significant interannual variations occur in spring, due to variations in baseline ozone, and in summer, due to variations in wildfires. In the Rocky Mtn region there are significant correlations between the surface and free tropospheric mixing ratios using both daily and monthly means for spring and summer months (see Jaffe 2011; DOI: 10.1021/es1028102). This implies that free tropospheric ozone mixes down and frequently contributes to elevated ozone at the surface. In the Pacific Northwest, we are using data from suburban and rural monitoring locations to identify ozone from local and non-local sources. The approach for both the Rocky Mtn and Pacific Northwest sites is to develop a statistical model for ozone using all available observations, (e.g. PM, meteorology, satellite data and trajectories) to quantify the variable contribution from baseline ozone. The statistical model can then be evaluated against actual observations of baseline ozone. The statistical model can also be useful to help identify "exceptional events" as defined by the US EPA.