Combining model projections with site-level observations to estimate changes in distributions and seasonality of ozone in surface air over the USA

While compliance with air quality standards is evaluated at individual monitoring stations, projections of future ambient air quality for global climate and emission scenarios often rely on coarse resolution models. We describe a statistical transfer approach that bridges the spatial gap between air quality projections, averaged over four broad U.S. regions, from a global chemistry-climate model and the local level (at specific U.S. CASTNet sites). Our site-level projections are intended as a line of evidence in planning for possible futures rather than the sole basis for policy decisions. We use a set of transient sensitivity simulations (2006-2100) from the Geophysical Fluid Dynamics Laboratory (GFDL) chemistry-climate model CM3, designed to isolate the effects of changes in anthropogenic ozone (O-3) precursor emissions, climate warming, and global background CH4 on surface O-3. We find that surface maximum daily 8-h average (MDA8) O-3 increases despite constant precursor emissions in a warmer climate during summer, particularly in the low tail of the MDA8 O-3 distribution for the Northeastern U.S., while MDA8 O-3 decreases slightly throughout the distribution over the West and Southeast during summer and fall. Under scenarios in which non-methane O-3 precursors decline as climate warms (RCP4.5 and RCP8.5), summertime MDA8 O-3 decreases with NOx emissions, most strongly in the upper tail of the MDA8 O-3 distribution. In a scenario where global methane abundances roughly double over the 21st century (RCP8.5), winter and spring MDA8 O-3 increases, particularly in the lower tail and over the Western U.S. In this RCP8.5 scenario, the number of days when MDA8 O-3 exceeds 70 ppb declines in summer with NOx emissions, but increases in spring (and winter); by the end of the century, the majority of sites in the WE and NE show probabilistic return values of the annual 4th highest MDA8 O-3 concentration above 70 ppb (the current O-3 NAAQS level). Continued increases in global CH4 abundances can be thought of as a methane penalty, offsetting benefits otherwise attainable by controlling non-CH4 O-3 precursors.