JUNE 2001: Background ozone in surface air over the U.S.: origin and contribution to pollution episodes

The top panel shows the probability distribution of afternoon (1-5 p.m.) background ozone concentrations in surface air over the U.S. during the summer of 1995 in the GEOS-CHEM model for two populations: (1) the ensemble of data from all U.S. grid squares on all summer days (black line) and (2) the data subset from highly polluted days, when total afternoon surface ozone exceeds 80 ppbv (green line). The background is defined here as ozone produced outside the North American boundary layer (surface to 700 hPa) using a tagged tracer method [Wang et al., 1998]. The figure shows that the background contribution is generally much smaller on polluted days, which are often associated with stagnation and subsidence inversions that suppress mixing from the free troposphere. This result indicates that inferring a background contribution from observations under clean conditions severely overestimates the actual background under the heavily polluted conditions associated with exceedances of the national air quality standard for ozone. A small population of points with high background concentrations and ozone above 80 ppbv is evident from the figure. Our analysis indicates that these values occur following subgrid-scale convective activity that transports ozone from the free troposphere to the surface.

The bottom panel shows the enhancement to background ozone due to anthropogenic emissions in Asia and Europe, plotted as a function of total ozone concentrations in the model. Points represent daily afternoon (1-5 p.m.) values for all U.S. grid squares on all summer days. The enhancement from Asian and European emissions is defined here as the difference between a GEOS-CHEM simulation with anthropogenic emissions within North America turned off , and one with all anthropogenic emissions turned off. The average enhancement in surface air over the U.S. is 4-7 ppbv, but as shown in the figure, this enhancement is particularly large (up to 14 ppbv) under moderately high ozone concentrations (50-70 ppbv), reflecting a combination of convective transport from the free troposphere and rapid photochemical production within the U.S. boundary layer. Please see our recently submitted paper Fiore et al. [2001] for more details.