Reversal of Long-Term Trend in Baseline Ozone Concentrations at the North American West Coast

Changes in baseline (here understood as representative of continental to hemispheric scales) tropospheric ozone concentrations that have occurred over western North American and eastern North Pacific are analyzed based on data from three measurement records: (1) sites in the U.S. Pacific coast marine boundary layer, (2) an inland, higher altitude site at Lassen Volcanic National Park, CA, and (3) springtime airborne measurements in the free troposphere between 3 and 8 km altitude. Consistent with previously published results, we find increasing ozone prior to the year 2000, but that rate of increase has slowed and now reversed in these data sets in all seasons. The past ozone increase has been identified as a significant difficulty to overcome in achieving U.S. air quality goals; this difficulty has now eased. Global models only poorly reproduce the observed baseline ozone and trends; policy guidance from such models must be considered very cautiously. Plain Language Summary In U.S. urban and rural areas, ozone transported into the country from the Pacific (i.e., baseline ozone) makes substantial contributions to exceedances of the ozone national ambient air quality standard. Over past decades, baseline ozone concentrations increased, which made achievement of ozone air quality goals increasingly difficult. However, that increase ended in the early to mid-2000s, and the baseline concentrations have begun to decrease, thus easing this particular difficulty. Global modes are relied upon to quantify the influence of this ozone transport, but they poorly reproduce observed baseline ozone concentrations.