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A regional modeling system that integrates the state-of-the-art emissions processing (SMOKE), climate (CWRF), and air quality (CMAQ) models has been combined with satellite measurements of fire activities to assess the impact of fire emissions on the contiguous United States (CONUS) air quality during 1997–2016. The system realistically reproduced the spatiotemporal distributions of the observed meteorology and surface air quality, with a slight overestimate of surface ozone (O3) by ~4% and underestimate of surface PM2.5 by ~10%. The system simulation showed that the fire impacts on primary pollutants such as CO were generally confined to the fire source areas but its effects on secondary pollutants like O3 spread more broadly. The fire contribution to air quality varied greatly during 1997-2016 and occasionally accounted for more than 100 ppbv of monthly mean surface CO and over 20 µg m−3 of monthly mean PM2.5 in the Northwest U.S. and Northern California, two regions susceptible to frequent fires. Fire emissions also had implications on air quality compliance. From 1997 to 2016, fire emissions increased surface 8-hour O3 standard exceedances by 10% and 24-hour PM2.5 exceedances by 33% over CONUS. 

Abstract. We investigated the ozone pollution trend and its sensitivity to keyprecursors from 1990 to 2015 in the United States using long-term EPA Air Quality System (AQS)observations and mesoscale simulations. The modeling system, a coupledregional climate–air quality model (CWRF-CMAQ; Climate-Weather Research Forecast andthe Community Multiscale Air Quality), captured well the summersurface ozone pollution during the past decades, having a mean slope oflinear regression with AQS observations of ∼0.75. While theAQS network has limited spatial coverage and measures only a few keychemical species, CWRF-CMAQ provides comprehensive simulations to enablea more rigorous study of the change in ozone pollution and chemicalsensitivity. Analysis of seasonal variations and diurnal cycle of ozoneobservations showed that peak ozone concentrations in the summer afternoondecreased ubiquitously across the United States, up to 0.5 ppbv yr−1 in majornon-attainment areas such as Los Angeles, while concentrations at certainhours such as the early morning and late afternoon increased slightly.Consistent with the AQS observations, CMAQ simulated a similar decreasingtrend of peak ozone concentrations in the afternoon, up to 0.4 ppbv yr−1, andincreasing ozone trends in the early morning and late afternoon. A monotonicallydecreasing trend (up to 0.5 ppbv yr−1) in the odd oxygen (Ox=O3+NO2) concentrations are simulated by CMAQ at all daytime hours.This result suggests that the increased ozone in the early morning and lateafternoon was likely caused by reduced NO–O3 titration, driven bycontinuous anthropogenic NOx emission reductions in the past decades.Furthermore, the CMAQ simulations revealed a shift in chemical regimes ofozone photochemical production. From 1990 to 2015, surface ozone productionin some metropolitan areas, such as Baltimore, has transited from aVOC-sensitive environment (>50 % probability) to aNOx-sensitive regime. Our results demonstrated that the long-termCWRF-CMAQ simulations can provide detailed information of the ozonechemistry evolution under a changing climate and may partially explain theUS ozone pollution responses to regional and national regulations.