Drought Impacts on PM2.5 Composition and Amount Over the US During 1988–2018

Yi Wang; Jun Wang; Yuxuan Wang; Wei Li

doi:10.1029/2022JD037677

This study uses PM2.5 species concentrations from Interagency Monitoring of Protected Visual Environments and Standardized Precipitation Evapotranspiration Index (SPEI) during 1988–2018 over the continental US to investigate the association of spatial‐temporal variations of surface PM2.5 species with droughts. Ubiquitous decreasing trends in seasonal mean reconstructed PM2.5 are detected in all seasons except for the summer over the northwestern US. The increasing trend in the reconstructed PM2.5 in summer over northwestern US is primary due to the positive trend in total carbonaceous aerosols (TCAs), which more than offsets the negative trends in sulfate and nitrate aerosols. This also causes the contribution of TCA to the reconstructed PM2.5 to show an increasing trend in summer over the same region. The positive trend in TCA in summer over northwestern US is stronger under the drought than the wet conditions, hence resulting in a stronger positive trend in the contribution of TCA to the reconstructed PM2.5 in the drought conditions. Drought enhances TCA over northwestern US through its impact on wildfire, and the temporal change of TCA mainly follows the non‐linear variation of SPEI. Although droughts are found to have statistically significant impacts on the trends in sulfate, nitrate, and dust in other regions and seasons, the small contributions of these species to the reconstructed PM2.5 make their trend variations not sufficient to affect trends in the reconstructed PM2.5. Plain Language Summary Ubiquitous decreasing trends in surface mass concentrations of fine aerosol particles are observed in the past ∼30 years in all seasons and regions except for the northwestern US in summer. The increasing trend in fine aerosol particle concentrations over the northwestern US in summer during 1988–2018 is primarily due to the positive trend in total carbonaceous aerosol concentrations associated with large fires in drought years. This positive trend is greater than the decreasing trends in sulfate and nitrate aerosols. As the frequency, duration, and spatial converge of droughts are expected to increase in a warming climate, the drought‐induced air pollution through wildfire cannot be overlooked and could partly offset the benefit from reduction of anthropogenic emissions. To circumvent the model deficiencies of predicting wildfires, drought index has the potential to be a predicator for forecasting long‐term trend of air pollution in future. Key Points The increase in PM2.5 in the northwestern US in summer is mainly due to the enhancement of total carbonaceous aerosols (TCAs) related to droughts Droughts affect TCA variations over the northwestern US mainly through its impact on wildfires The impacts of droughts on species concentrations in other regions and seasons are not sufficient to change PM2.5 trends

Drought Impacts on PM2.5 Composition and Amount Over the US During 1988–2018

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