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

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.

Automated summary

This study explores the association between spatial-temporal variations of surface PM2.5 species and droughts using PM2.5 concentrations from Interagency Monitoring of Protected Visual Environments and Standardized Precipitation Evapotranspiration Index (SPEI) data. The study reveals an increasing trend in summer PM2.5 over the northwestern US, driven primarily by total carbonaceous aerosols (TCAs). Drought conditions enhance TCAs and their contribution to PM2.5. Although droughts have significant impacts on sulfate, nitrate, and dust trends in other regions and seasons, their contributions to PM2.5 are relatively small.