This study investigates the impacts of the 2011 severe drought in the southern United States on ground-level fine aerosol (PM 2.5 ) concentrations in the summer. The changes in surface concentrations and planetary boundary layer (PBL) budget of PM 2.5 between June 2010 (near-normal rainfall) and June 2011 (severe drought) are quantified using surface observations and the GEOS-Chem model. Observations show an average enhancement of 26% (p , 10 24 ) in total PM 2.5 over the southern U.S. (SUS)

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... ion during the drought, which is largely attributed to a ;120% increase in organic carbon (OC). Over Texas (TX) under extreme drought conditions, surface PM 2.5 shows a mean decrease of 10.7% (p , 0.15), which is mainly driven by a decrease of 26% (p , 0.03) in sulfate. Model simulations reproduce the observed relative changes in total PM 2.5 , OC, and sulfate during the drought. The model correctly identifies OC as the major contributor to the overall PM 2.5 increase over SUS and sulfate as the key driver of the PM 2.5 decrease over TX. Budget analysis suggests that increased OC emissions from wildfires (158 kt C month 21 ), enhanced SOA production (11.1 kt C month 21 ), and transboundary inflow from Mexico (18.6 kt C month 21 ) are major contributors to the increase in atmospheric OC contents over SUS. Over TX, a 70% decrease of aqueous-phase oxidation of sulfate, driven by decreasing low clouds, outweighs the combined effects of reduced wet deposition and decreased outflow as the key driver of sulfate decrease both at the surface and within the PBL.