Significant Decrease in Wet Deposition of Anthropogenic Chloride Across the Eastern United States, 1998-2018

Using deposition observations from precipitation samples collected by the National Atmospheric Deposition Program at 125 sites across the United States, we show that the mean wet deposition flux of non-sea-salt chloride (NSS Cl-) has decreased by 83% throughout the eastern United States between 1998 and 2018. We find that 30% of the sites switch from having excess Cl- to being depleted in Cl-. We attribute the observed decreases in NSS Cl- deposition to a 95% decrease in U.S. anthropogenic HCl emissions since 1998. We propose that industry emission controls that remove HCl as a cobenefit of NOx and SO2 have caused significant decreases in NSS Cl- deposition throughout the eastern United States, in addition to shifts from coal to natural gas and to coal with lower Cl- content. Our analysis implies that the lower tropospheric reactive inorganic chlorine burden was larger over the United States in the past than it is today.