Significant decrease in wet deposition of anthropogenic chloride across the Eastern U.S., 1998-2018

Abstract:

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 an average of 83% in the last two decades at sites with significant trends throughout the Eastern United States. In the eastern US, wet deposition of NSS Cl^- decreased from an average +0.12 kg ha^-1 (1998-2000) to -0.06 kg ha^-1 (2016-2018) with 30% of sites switching from having excess Cl^- relative to Na⁺ deposited, to being depleted in Cl^- over this 21-year span. We show that these decreases in NSS Cl^- wet deposition are spatially correlated with sulfate deposition decreases, and temporally correlated with anthropogenic HCl emissions decreases. By combining observed trends in sulfate wet deposition with the HCl/SO₂ anthropogenic emission ratio, we attribute the observed decreases in NSS Cl^- deposition in the eastern US to changes in U.S. anthropogenic HCl emissions, which have decreased by 95% since 1998. We propose that industry emissions controls that remove HCl as a co-benefit of NO_x and SO₂ have caused significant decreases in NSS Cl^- deposition throughout the Eastern U.S. in the past 2 decades. In the Southeastern United States, we see evidence of weakened depletion of Cl^- relative to Na⁺ (and hence an increase in NSS Cl^- wet deposition) over the 1998-2018 period, suggesting decreased mobilization of Cl^- from sea salt aerosol because of decreasing NO_x and SO₂ concentrations. Our analysis implies that the lower tropospheric reactive inorganic chlorine burden was distinctly larger over the U.S. in the past than it is today, both from higher direct HCl emissions and, perhaps, larger mobilization of sea salt Cl^- via acid displacement and heterogenous reactions.