Anthropogenic Impacts on Tropospheric Reactive Chlorine Since the Preindustrial

Tropospheric reactive gaseous chlorine (Cl_y) impacts the atmosphere's oxidation capacity with implications for chemically reduced gases such as methane. Here we use Greenland ice-core records of chlorine, sodium, and acidity, and global model simulations to show how tropospheric Cl_y has been impacted by anthropogenic emissions since the 1940s. We show that anthropogenic contribution of non-sea-salt chlorine significantly influenced total chlorine and its trends after the 1940s. The modeled regional 170% Cl_y increase from preindustrial to the 1970s was driven by acid displacement from sea-salt-aerosol, direct emission of hydrochloric acid (HCl) from combustion, and chemical reactions driven by anthropogenic nitrogen oxide (NO_x) emissions. Since the 1970s, the modeled 6% Cl_y decrease was caused mainly by reduced anthropogenic HCl emissions from air pollution mitigation policies. Our findings suggest that anthropogenic emissions of acidic gases and their emission control strategies have substantial impacts on Cl_y with implications for tropospheric oxidants, methane, and mercury.