Impacts of anthropogenic emissions on tropospheric reactive chlorine: Implications for Greenland ice core records of chlorine

Abstract:

Anthropogenic activity may have influenced tropospheric reactive chlorine abundance, with potential implications for Arctic ice core records of chlorine since the Industrial Revolution. Anthropogenic impacts on chlorine include direct emissions of HCl from combustion and the mobilization of HCl from sea salt aerosol (SSA), which is driven by atmospheric acidity. Greenland ice core chlorine may also be influenced by the decline of Arctic sea ice extent (SIE) in the late 20^th century, since SIE controls the magnitude of open ocean and blowing snow source of SSA. Greenland ice core records suggest that anthropogenic emissions of acidic gases (SO₂, NO_x) started to increase in the 1850s due to combustion and declined or leveled off after the 1970s due to air pollutant mitigation policies in North America and Europe. Reduction in chlorine deposition in the past 20 years in the US indicates that the pollution mitigation technology also reduces HCl emissions. We use historical simulations from a global chemical transport model (GEOS-Chem), two new ice core records from Summit, Greenland, and ice core records from coastal Arctic locations (SE-Dome and Akademii Nauk) to investigate the impacts of anthropogenic emissions on ice core chlorine records, and the implications for tropospheric reactive chlorine. Summit chlorine records increased starting from ~1940, peaked in the 1970s, and declined since then. In contrast, no significant trends have been found at the coastal sites. Strong relationships between chlorine and sodium in coastal Arctic locations suggest that the variation of sea salt emissions dominate chlorine records there, while relationships between acidity and non-sea salt chlorine (nssCl) suggest that anthropogenic emissions impact ice cores from inland Greenland. Model results show that the increase of HCl since pre-industrial time is dominated by acidity-driven mobilization of HCl from accumulation mode sea salt aerosol, with a small but important contribution from direct emissions of HCl from combustion. Model results show that anthropogenic emissions can explain the increasing trends of nssCl from pre-industrial time to the 1970s at Summit, Greenland, but the decreasing trends afterwards cannot be fully explained by the model, suggesting other influences such as decline in Arctic sea ice extent.