Key role of continental inorganic halogens in the evolution of global air quality

Halogen (chlorine, bromine, and iodine) species actively interact with other atmospheric constituents such as nitrogen oxides, organic gases, and ozone. Previous field observations confirmed the ubiquitous existence of halogens in various environments and modeling simulations quantified halogen impacts on air pollutants at present time. However, the abundance and impacts of continental inorganic halogens (CIH) on global air quality throughout history remain unexplored. Here, we present a global inventory of CIH emissions from anthropogenic and biomass-burning sources from 1970 to 2015, with average fluxes of 4302, 207, and 98 kt/yr for chlorine, bromine, and iodine, respectively, representing ~15%, 5-10%, and ~3% of the corresponding global short-lived chlorine, bromine, and iodine emissions. Incorporating these emissions into a global chemistry-climate model reveals a substantial increase (> 50%) in inorganic halogen levels over continents, helping to reproduce the observed halogen levels. Our results also suggest substantial effects of CIH on air pollutant concentrations with significant spatio-temporal variations, e.g., the largest perturbations in nitrogen oxides and secondary aerosols migrating from Europe and North America in the 1980s-1990s, to East Asia in the 2000s, then to South Asia in the 2010s, suggesting a movement of CIH-associated air quality impact hotspots from the developed regions to the developing areas. This study highlights the under-appreciated role of halogen chemistry in air quality and its evolution in the recent four decades and calls for attention to the potential CIH impacts in emerging regions where field observations are not yet available.