Halogen chemistry and implications for air quality and climate

Reactive halogen chemistry significantly impacts air quality and climate by influencing the levels of methane, ozone, and aerosols. In the lower atmosphere, reactive halogen species (Cl, Br, and I) are typically present in very low concentrations, making them challenging to detect and study. Over the past two decades, advancements in chemical ionization mass spectrometry have led to an increasing number of studies measuring these species in both field and laboratory settings to better understand their abundances and formation mechanisms. Global models have highlighted the extensive environmental impacts of reactive halogen chemistry. This talk will explore recent field measurements, laboratory experiments, and modeling of reactive halogen species. In particular, high levels of reactive chlorine species have been observed in Hong Kong and Mainland China, primarily due to anthropogenic activities such as coal combustion, agricultural fires, and waste incineration. Current atmospheric chemistry models struggle to accurately simulate reactive chlorine species in China. We propose that iron photochemistry on aerosols could efficiently release molecular chlorine gas, which photolyzes into chlorine atoms, oxidizing volatile organic compounds to produce secondary organic aerosols and ozone. The uptake of ozone by chloride-containing aerosols may also be a significant source of molecular chlorine gas. Additionally, our recent research highlights a substantial contribution of reactive halogen species to sulfate aerosol formation in Hong Kong.