Atmospheric Chlorine Wet Deposition in China: National Decreases but Localized Increases in Urban Agglomerations

Chlorine (Cl) is a critical halogen and acidic component that influences atmospheric oxidation capacity, air quality, and acidifies ecosystems. Emission reduction measures in China are expected to similarly reduce Cl deposition as with sulfur (S). However, large knowledge gaps remain regarding the trends and drivers of China's Cl deposition. By integrating data from an observational network and published literature, we developed the first national dataset of Cl wet deposition in China from 1990 to 2020. This dataset revealed that total Cl^- and nonsea-salt (NSS) Cl^- deposition initially increased, reaching an inflection point around 2005, and then decreased, with an overall reduction of approximately 52%. As anticipated, Cl and S deposition decreased synchronously nationwide, attributable to reduced precursor emissions from energy structure adjustments and emission reduction technologies. However, in China's three major urban agglomerations, Cl did not decrease in parallel with S and even increased in the Pearl River Delta at a rate of +8% yr^-1. This mainly stems from rising waste incineration emissions driven by rapid economic and urban development, population and consumption expansion, which offset the mitigation achieved from coal combustion. Our study underscores the effectiveness of emission reduction measures, yet targeted control of emerging anthropogenic Cl pollution sources in urban hotspot areas is urgently needed to achieve regional sustainable development.