Impact and pathway of halogens on atmospheric oxidants in coastal city clusters in the Yangtze River Delta region in China

Halogens (chlorine, bromine, and iodine) are known to profoundly influence atmospheric oxidants (hydroxyl radical (OH), hydroperoxyl radical (HO2), ozone (O3), and nitrate radical (NO3)) in the troposphere and subsequently affecting air quality. However, their impact on atmospheric oxidation and air pollution in coastal areas in China is poorly characterized. In this study, we use the WRF-CMAQ (Weather Research and ForecastingCommunity Multiscale Air Quality) model with full halogen chemistry and process analysis to assess the influences and pathways of halogens on atmospheric oxidants in the Yangtze River Delta (YRD) region, a typical coastal city cluster in China. Halogens cause the annual OH radical increase by up to 16.4% and NO3 decrease by up to 45.3%. O3 increases by 2.0% in the YRD but decreases by 3.3% in marine environment. Halogen induced changes in atmospheric oxidants lead to a general increase of atmospheric oxidation capacity by 5.1% (maximum 48.4%). The production rate of OH (POH) in the YRD is enhanced by anthropogenic chlorine through both increased HO2 pathway and hypohalous acid photolysis pathway, while POH over ocean is enhanced by oceanic halogens through converting HO2 into hypohalous acid. Anthropogenic chlorine enhances both O3 and NO3 production (PNO3) rates through influencing their precursors while oceanic halogens reduce PNO3 and directly destroy ozone. Iodine contributed most (on average of 91% in oceanic halogens) in reducing production rates of oxidants. Thus, halogen emissions and potential effects of halogens on air quality need to be considered in air quality policies and regulations in the YRD region.