We present a new chemical mechanism for Hg0/HgI/HgII atmospheric cycling, including recent laboratory and computational data, and implement it in the GEOS-Chem global atmospheric chemistry model for comparison to observations. Our mechanism includes the oxidation of Hg0 by Br and OH, subsequent oxidation of HgI by ozone and radicals, respeciation of HgII in aerosols and cloud droplets, and speciated HgII photolysis in the gas and aqueous phases. The tropospheric Hg lifetime against deposition in the model is 5.5 months, consistent with observational constraints. The model reproduces the observed global surface Hg0 concentrations and HgII wet deposition fluxes. Br and OH make comparable contributions to global net oxidation of Hg0 to HgII. Ozone is the principal HgI oxidant, enabling the efficient oxidation of Hg0 to HgII by OH. BrHgIIOH and HgII(OH)2, the initial HgII products of Hg0 oxidation, respeciate in aerosols and clouds to organic and inorganic complexes, and volatilize to photostable forms. Reduction of HgII to Hg0 takes place largely through photolysis of aqueous HgII–organic complexes. 71% of model HgII deposition is to the oceans. Major uncertainties for atmospheric Hg chemistry modeling include Br concentrations, stability and reactions of HgI, and speciation and photoreduction of HgII in aerosols and clouds.

Ozone (O) pollution is becoming increasingly severe in China. In addition, our limited understanding of the relationship between O and volatile organic compounds (VOCs), is an obstacle to improving air quality. By developing an improved source-oriented speciated VOC emission inventory in 2013, we estimated the ozone formation potential (OFP) and investigated its characteristics in China. Besides, a comparison was made between our estimates and space-based observations from the ozone monitoring instrument (OMI) on the National Aeronautics and Space Administration (NASA)'s Aura satellite. According to our estimates, m-/p-xylene, ethylene, formaldehyde, toluene, and propene were the five species that had the largest potential to form ozone, and on-road vehicles, industrial processes, biofuel combustion, and surface coating were the key contributing sectors. Among different regions of China, the North China Plain, Yangtze River Delta, and Pearl River Delta had the highest OFP values. Our results suggest that O formation is VOC-limited in major urban areas of China. Additionally, considering the different photochemical reactivities of various VOC species and the disparate energy and industry structures in the different regions of China, more efficient OFP-based and localized VOC control measures should be implemented, instead of the current mass-based and nationally uniform policies.

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