Abstract
Chlorine is an important atmospheric photochemical oxidant, but few studies have focused on atmospheric chlorine. In this study, PM2.5 samples were collected from urban and rural sites across China in January 2018, and concentrations of Cl− and other water-soluble ions in PM2.5 were analyzed. The size-segregated aerosol Cl− data measured across Chinese cities by other studies were compiled for comparison. The observed data demonstrated that the Cl− concentrations of PM2.5 in northern cities (5.0 ± 3.7 µg/m3) were higher than those in central (1.9 ± 1.2 µg/m3) and southern cities (0.84 ± 0.54 µg/m3), suggesting substantial chlorine emissions in northern cities during winter. The concentrations of Cl− in aerosol were significantly higher in urban regions (0.11–26.7 µg/m3) compared to than in rural regions (0.03–0.61 µg/m3) across China during winter, implying strong anthropogenic chlorine emission in cities. Based on the mole ratios of Cl−/Na+, Cl−/K+ and Cl−/ SO 4 2 − and the PMF model, Cl− in northern and central cities was mainly sourced from the coal combustion and biomass burning, but in southern cities, Cl− in PM2.5 was mainly affected by the equilibrium between gas-phase HCl and particulate Cl−. The size-segregated statistical data demonstrated that particulate Cl− had a bimodal pattern, and more Cl− was distributed in the fine model than that in the coarse mode in winter, with the opposite pattern was observed in summer. This may be attributed to both sources of atmospheric Cl− and Cl− involved in chemical processes. This study reports the concentrations of aerosol Cl− on a national scale, and provides important information for modeling the global atmospheric reactive chlorine distribution and the effects of chlorine on atmospheric photochemistry.
Highlights
Chlorine is one of the most abundant halogen species in the atmosphere, where it exists as both inorganic Cl and organic Cl, and participates in many atmospheric photochemical reactions [1].Cl− -containing inorganic and organic compounds are considered to be one of the factors driving the global climate change [2]
The Cl− concentrations in PM2.5 ranged from 0.11 to 26.7 μ g m−3 over the 20 urban sites (Table 1; Figure 2a), which is consistent with previous observations of aerosol Cl− concentrations across China during winter [52,53,54,55]
The concentrations of Cl− in PM2.5 across Chinese cities in our observations were significantly higher than previous studies of aerosol Cl− during wintertime in the northeastern United States (0.06 ~ 0.23 μ g/m3) [14], in Iasi (0.32 ± 0.15 μ g/m3) [30], and in Sao Paulo
Summary
Chlorine is one of the most abundant halogen species in the atmosphere, where it exists as both inorganic Cl (such as particulate Cl− and gas-phase HCl) and organic Cl (such as CH3 Cl and chlorofluorocarbons), and participates in many atmospheric photochemical reactions [1]. Cl− -containing inorganic and organic compounds are considered to be one of the factors driving the global climate change [2]. Using the GEOS-Chem model, halogens were estimated to account for as much as ~15–27% of the global oxidation of volatile organic compounds [7]. Chlorine depletion is a common phenomenon for marine aerosol and the volatilization of gas-phase HCl from the surface of particles. The Cl− concentration in aerosol has been incidentally reported in many studies, very few studies have paid attention to Cl− in terrestrial aerosols as compared to marine aerosol, let alone in situ observations on a large spatial scale
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