Chlorine is an important atmospheric photochemical oxidant, but few studies have focused on atmospheric chlorine. In this study, PM 2.5 samples were collected from urban and rural sites across China in January 2018, and concentrations of Cl − and other water-soluble ions in PM 2.5 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 PM 2.5 in northern cities (5.0 ± 3.7 µg/m 3 ) were higher than those in central (1.9 ± 1.2 µg/m 3 ) and southern cities (0.84 ± 0.54 µg/m 3 ), 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/m 3 ) compared to than in rural regions (0.03-0.61 µg/m 3 ) across China during winter, implying strong anthropogenic chlorine emission in cities. Based on the mole ratios of Cl − /Na + , Cl − /K + and Cl − /SO 2− 4 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 PM 2.5 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.Atmosphere 2019, 10, 505 2 of 16 may disturb the aerosol acidity [11][12][13][14], which further affects the water-soluble ion concentrations aerosol [15,16]. Although 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.The sources of atmospheric Cl − have been identified using several different methods, such as determining stable isotopic compositions of Cl − [17][18][19], monitoring the relationships between Cl − and OC, EC, Na + , and K + [16,[20][21][22], and model simulations [23,24]. Atmospheric Cl − has many natural sources, including sea water, wildfires, dust storms, and volcanic eruptions [25][26][27][28], as well as anthropogenic sources, such as coal combustion, biomass burning, industrial emissions, and the use of sodium chloride on icy roads [20,21,[29][30][31][32][33][34][35]. The chlorine emissions from sea water (both particulate Cl − and gas-phase HCl) were estimated to be 1792.6 Tg Cl yr −1 [23], which is considerably higher than emissions from other sources, such as dust (15 Tg Cl yr −1 as particulate Cl − ) and volcanic eruption...
