Molecular characterization of atmospheric water-soluble organic carbon in contrasting remote, rural, and urban environments in northern China: Comparison of normal and dust days

   Atmospheric water-soluble organic carbon (WSOC) is a critical component of airborne particulates, plays an important role in Earth’s energy balance, air quality, and human health. Until now, molecular composition and potential sources of WSOC in non-urban areas under different weather conditions are poorly understood in China. In this study, aerosol samples were collected at three sites in northern China representing remote (Erenhot), rural (Zhangbei), and urban (Jinan) environments during springtime. The WSOC components were analyzed by high-performance liquid chromatography coupled with high-resolution mass spectrometry. During normal days, the results showed that the numbers of assigned formulas of WSOC present the trend of urban > rural > remote sites. The CHO compounds were the most abundant formula category, followed by the CHON compounds at all three sites. The CHO compounds detected at the remote site Erenhot were dominated by oxidized unsaturated organic compounds and biomass-burning-related organic aerosol. The CHON compounds are majorly low-oxygenated-aliphatic species. All these results indicate primary anthropogenic emissions are significant sources at the remote site. However, higher contributions of highly oxygenated CHO and CHON compounds were observed at the site of Jinan, reflecting more insensitive secondary oxidation processes in the urban area. Reduced sulfur-containing species from the combustion of coal or diesel were abundant at Erenhot and Zhangbei, while the aliphatic organosulfates likely from traffic emission and nitrooxy-organosulfates from biogenetic sources were dominant at Jinan. Dust storms significantly changed the molecular composition of WSOC at Erenhot and Zhangbei. The dust-bounded WSOC was dominated by lignin-like species from plant debris, such as flavonoids. This type of organic species presents high volatility and viscosity and is likely an efficient ice-nucleating substance.