Molecular characterization of atmospheric particulate organosulfates in three megacities at the middle and lower reaches of the Yangtze River

Abstract. PM_2.5 filter samples have been collected in three megacities at the middle and lower reaches of the Yangtze River: Wuhan (WH), Nanjing (NJ), and Shanghai (SH). The samples were analyzed using ultra-high-performance liquid chromatography (UHPLC) coupled with Orbitrap mass spectrometry (MS), which allowed for detection of about 200 formulas of particulate organosulfates (OSs), including dozens of formulas of nitrooxy-organosulfates, with various numbers of isomers for each tentatively determined formula at each location. The number of aliphatic OS formulas represented more than 78 % of the detected OSs at the three locations, while aromatic OSs were much less numerous. OSs with two to four isomers accounted for about 50 % of the total OSs on average in these megacity samples, and the percentage of OSs with six and more isomers in the WH sample was more significant than those in the SH and NJ samples. Additionally, the molecular formula, average molecular weight, and degrees of oxidation and unsaturation of tentatively assigned OSs were compared. The results indicate that the OSs between NJ and SH shared higher similarity, and the characteristics of OSs in SH varied diurnally and seasonally. OSs derived from isoprene, monoterpenes, and sesquiterpenes were abundant in samples from the three megacities and could be produced through both daytime photochemistry and NO₃ night-time chemistry. The reaction pathways leading to isoprene-derived OSs probably varied in those locations because of the different NO_x levels. In addition, a number of OSs that might be formed from polycyclic aromatic hydrocarbons were also detected, which underlies the importance of anthropogenic sources for this class of compounds.