Molecular Characteristics of Atmospheric Organosulfates During Summer and Winter Seasons in Two Cities of Southern and Northern China

Organosulfates in atmospheric PM_2.5 were investigated at two urban sites in Hefei and Shijiazhuang of southern and northern China during the summer and winter seasons of 2019–2020. Organic molecular composition was characterized using ultrahigh-performance liquid chromatography coupled with Orbitrap mass spectrometry in negative electrospray ionization mode. There were overall 2,516–6,190 and 2,417–3,474 organic molecular formulas assigned on individual days in Hefei and Shijiazhuang, respectively, among which approximately 25%–41% were sulfur-containing CHOS and CHONS compounds. The number of total organosulfates (including nitrooxy-organosulfates) accounted for on average 84% and 79% of sulfur-containing compounds during summer and winter in Shijiazhuang, which were much higher than those of 36% and 57% in Hefei. The higher fractions of organosulfates in the northern city of Shijiazhuang might be closely associated with the higher prevalence of SO₂ and particulate sulfate. More than 96% in the peak areas of total organosulfates were present in aliphatic molecular structures instead of aromatics in both cities, suggesting the substantial contributions from biogenic precursors and anthropogenic long-chain alkanes. The positive correlations (r: 0.33–0.96) of inorganic sulfate with the peak area abundance of organosulfates derived from isoprene and monoterpenes revealed the significant role of sulfate particles in the production of organosulfates. Particle acidity also correlated positively with these species during the winter in Hefei and summer in Shijiazhuang, yet negatively during other periods, indicating that additional reaction pathways would compete with acid-catalyzed chemistry under atmospheric conditions limited by particle acidity or hydrocarbon precursors.