Urban particulate water-soluble organic matter in winter: Size-resolved molecular characterization, role of the S-containing compounds on haze formation

Abstract

Water-soluble organic matter (WSOM), as a group of ubiquitous components in atmospheric PM, plays a crucial role in global climate change and carbon cycle. In this study, the size-resolved molecular characterization of WSOM in the range of 0.010–18 μm PM was studied to gain insights into their formation processes. The CHO, CHNO, CHOS, CHNOS compounds were identified by the ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry in ESI source mode. A bimodal pattern of the PM mass concentrations was found in the accumulation and coarse mode. The increasing mass concentration of PM was mainly attributed to the growth of large-size PM with the occurrence of haze. Both Aiken-mode (70.5–75.6 %) and coarse-mode (81.7–87.9 %) particles were proven the main carriers of the CHO compounds, the majority of which were indicated to be the saturated fatty acids and their oxidized derivatives. The S-containing (CHOS and CHNOS) compounds in accumulation-mode (71.5–80.9 %) increased significantly in hazy days, where organosulfates (C11H20O6S, C12H22O7S) and nitrooxy-organosulfates (C9H19NO8S, C9H17NO8S) were confirmed in majority. The S-containing compounds in accumulation-mode particle with high oxygen content (6–8 oxygen atoms), unsaturation degree (DBE < 4), and reactivity could facilitate the particle agglomeration and accelerate the haze formation.