Increase of High Molecular Weight Organosulfate With Intensifying Urban Air Pollution in the Megacity Beijing

Abstract Organosulfates (OSs), a key component of secondary organic aerosols (SOA), account for up to one third of organic matter in the atmosphere. However, high molecular weight (HMW, 500–800 Da) OSs in ambient aerosols are poorly characterized at a molecular level, due to experimental difficulties. With Fourier transform‐ion cyclotron resonance mass spectrometry (FT‐ICRMS), we are able to identify more than 8,000 OSs in wintertime aerosols in Beijing. We found that both the number and signal magnitudes of HMW OSs with low H/C and O/C ratios and degrees of unsaturation were greatly enhanced during hazy days, indicating that most HMW OSs were freshly formed during stagnant air pollution episodes. They are most likely to be the oxidation products of semivolatility to low‐volatility precursors (e.g., polycyclic aromatic hydrocarbons and fatty acids) and have showed a strong influence of anthropogenic emissions. The molecular corridor analysis suggests that the high abundance of HMW aromatic‐like and aliphatic OSs considerably decreases the volatility of organic aerosols in the urban atmosphere. Key Points Large numbers of high molecular weight organosulfates with low volatility are produced in urban aerosols with increased air pollution Numerous aliphatic‐ and aromatic‐like OSs are potentially generated from anthropogenic precursors affected by urban emissions The high abundance of high molecular weight OSs decreases the volatility of urban organic aerosols