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

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 aerosolsare 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 eight thousand OSsin wintertime aerosols in Beijing. We found that both the number and signal magnitudesof 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 semi- to low-volatility precursors (e.g., polycyclic aromatic hydrocarbons and fatty acids) and have showed a strong influence of anthropogenic emissions.The molecular corridor analysissuggests that the high abundance of HMWaromatic-like and aliphatic OSsconsiderably decreases the volatility of organic aerosols in the urban atmosphere.