Molecular composition of urban organic aerosols on clear and hazy days in Beijing: a comparative study using FT-ICR MS

Haze has frequently affected many cities and threatened human health in China. Detailed knowledge of the chemical composition of secondary organic aerosol provides fundamental information in the study of the formation mechanism of haze and its adverse effects on human health. In our work, dichloromethane and water extracts of ambient aerosols collected on hazy and clear days in Beijing were characterised by negative-ion electrospray ionisation and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Formulae in four elemental compositional groups, namely CHOS, CHONS, CHO and CHON, were identified relying on the ultrahigh resolution and mass accuracy of FT-ICR MS. Significantly more compounds were detected and the peaks were much more intense in the hazy day samples, especially for the CHOS, CHONS and CHON formula groups. Organosulfates (OS) and nitrooxy-organosulfates (nitrooxy OS) were the major forms of CHOS and CHONS formulae respectively, and their numbers more than tripled on the hazy days. Under the severely polluted conditions in Beijing, the compositional distribution of the OS and nitrooxy OS exhibited distinct features such as intense peaks of low double-bond equivalent (DBE) (DBE = 0, 1 for OS and DBE = 1, 2 for nitrooxy OS) and low degree oxidation, of medium DBE (DBE = 2, 3 for OS and DBE = 3, 4 for nitrooxy OS), and of high DBE (DBE >= 4 for OS and DBE >= 5 for nitrooxy OS). The likely respective candidates for these could be aliphatic OS having a low degree of oxidation, biogenic OS and many aromatics and polycyclic aromatic hydrocarbon (PAH)derived OS. The CHON formulae observed on hazy days were double those on clear days and had higher DBE values and larger O/N ratios. Slightly more CHO compounds were detected in the hazy-day samples and they had higher DBE values and more oxygen atoms. The comparative study suggests that oxidation, sulfation and nitrification processes actively involve precursors of anthropogenic origin in the Beijing polluted urban atmosphere.