Molecular Characterization Of Organic Compounds In PM_2.5 By High Resolution Mass Spectrometry

The molecular composition of organic aerosols is very complex.Only a small fraction of the organic mass in aerosols has been identified by using conventional analysis methods.The composition of some organic compounds in aerosols with high molecular weight,low volatility,high polarity,as well as their formation mechanism are still not clear.In this paper,molecular composition analytical methods of organic aerosols were established by using Fourier transform ion cyclotron resonance mass spectrometry?FTICR MS?combining with solvent extraction,solid phase extraction,high performance liquid chromatography?HPLC?and other separation methods.The methods were used for detailed characterization of molecular composition of the Beijing urban organic aerosols.Analytical methods based on FT-ICR MS for the molecular composition analysis of organic compounds in fine particle matters(PM_2.5,particles with aerodynamic diameter ?2.5 ?m)were established.The applicability of different ionization sources for various classes of compounds and the selective extraction of different solvents were investigated.The establishment of analytical methods were based on extraction conditions,the ionization source type and operating conditions for different classes of compounds.Aromatic fractions derived from aerosol samples were characterized by gas chromatography and mass spectrometry?GC-MS?,high temperature simulated distillation?SIMDIS?,and positive-ion atmospheric pressure photoionization?APPI?FTICR MS.Highly condensed hydrocarbons with 6 or more aromatic rings as well as their corresponding S,N,O hetero-atom compounds were found accounting for a large portion of total PAHs in aerosols,which were neglected by conventional analysis.The findings implies that the environmental toxicity of aerosol PAHs could be underestimated.Molecule-level characterization of organic compounds of ambient aerosols collected on hazy and clear days in Beijing was performed by FT-ICR MS combining with different ionization sources.With ultra-high mass resolution and mass accuracy of the FT-ICR MS,many different types of hydrocarbons and heteroatom-containing?oxygen,nitrogen and sulfur?compounds were characterized at molecular level.The results indicated that the molecular composition of organic aerosols on hazy days was far more complex than that on clear days.More organic compounds with higher molecular condensation and oxidation degree were found on hazy days.Abundant organosulfates?OS?and nitrooxyorganosulfates?nitrooxy OS?were found as the most obvious characteristics of hazy days.A methodology of separation and enrichment of organosulfates in PM_2.5 was developed by HPLC combined with solvent extraction and solid phase extraction.Abundant aromatic organosulfates with high degree of condensation and aliphatic organosulfates with low-degree of oxidation were found as a major feature of atmospheric aerosols in Beijing.The comparative study suggests that the formation of organic aerosols in Beijing is strongly related with fossil fuel combustion.This work developed analytical methods for molecular composition analysis of organic compounds in aerosols.Chemical composition of organic aerosols in Beijing were analyzed at molecular level.Compositional characteristics and source analysis of characteristic compounds like PAHs and organosulfates were discussed.The new findings would be significant for providing analytical methods and a theoretical basis for haze formation mechanism,source apportionment of pollutants and fossil fuels combustion to environmental pollution.