The role of functional groups in the understanding of secondary organic aerosol formation mechanism from α-pinene

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The infrared spectra (IR) analysis in combination with electrospray ionization high-resolution orbitrap mass spectra (ESI-HRMS) can provide new insight into the overall structural feature and specific molecules of secondary organic aerosol (SOA). In this study, the functional group signature of SOA produced from OH and O3 channel oxidation of α-pinene is characterized based on the IR and ESI-HRMS. The IR spectra of SOA from the OH channel show strong absorptions of hydrogen bonded OH groups and weak absorptions of CO groups, while the absorptions of CO are more abundant than OH in the O3 channel. A linear relationship between the ratio of functional group absorption area (SO−H/SC=O) and the group number ratio of nO−H/nC=O is obtained. The ratios of nO−H/nC=O in the O3 and H2O2 systems of SOA are estimated to be 0.60 and 3.91, respectively. The ESI-HRMS results show that organic acids are the major products in both the O3 and NO2 systems. In contrast to the O3 channel, alcohols are more abundant from the OH channel. The major compounds of SOA from the H2O2 system are confirmed to be formed by autoxidation of first generation RO2 radicals. The nO−H/nC=O ratio obtained by IR is in good agreement with that by MS. Thus, the ratio of nO−H/nC=O can be used to characterize SOA formation from different oxidation channels. In α-pinene-NO2 irradiations, the ratio of nO−H/nC=O is 0.83, which is quite close to that from the O3 system, but totally different from that in the H2O2 system. This strongly supports that the O3 channel plays a key role in the formation of SOA from the α-pinene-NO2 system. The similarity of both products and the nO−H/nC=O ratios between the α-pinene-O3 and α-pinene-NO2 systems strongly states that a stabilized Criegee intermediate (SCI) is a key factor controlling SOA formation.