Effects of NO_x on the molecular composition of secondary organic aerosol formed by the ozonolysis and photooxidation of Α-pinene

The molecular composition of secondary organic aerosol (SOA), obtained from the ozonolysis and photooxidation of α-pinene, was investigated using ultrahigh-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) in negative ion mode electrospray ionization (ESI). SOA formation was performed in an indoor smog chamber. The molecular formulae of individual species were identified on the basis of the measured ionic mass using guidelines, such as number of atoms, elemental ratios, and the nitrogen rule. In each of the SOAs obtained, 815–3501 monomeric and oligomeric (mainly dimeric) species were identified below m/z 800. From ozonolysis, mainly 95% of the typical oxygenated species (CHO) were detected, whereas from photooxidation under high NO_x conditions, 32% of nitrogen-containing species (CHON) were detected. Several common intense species (e.g., C₉H₁₄O₆, C₁₀H₁₄O₆, C₁₀H₁₆O₅, C₁₇H₂₆O₇, C₁₉H₂₈O₉, C₁₀H₁₅NO₈, and C₁₀H₁₅NO₉) could be listed as candidate tracers for the conventional tracers for α-pinene SOA. The increased percentage of CHON as a primary effect of NO_x on the SOA composition evidently affected other physicochemical parameters, such as elemental ratios (i.e., O/C, H/C, and N/C), the double-bond equivalent (DBE), the carbon oxidation state (OS_C), and the organic-mass-to-carbon ratio (OM/OC). The O/C and OM/OC for CHON were greater than those observed for CHO, indicating that nitrogen preferentially exists in the oxidized form (e.g., -ONO₂). The complexity of oligomerization was observed in DBE and OM/OC according to the number of carbon atoms.