Molecular Composition and Photochemical Evolution of Water Soluble Organic Carbon (WSOC) Extracted from Field Biomass Burning Aerosols using High Resolution Mass Spectrometry

Abstract. Photochemistry plays an important role in the evolution of atmospheric water soluble organic carbon (WSOC), which dissolves into clouds, fogs and aerosol liquid water. In this study, we examined the molecular composition and evolution of a WSOC mixture extracted from fresh biomass burning aerosols upon photolysis, using direct infusion electrospray ionization high-resolution mass spectrometry (ESI-HRMS) and liquid chromatography coupled with mass spectrometry (LC/ESI-HRMS). For comparison, two typical phenolic compounds (i.e., phenol and guaiacol) emitted from lignin pyrolysis in combination with hydrogen peroxide (H2O2) as a typical OH radical precursor, were exposed to simulated sunlight irradiation. The photochemistry of both, the phenols (photo-oxidation) and WSOC mixture (direct photolysis) can produce a series of highly oxygenated compounds which in turn increases the degree of oxidation of organic composition and acidity of the bulk solution. In particular, the LC/ESI-HRMS technique revealed significant photochemical evolution on the WSOC composition, e.g., the photodegradation of low oxygenated species and the formation of highly oxygenated products. We also tentatively compared the mass spectra of photolytic time-profile extract with each other for a more comprehensive description of the photolytic evolution. The calculated average oxygen-to-carbon (O / C) ratios of oxygenated compounds in bulk extract increases from 0.38 ± 0.02 to 0.44 ± 0.02 (mean±standard deviation) while the intensity (S / N)-weighted average O / C (O / Cw) increases from 0.45 ± 0.03 to 0.53 ± 0.06 as the time of irradiation extends from 0 to 12 h. These findings indicate that the water soluble organic fraction of fresh combustion-derived aerosols have the potential to form more oxidized organic matter, accounting for the highly oxygenated nature of atmospheric organic aerosols.