This work presents a systematic investigation of the molecular level composition and the extent of aqueous photochemical processing in different types of secondary organic aerosol (SOA) from biogenic and anthropogenic precursors including α-pinene, β-pinene, β-myrcene, d-limonene, α-humulene, 1,3,5-trimethylbenzene, and guaiacol, oxidized by ozone (to simulate a remote atmosphere) or by OH in the presence of NOx (to simulate an urban atmosphere). Chamber- and flow-tube-generated SOA samples were collected, extracted in a methanol/water solution, and photolyzed for 1 h under identical irradiation conditions. In these experiments, the irradiation was equivalent to about 3-8 h of exposure to the sun in its zenith. The molecular level composition of the dissolved SOA was probed before and after photolysis with direct-infusion electrospray ionization high-resolution mass spectrometry (ESI-HR-MS). The mass spectra of unphotolyzed SOA generated by ozone oxidation of monoterpenes showed qualitatively similar features and contained largely overlapping subsets of identified compounds. The mass spectra of OH/NOx-generated SOA had more unique visual appearance and indicated a lower extent of product overlap. Furthermore, the fraction of nitrogen-containing species (organonitrates and nitroaromatics) was highly sensitive to the SOA precursor. These observations suggest that attribution of high-resolution mass spectra in field SOA samples to specific SOA precursors should be more straightforward under OH/NOx oxidation conditions compared to the ozone-driven oxidation. Comparison of the SOA constituents before and after photolysis showed the tendency to reduce the average number of atoms in the SOA compounds without a significant effect on the overall O/C and H/C ratios. SOA prepared by OH/NOx photooxidation of 1,3,5-trimethylbenzene and guaiacol were more resilient to photolysis despite being the most light-absorbing. The composition of SOA prepared by ozonolysis of monoterpenes changed more significantly as a result of the photolysis. The results indicate that aqueous photolysis of dissolved SOA compounds in cloud/fog water can occur in various types of SOA, and on atmospherically relevant time scales. However, the extent of the photolysis-driven change in molecular composition depends on the specific type of SOA.

Formation of secondary organic aerosols (SOA) from biogenic volatile organic compounds (BVOC) occurs via O- and OH-initiated reactions during the day and reactions with NO during the night. We explored the effect of these three oxidation conditions on the molecular composition and aqueous photochemistry of model SOA prepared from two common BVOC. A common monoterpene, α-pinene, and sesquiterpene, α-humulene, were used to form SOA in a smog chamber via BVOC + O, BVOC + NO, and BVOC + OH + NO oxidation. Samples of SOA were collected on filters, water-soluble compounds from SOA were extracted in water, and the resulting aqueous solutions were photolyzed to simulate the photochemical aqueous processing of SOA. The extent of change in the molecular level composition of SOA over 4 h of photolysis (approximately equivalent to 64 h of photolysis under ambient conditions) was assessed with high-resolution electrospray ionization mass spectrometry. The analysis revealed significant differences in the molecular composition between SOA formed by the different oxidation pathways. The composition further evolved during photolysis with the most notable change corresponding to the nearly complete removal of nitrogen-containing organic compounds. Hydrolysis of SOA compounds also occurred in parallel with photolysis. The preferential loss of larger SOA compounds during photolysis and hydrolysis made the SOA compounds more volatile on average. This study suggests that aqueous processes may under certain conditions lead to a reduction in the SOA loading as opposed to an increase in SOA loading commonly assumed in the literature.

Mass absorption coefficient (MAC) values were measured for secondary organic aerosol (SOA) samples produced by flow tube ozonolysis and smog chamber photooxidation of a wide range of volatile organic compounds (VOC), specifically: α-pinene, β-pinene, βmyrcene, d-limonene, farnesene, guaiacol, imidazole, isoprene, linalool, ocimene, p-xylene, 1-methylpyrrole, and 2-methylpyrrole. Both low-NO x and high-NO x conditions were employed during the chamber photooxidation experiments. MAC values were converted into effective molecular absorption cross sections assuming an average molecular weight of 300 g/mol for SOA compounds. The upper limits for the effective photolysis rates of SOA compounds were calculated by assuming unity photolysis quantum yields and convoluting the absorption cross sections with a time-dependent solar spectral flux. A more realistic estimate for the photolysis rates relying on the quantum yield of acetone was also obtained. The results show that condensed-phase photolysis of SOA compounds can potentially occur with effective lifetimes ranging from minutes to days, suggesting that photolysis is an efficient and largely overlooked mechanism of SOA aging.

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Biomass burning emissions have substantially increased with continued warming and drying in the southwestern U.S., impacting air quality and atmospheric processes. To better quantify impacts of biomass burning aerosols, an extensive laboratory study of fresh smoke emissions was conducted at Los Alamos National Laboratory. Laboratory burn experiments with selected native and invasive southwestern U.S. fuels were used to elucidate the role of fuel type, chemical composition, and ignition method on the hygroscopicity of smoke. Here we focus on a custom controlled relative humidity (RH) nephelometry system using the direct measurement of aerosol light scattering with two nephelometers—one at dry conditions and one at a controlled high RH (RH ~ 85%). Aerosol hygroscopicity was highly variable with the enhancement in light scattering coefficient in the range of 1.02 < f(RH = 85%) < 2.1 and corresponding to the kappa parameter (κneph) ranging from ~0 to 0.18. Hygroscopicity is determined primarily by the fuel's inorganic ion content. For example, invasive halophytes with high inorganic salt content exhibit much greater water uptake than native coniferous species with low inorganic content. Combustion temperature and phase, flaming or smoldering, play a secondary role in the water uptake of smoke. High‐temperature ignition methods create flaming conditions that enhance hygroscopicity while lower‐temperature smoldering conditions diminish hygroscopicity. Our results construct an empirical relation between κneph and the inorganic content of the fuel and smoke to predict water uptake.