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Formation of secondary organic aerosols (SOA) from biogenic volatile organic compounds (BVOC) occurs via O3- and OH-initiated reactions during the day and reactions with NO3 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 + O3, BVOC + NO3, and BVOC + OH + NOx 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. [ABSTRACT FROM AUTHOR]

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, β-myrcene, 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. [ABSTRACT FROM AUTHOR]

Molar absorption coefficients were measured for select alkyl nitrates and β-hydroxyalkyl nitrates in methanol. The presence of the β-hydroxyl group has a relatively minor effect on the absorption spectrum in the vicinity of the weakn→ π* transition, which is responsible for photolysis of organic nitrates in the atmosphere. For both alkyl nitrates and β-hydroxyalkyl nitrates, there is an enhancement in the absorption coefficients in solution compared to the gas-phase values. The effect of the β-hydroxyl group on the spectra was modelled with molecular dynamics simulations using an OM2/GUGA-CI Hamiltonian for ethyl nitrate and β-hydroxyethyl nitrate. The simulation provided a qualitatively correct shape of the low energy tail of the absorption spectrum, which is important for atmospheric photochemistry. The role of direct aqueous photolysis in removal of β-hydroxyalkyl nitrates in cloud and fog water was modelled using a relative rate approach, and shown to be insignificant relative to gas-phase photochemical processes and aqueous OH oxidation under typical atmospheric conditions. [ABSTRACT FROM AUTHOR]

This work presents a systematic investigationof the molecularlevel composition and the extent of aqueous photochemical processingin different types of secondary organic aerosol (SOA) from biogenicand 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 simulatean urban atmosphere). Chamber- and flow-tube-generated SOA sampleswere collected, extracted in a methanol/water solution, and photolyzedfor 1 h under identical irradiation conditions. In these experiments,the irradiation was equivalent to about 3–8 h of exposure tothe sun in its zenith. The molecular level composition of the dissolvedSOA was probed before and after photolysis with direct-infusion electrosprayionization high-resolution mass spectrometry (ESI-HR-MS). The massspectra of unphotolyzed SOA generated by ozone oxidation of monoterpenesshowed qualitatively similar features and contained largely overlappingsubsets of identified compounds. The mass spectra of OH/NOx-generated SOA had more unique visual appearanceand indicated a lower extent of product overlap. Furthermore, thefraction of nitrogen-containing species (organonitrates and nitroaromatics)was highly sensitive to the SOA precursor. These observations suggestthat attribution of high-resolution mass spectra in field SOA samplesto specific SOA precursors should be more straightforward under OH/NOxoxidation conditions compared to the ozone-drivenoxidation. Comparison of the SOA constituents before and after photolysisshowed the tendency to reduce the average number of atoms in the SOAcompounds without a significant effect on the overall O/C and H/Cratios. SOA prepared by OH/NOxphotooxidationof 1,3,5-trimethylbenzene and guaiacol were more resilient to photolysisdespite being the most light-absorbing. The composition of SOA preparedby ozonolysis of monoterpenes changed more significantly as a resultof the photolysis. The results indicate that aqueous photolysis ofdissolved SOA compounds in cloud/fog water can occur in various typesof SOA, and on atmospherically relevant time scales. However, theextent of the photolysis-driven change in molecular composition dependson the specific type of SOA. [ABSTRACT FROM AUTHOR]