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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.

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]