Effects of Photolysis on the Chemical and Optical Properties of Secondary Organic Material Over Extended Time Scales

As organic aerosol particles age in the atmosphere, their chemical and physical properties can change which impacts their loss rates and atmospheric lifetimes. Here we investigate 4 days of photolytic aging of α-pinene secondary organic aerosol particles collected on Teflon filters. Changes in the chemical and optical absorption properties of this secondary organic material were measured using UV/vis absorption spectroscopy, offline-aerosol mass spectrometry (AMS), electrospray ionization ultrahigh resolution mass spectrometry (UHR-MS), and attenuated total reflectance-Fourier transform infrared spectroscopy. For water-soluble extracts, our results show an initial decrease in the absorption cross section at ∼280 nm corresponding to the removal of carbonyls. The majority of this decrease occurs over the first 2 days with much smaller changes on days three and four. Offline-AMS shows a shift in ion intensity from C₂H₃O⁺ to CO₂⁺ suggesting a corresponding increase in carboxylic acids or esters. Results from soft ionization UHR-MS indicate that oligomerization occurs concurrent with photolysis, leading to a shift toward the center of the mass distribution range (∼320 amu). Aged dimers and trimers are observed, generated from the photo products formed over the first 2 days. These results demonstrate that extended experimental time frames and complementary analytical techniques provide new insights into multigenerational aging processes of atmospheric aerosol particles.