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.