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 C 2 H 3 O + to CO 2 + 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.

Organic films on indoor surfaces serve as a medium for reactions and for partitioning of semi-volatile organic compounds and thus play an important role in indoor chemistry. However, the chemical...

In the atmosphere, brown carbon (BrC) molecules can contribute to photochemistry and to the warming and evaporation of cloud droplets. The lifetimes of BrC molecules in these cloud droplets are not well constrained, especially considering the droplets can contain mixtures of organic and inorganic molecules. Here we investigated the kinetics of photodegradation for a representative brown carbon molecule, 4-nitrophenol (4NP), in aqueous solutions at low pH. Three different solutions were compared: one containing 4NP alone and the other two additionally containing either fresh or photo-recalcitrant (pre-photobleached) α-pinene secondary organic aerosol (SOA). Fresh SOA slightly accelerates the photodegradation compared to that with 4NP alone. In contrast, photo-recalcitrant SOA decelerates photodegradation, increasing the estimated atmospheric lifetime of 4NP by 4 h (from ∼11 h for BrC alone to ∼14 h when it is mixed with photo-recalcitrant SOA). Photo-recalcitrant SOA also shows minimal chemical changes after irradiation for 20 h in aqueous solution. These results suggest that, in contrast to fresh SOA, aqueous photorecalcitrant SOA does not serve as a source of OH radicals when irradiated but may instead act as an OH radical scavenger. The presence of photo-recalcitrant SOA could have important effects on atmospheric lifetimes of organics in mixed aqueous systems.