Fresh versus Photo-recalcitrant Secondary Organic Aerosol: Effects of Organic Mixtures on Aqueous Photodegradation of 4-Nitrophenol

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-photo-bleached) alpha-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 similar to 11 h for BrC alone to similar 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 photo-recalcitrant 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.