Abstract
Abstract. This paper demonstrates that OH radicals are formed by photolysis of secondary organic aerosol (SOA) material formed by terpene ozonolysis. The SOA is collected on filters, dissolved in water containing a radical trap (benzoic acid), and then exposed to ultraviolet light in a photochemical reactor. The OH formation rates, which are similar for both α-pinene and limonene SOA, are measured from the formation rate of p-hydroxybenzoic acid as measured using offline HPLC analysis. To evaluate whether the OH is formed by photolysis of H2O2 or organic hydroperoxides (ROOH), the peroxide content of the SOA was measured using the horseradish peroxidase-dichlorofluorescein (HRP-DCF) assay, which was calibrated using H2O2. The OH formation rates from SOA are 5 times faster than from the photolysis of H2O2 solutions whose concentrations correspond to the peroxide content of the SOA solutions, assuming that the HRP-DCF signal arises from H2O2 alone. The higher rates of OH formation from SOA are likely due to ROOH photolysis, but we cannot rule out a contribution from secondary processes as well. This result is substantiated by photolysis experiments conducted with t-butyl hydroperoxide and cumene hydroperoxide which produce over 3 times more OH than photolysis of equivalent concentrations of H2O2. Relative to the peroxide level in the SOA and assuming that the peroxides drive most of the ultraviolet absorption, the quantum yield for OH generation from α-pinene SOA is 0.8 ± 0.4. This is the first demonstration of an efficient photolytic source of OH in SOA, one that may affect both cloud water and aerosol chemistry.
Highlights
Given the importance of secondary organic aerosol (SOA) to both climate and air quality, considerable attention has been given to studying the formation pathways and composition of SOA both in the lab and the field (Hallquist et al, 2009)
We find that the OH production rates from SOA are substantially higher than from the pure H2O2 solutions, implying that species other than hydrogen peroxide – likely organic hydroperoxide – are photolyzing into OH
The yields have been reported in three ways: (1) mole percentage, moles of peroxides/moles of SOA × 100 %, where the molecular weight of SOA is assumed to be 200 g mole−1 (Bateman et al, 2011); (2) mass percentage, mass of peroxides/mass of SOA collected × 100 %, where the molecular weight of peroxides is assumed to be 34 g mole−1; and (3) normalized yield, moles of peroxides/mass of SOA collected, where the peroxides are assumed to be H2O2
Summary
Given the importance of secondary organic aerosol (SOA) to both climate and air quality, considerable attention has been given to studying the formation pathways and composition of SOA both in the lab and the field (Hallquist et al, 2009). If the particles become more hygroscopic, they will be wet scavenged more Their resultant health effects may transform if reactive functional groups are either produced or lost during processing. One direction has been to address the multi-phase oxidation processes in which SOA participates, primarily via oxidation by gas-phase OH radicals (George and Abbatt, 2010). The conclusions from these studies are that heterogeneous exposure to OH leads to a more oxidized and hygroscopic aerosol, with small amounts of mass loss through fragmentation reactions occurring on a timescale of a few days of equivalent OH exposure in the atmosphere
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