Particle-Phase Photosensitized Radical Production and Aerosol Aging

Atmospheric aerosol particles may contain light absorbing (brown carbon, BrC), triplet forming organic compounds that can sustain catalytic radical reactions and thus contribute to oxidative aerosol aging. We quantify UVA induced radical production initiated by imidazole-2-carboxaldehyde (IC), benzophenone (BPh). and 4-benzoylbenzoic acid (BBA) in the presence of the nonabsorbing organics citric acid (CA), shikimic acid (SA), and syringol (Syr) at varying mixing ratios. We observed a maximum HO₂ release of 10¹³ molecules min^–1 cm^–2 at a mole ratio <i>X</i>_BPh < 0.02 for BPh in CA. Mixtures of either IC or BBA with CA resulted in 10¹¹–10¹² molecules min^–1 cm^–2 of HO₂ at mole ratios (<i>X</i>_IC and <i>X</i>_BBA) between 0.01 and 0.15. HO₂ release was affected by relative humidity (<i>RH</i>) and film thickness suggesting coupled photochemical reaction and diffusion processes. Quantum yields of HO₂ formed per absorbed photon for IC, BBA and BPh were between 10^–7 and 5 × 10^–5. The nonphotoactive organics, Syr and SA, increased HO₂ production due to the reaction with the triplet excited species ensuing ketyl radical production. Rate coefficients of the triplet of IC with Syr and SA measured by laser flash photolysis experiments were <i>k</i>_Syr = (9.4 ± 0.3) × 10⁸ M^–1 s^–1 and <i>k</i>_SA = (2.7 ± 0.5) × 10⁷ M^–1 s^–1. A simple kinetic model was used to assess total HO₂ and organic radical production in the condensed phase and to upscale to ambient aerosol, indicating that BrC induced radical production may amount to an upper limit of 20 and 200 M day^–1 of HO₂ and organic radical respectively, which is greater or in the same order of magnitude as the internal radical production from other processes, previously estimated to be around 15 M per day