[pam-users] Kasthuriarachchi et al., ACS E&S Air, 2025; Edwards et al., ACS ES&T Air, 2025

[pam-users]

Kasthuriarachchi, Nethmi Y., Rivellini, Laura-Helena, and Lee, Alex K. Y., Formation of Brown Carbon from Aqueous-Phase Limonene SOA Reactions with Reduced Nitrogen Species: Effects of Gas-Phase Precursors and Drying Conditions, ACS ES&T Air, https://doi.org/10.1021/acsestair.5c00054, 2025.

Abstract. Aqueous-phase processing of secondary organic aerosols (SOAs) from limonene oxidation can generate brown carbon (BrC) through reactions with reduced nitrogen species. This study provides laboratory evidence that dark ozonolysis of limonene, in the presence of biogenic (α-pinene) or anthropogenic (toluene) precursors, generates SOAs more effectively in BrC formation when subsequently dried with (NH4)2SO4/glycine in bulk solutions, and toluene had a stronger effect than α-pinene in promoting oligomeric BrC formation. BrC absorption was significantly enhanced in limonene-SOA and the (NH4)2SO4/glycine mixed droplet upon evaporation at ∼75% RH but was suppressed at lower RH, likely due to reduced aerosol liquid water and precursor evaporation. RH-dependent trends were also observed in SOA generated by limonene and toluene mixtures (lim/tol-SOA), with the strongest oligomeric BrC absorption at ∼85% RH for both (NH4)2SO4 and glycine systems. While bulk-dried experiments showed 1-2 orders of magnitude differences in BrC absorption between limonene-SOA and lim/tol-SOA, evaporated droplets exhibited no significant differences. This study underscores the importance of studying the effects of SOA precursors, atmospherically relevant drying conditions, and volatility of BrC precursors on the light absorption properties of complex SOAs via aqueous-phase processing.

Edwards, Kasey C., Gerritz, Lena, Schervish, Meredith, Canagaratna, Manjula, Avery, Anita M., Alton, Mitchell W., Wingen, Lisa M., Ryan, Jackson T., Faiola, Celia L., Lambe, Andrew T., Nizkorodov, Sergey A., and Shiraiwa, Manabu. Dependence of Reactive Oxygen Species Formation on the Oxidation State of Biogenic Secondary Organic Aerosols,  ACS ES&T Air, https://doi.org/10.1021/acsestair.5c00133, 2025.

Abstract. Reactive oxygen species (ROS) play a central role in the chemical aging of organic aerosols and adverse aerosol health effects upon respiratory deposition. Previous research has shown that biogenic secondary organic aerosols (SOA) form ROS, including hydroxyl radicals and superoxide, via reactions of reactive compounds, including organic hydroperoxides and alcohols in the aqueous phase. However, the influence of oxidative aging and the SOA oxidation state on the ROS yield has not been systematically investigated. In this study, we quantify ROS yields in d-limonene SOA and β-caryophyllene SOA generated via *OH and *Cl oxidation in an oxidation flow reactor at equivalent atmospheric aging times ranging from 4 h to 22 days. We quantify radical formation using electron paramagnetic resonance spectroscopy combined with a spin-trapping technique and characterize the molecular composition of the SOA samples with high-resolution mass spectrometry. We observe maximum radical formation at an oxygen-to-carbon ratio (O/C) of ∼0.5. Thereafter, we observe a >90% decrease in radical yield as the O/C increases to 1.2 for both d-limonene SOA and β-caryophyllene SOA. Similarly, the radical yield in d-limonene and β-caryophyllene SOA is reduced by >80% after on-filter photoirradiation. Peroxide yields are found to decrease with increasing O/C values and irradiation, suggesting that the aging-induced fragmentation and/or photolysis of hydroperoxides contribute to a decrease of radical formation in aged SOA.

PAM Wiki - Publications Using the PAM Oxidation Flow Reactor<https://sites.google.com/site/pamwiki/publications-using-the-pam-oxidation-flow-reactor?authuser=0>

Andrew Lambe
Principal Scientist
Aerodyne Research, Inc.

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