Tropospheric Multiphase Chemistry: Excited Triplet States Compete with OH Radicals and Singlet Molecular Oxygen

Abstract

Excited triplet states of organic molecules (³C*) present in aerosols have been investigated for their ability as oxidants in the aqueous phase. Aerosols were collected in Grenoble (France), in winter and summer times during the period December 2021 through June 2022, and steady-state concentrations and quantum yields of ³C*, ¹O₂, and ^•OH were determined under solar-simulated conditions using various specific chemical probes. For comparison purposes, the steady-state concentrations of these three oxidants in these aerosols were determined at sample concentrations of 10 mg C L^–1. The resulting steady-state concentrations of all oxidants were larger in the winter samples than in the summer ones and ranked as follows [¹O₂]_ss > [³C*]_ss > [OH]_ss in agreement with previous reports. However, those oxidants do exhibit different reactivities with different classes of organic compounds, which can be ranked in a generic way as k_OH,ORG > k_3C*,ORG > k_1O2,ORG. If we combine the steady-state concentrations and the reactivity of these three oxidants, it appears that triplet states of organic matter are the main oxidants for most classes of organic compounds. This study emphasizes the relevance of excited triplet states compared to singlet oxygen and OH radicals in aqueous aerosols and the need for additional studies under various conditions.