Excited triplet states from aerosol extracts competing with OH radical and singlet oxygen

Recently, excited triplet states of organic molecules (³C*) have been investigated for their abilities as oxidants in the aqueous aerosol phase. Here, we aim to understand if triplet states could have an influence on aqueous chemistry that is comparable to or even larger than two of the main oxidants in this environment, singlet oxygen (¹O₂) and hydroxyl radicals (•OH).

Aerosols were collected in Grenoble, France both in winter and summer time during the period 02/12/2021 – 17/06/2022, and analyzed for their ability to produce ³C*, ¹O₂, and •OH. The optical properties of the aerosol samples show that compared to the summer samples, the winter samples have higher absorbance in the UV/Vis and likewise higher fluorescence intensity in the two major peaks in the excitation emission matrix (EEM), with λ_ex/em = 225/415 nm and 320/425 nm. There was a good correlation between the mass absorption efficiency at 365 nm (MAE₃₆₅), which is a common parameter for light absorption from BrC extracts, and the fluorescence intensity of the two peaks.

From the degradation of various chemical probes and analysis by liquid chromatography (LC) with UV or fluorescence detection, we estimated the steady-state concentration of these three oxidants in aerosols at sample concentrations of 10 mgC L^-1. The resulting concentrations of all oxidants were found to be larger in the winter samples than in summer samples. [³C*]_ss showed an average of (1.87 ± 0.53) ∙ 10^-13 M in winter samples, and (9.95 ± 2.9) ∙ 10^-14 M in summer samples. [¹O₂]_ss was found to have an average of (6.51 ± 0.32) ∙ 10^-13 M in winter samples and (4.40 ± 0.28) ∙ 10^-13 M in summer samples. For •OH the steady-state concentrations were (2.01 ± 0.52) ∙ 10^-16 M in winter samples and (1.07 ± 0.30) ∙ 10^-16 M in summer samples. These values give a trend of oxidant concentrations of [¹O₂]_ss > [³C*]_ss > [•OH]_ss. Through a literature search for values of second-order rate constants between the mentioned oxidants with organic species in aqueous media, we observed a trend of k_OH,ORG > k_3C*,ORG > k_1O2,ORG. When considering both steady-state concentrations and second-order rate constants, the triplet states appear to be highly important in aqueous media. By further experiments, the concentration of a single aerosol extract sample was varied (2.5 – 20 mgC L^-1), and we observed a plateau of ³C* concentrations at approximately 2∙10^-13 M at 7.5 mgC L^-1 (corresponding to 3 ∙ 10^-5 µg PM / µg H₂O). From these observations, we suggest that in a range of similar particulate matter to water ratios (µg PM / µg H₂O ≈ (0.3 – 1) ∙ 10^-4), this could serve as an estimate of steady-state triplet state concentrations in aerosols.

Overall, this study emphasizes the relevance of excited triplet states compared to singlet oxygen and OH radicals in aqueous aerosols by including both estimated steady-state concentrations and literature second-order rate constants in aqueous solution for the oxidants with organic species.