In atmospheric drops and particles (as well as surface waters), photochemically generated oxidants - including hydroxyl radical (OH), singlet molecular oxygen (1O2*), and oxidizing triplet excited states of organic matter (3C*) - drive many aqueous reactions. In fog and cloud waters, 3C* are important oxidants, with concentrations 10 - 100 times higher than hydroxyl radical; concentrations are even higher (by a few orders of magnitude) in aerosol liquid water (ALW). Therefore, knowing oxidizing triplet concentrations in ALW is important to understand reactivity in aerosol water. However, triplets represent a complex mixture with a wide range of reactivities, which makes it difficult to quantify this group of oxidants. Electron transfer probes such as phenols are commonly used to assess 3C* steady-state concentrations. However, phenols and aniline probes can be problematic at the high concentrations of dissolved organic matter (DOM) in ALW, since the DOM can inhibit probe degradation by 3C*, leading to an underestimate of triplet concentrations. A good triplet probe would also have relatively low reactivity with 1O2* so that it is more specific for triplets. Our goal is to find a triplet probe that is minimally inhibited by high concentrations of DOM and reacts much more quickly with 3C* compared to 1O2*. Our work shows that the net oxidation of phenols and anilines by triplets can be heavily inhibited by an aerosol water mimic containing Cu(II) and syringol (SYR), a biomass-burning phenol. To evaluate possible triplet probes we measured their inhibition by Cu(II) and SYR and quantified their rate constants with triplets and 1O2*. After examining the results for 12 potential probes, we find that (phenylthiol)acetic acid (PTA) is an appropriate triplet probe, with mild inhibition by DOM and high rate constants with triplets. However, PTA also has some disadvantages, including a pH-dependent rate constant with triplets and low sensitivity to low energy triplets. Given this, we also examine whether we can use measured inhibition factors to correct the DOM inhibition of syringol as a triplet probe. .

Evaluating probes of oxidizing triplet excited states of organic matter in aerosol liquid water

ACS Spring 2022

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