An evaluation of probes for oxidizing triplet excited states of organic matter in aerosol liquid water

Abstract:

In cloud/fog drops and aerosol water, photochemically generated oxidants - including hydroxyl radical (OH), singlet molecular oxygen (¹O₂*), and oxidizing triplet excited states of organic matter (³C*) - drive many aqueous reactions, some of which form aqueous secondary organic aerosol (aqSOA). ³C* are important oxidants in fog and cloud waters, with concentrations 10 - 100 times higher than hydroxy radical, and even higher concentrations (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. In surface waters, one common probe to assess ³C* steady-state concentrations is 2,4,6-trimethylphenol (TMP) but the high concentrations of dissolved organic matter (DOM) in ALW might inhibit the decay of TMP, confounding results from this probe.

To address this problem, we are investigating current and alternative probes for oxidizing triplets that can be used at the high DOM concentrations in particle water. To evaluate probes we are measuring several factors, including: (1) inhibition of probe loss by DOM, (2) rate constants of probe with triplets, and (3) rate constants of probe with ¹O₂* and OH. We find that the decay of phenols by triplet benzophenone can be almost fully suppressed by DOM in particle water extracts, while inhibition of the loss of the alkene methyl jasmonate is much smaller (~ 30%) under the same conditions. DOM suppresses ¹O₂* formation by approximately the same percentage as MeJA, suggesting that DOM causes a minor reduction in triplet concentrations while extensively suppressing phenol loss through a separate mechanism. Based on our results, the chemical loss of alkenes appears to be more promising technique for measuring oxidizing triplets in ALW. We will discuss the reactivity and DOM sensitivity of several alkenes and describe which of these compounds appears to be the most robust probe for oxidizing triplets under high DOM conditions.