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Discussion papers
https://doi.org/10.5194/acp-2018-1259
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1259
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Dec 2018

Research article | 04 Dec 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Aqueous Reactions of Organic Triplet Excited States with Atmospheric Alkenes

Richie Kaur1,2, Brandi M. Hudson3, Joseph Draper1,a, Dean J. Tantillo3, and Cort Anastasio1,2 Richie Kaur et al.
  • 1Department of Land, Air, and Water Resources, University of California, Davis, California 95616, United States
  • 2Agricultural & Environmental Chemistry Graduate Group, University of California, Davis
  • 3Department of Chemistry, University of California, Davis, California 95616, United States
  • anow at: the Fresno Metropolitan Flood Control District, Fresno, California 93727, United States

Abstract. Triplet excited states of organic matter, a.k.a. triplets, are formed when brown carbon absorbs light. While triplets can be important photooxidants in atmospheric drops and particles (e.g., they rapidly oxidize phenols), very little is known about their reactivity toward many classes of organic compounds in the atmosphere. Here we measure the bimolecular rate constants of the triplet excited state of benzophenone (3BP*), a model species, with 17 water-soluble C3–C6 alkenes that have either been found in the atmosphere or are reasonable surrogates for identified species. Measured rate constants (kALK+3BP*) vary by a factor of 30 and are in the range of (0.24–7.5)×109M−1s−1. Biogenic alkenes found in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, and methyl jasmonate – react rapidly, with rate constants above 1×109M−1s−1. Rate constants depend on alkene characteristics such as the location of the double bond, stereochemistry, and alkyl substitution on the double bond. There is a reasonable correlation between kALK+3BP* and the calculated one-electron oxidation potential (OP) of the alkenes (R2=0.58); in contrast, rate constants are not correlated with bond dissociation enthalpies, bond dissociation free energies, or computed energy barriers for hydrogen abstraction. Using the OP relationship, we estimate aqueous rate constants for a number of unsaturated isoprene and limonene oxidation products with 3BP*: values are in the range of (0.080–1.7)×109M−1s−1, with generally faster values for limonene products. Using our predicted rate constants, along with values for other reactions from the literature, we conclude that triplets are probably minor oxidants for isoprene and limonene-related compounds in cloudy or foggy atmospheres, except in cases where the triplets are very reactive.

Richie Kaur et al.
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Latest update: 13 Dec 2018
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Short summary
Organic triplets are an important class of aqueous photooxidants but little is known about their reactions with atmospheric organic compounds. Here, we measured reaction rate constants of a model triplet with 17 aliphatic alkenes and using their correlation with oxidation potential, predicted rate constants for some atmospherically-relevant alkenes. Overall, depending on their reactivities, triplets can be minor-to-important sinks for isoprene and limonene-derived alkenes in cloud or fog drops.
Organic triplets are an important class of aqueous photooxidants but little is known about their...
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