Preprints
https://doi.org/10.5194/acp-2021-875
https://doi.org/10.5194/acp-2021-875

  01 Nov 2021

01 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

Enhanced photodegradation of dimethoxybenzene isomers in/on ice compared to in aqueous solution

Ted Hullar1, Theo Tran1, Zekun Chen2, Fernanda Bononi2, Oliver Palmer1,a, Davide Donadio2, and Cort Anastasio1 Ted Hullar et al.
  • 1Department of Land, Air and Water Resources, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
  • 2Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
  • anow at: TeraPore Technologies, 407 Cabot Road, South San Francisco, CA 94080

Abstract. Photochemical reactions of contaminants in snow and ice can be important sources and sinks for various organic and inorganic compounds. Snow contaminants can be found in the bulk ice matrix, in internal liquid-like regions (LLRs), or in quasi-liquid layers (QLLs) at the air-ice interface, where they can readily exchange with the firn air. Some studies have reported that direct photochemical reactions occur faster in LLRs and QLLs than in aqueous solution, while others have found similar rates. Here, we measure the photodegradation rate constants of the three dimethoxybenzene isomers under varying experimental conditions, including in aqueous solution, in LLRs, and at the air-ice interface of nature-identical snow. Relative to aqueous solution, we find modest photodegradation enhancements (3- and 6-fold) in LLRs for two of the isomers, and larger enhancements (15- to 30-fold) at the air-ice interface for all three isomers. We use computational modeling to assess the impact of light absorbance changes on photodegradation rate enhancements at the interface. We find small (2–5 nm) bathochromic (red) absorbance shifts at the interface relative to in solution, which increases light absorption, but this factor only accounts for less than 50 % of the measured rate constant enhancements. The major factor responsible for photodegradation rate enhancements at the air-ice interface appears to be more efficient photodecay: estimated dimethoxybenzene quantum yields are 6- to 24-fold larger at the interface compared to in aqueous solution and account for the majority (51–96 %) of the observed enhancements. Using a hypothetical model compound with an assumed Gaussian-shaped absorbance peak, we find that a shift in the peak to higher or lower wavelengths can have a minor to substantial impact on photodecay rate constants, depending on the original location of the peak and the magnitude of the shift. Changes in other peak properties at the air-ice interface, such as peak width and height (i.e., molar absorptivity) can also impact rates of light absorption and direct photodecay.

Ted Hullar et al.

Status: open (until 13 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-875', Anonymous Referee #1, 12 Nov 2021 reply
  • RC2: 'Comment on acp-2021-875', Dominik Heger, 26 Nov 2021 reply

Ted Hullar et al.

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Short summary
Chemicals are commonly found in snowpacks throughout the world, and may be degraded by sunlight; some previous research has reported faster decay rates for chemicals on the surface of snow and ice compared to in water. We found photodegradation on snow can be as much as 30 times faster than in solution for the three dimethoxybenzene isomers. Our computational modeling found light absorbance by dimethoxybenzenes increases on the snow surface, but this only partially explains the decay rate.
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