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Abstract. Photochemical reactions of contaminants in snow and ice can be importantsinks for organic and inorganic compounds deposited onto snow from theatmosphere and sources for photoproducts released from snowpacks into theatmosphere. Snow contaminants can be found in the bulk ice matrix, ininternal liquid-like regions (LLRs), or in quasi-liquid layers (QLLs) at theair–ice interface, where they can readily exchange with the firn air. Somestudies have reported that direct photochemical reactions occur faster inLLRs and QLLs than in aqueous solution, while others have found similarrates. Here, we measure the photodegradation rate constants for loss of thethree dimethoxybenzene isomers under varying experimental conditions,including in aqueous solution, in LLRs, and at the air–ice interface ofnature-identical snow. Relative to aqueous solution, we find modestphotodegradation enhancements (3- and 6-fold) in LLRs for two of theisomers and larger enhancements (15- to 30-fold) at the air–ice interfacefor all three isomers. We use computational modeling to assess the impact oflight absorbance changes on photodegradation rate enhancements at theinterface. We find small (2–5 nm) bathochromic (red) absorbance shifts atthe interface relative to in solution, which increases light absorption, butthis factor only accounts for less than 50 % of the measured rate constantenhancements. The major factor responsible for photodegradation rateenhancements at the air–ice interface appears to be more efficientphotodecay: estimated dimethoxybenzene quantum yields are 6- to 24-foldlarger at the interface compared to in aqueous solution and account for themajority (51 %–96 %) of the observed enhancements. Using a hypotheticalmodel compound with an assumed Gaussian-shaped absorbance peak, we find thata shift in the peak to higher or lower wavelengths can have a minor tosubstantial impact on photodecay rate constants, depending on the originallocation of the peak and the magnitude of the shift. Changes in other peakproperties at the air–ice interface, such as peak width and height (i.e.,molar absorption coefficient), can also impact rates of light absorption anddirect photodecay. Our results suggest our current understanding ofphotodegradation processes underestimates the rate at which some compoundsare broken down, as well as the release of photoproducts into theatmosphere. 

Snowpacks contain a wide variety of inorganic and organic compounds, including some that absorb sunlight and undergo direct photoreactions. How the rates of these reactions in, and on, ice compare to rates in water is unclear: some studies report similar rates, while others find faster rates in/on ice. Further complicating our understanding, there is conflicting evidence whether chemicals react more quickly at the air–ice interface compared to in liquid-like regions (LLRs) within the ice. To address these questions, we measured the photodegradation rate of guaiacol (2-methoxyphenol) in various sample types, including in solution, in ice, and at the air–ice interface of nature-identical snow. Compared to aqueous solution, we find modest rate constant enhancements (increases of 3- to 6-fold) in ice LLRs, and much larger enhancements (of 17- to 77-fold) at the air–ice interface of nature-identical snow. Our computational modeling suggests the absorption spectrum for guaiacol red-shifts and increases on ice surfaces, leading to more light absorption, but these changes explain only a small portion (roughly 2 to 9%) of the observed rate constant enhancements in/on ice. This indicates that increases in the quantum yield are primarily responsible for the increased photoreactivity of guaiacol on ice; relative to solution, our results suggest that the quantum yield is larger by a factor of roughly 3–6 in liquid-like regions and 12–40 at the air–ice interface.