Photon flux dependence on solute environment in water ices

Environmental context Anthropogenic pollutants deposited in and on snowpacks can undergo many sunlight-driven reactions. These processes have been studied, but typically without measuring the photon flux, the amount of light seen by the reactants, which is needed for comparing results across studies. This work investigates the effects of container albedo, solute location and mechanical ice crushing on the photon flux in laboratory ice samples to understand how these factors might affect photochemical rates. Abstract The photon flux directly affects the rates of both direct and indirect photodegradation reactions in water and ice. This flux might vary in the different solute reservoirs of water ice (e.g. between the bulk ice and air–ice interface), which might help explain reported differences in measured reaction rates. To address this possibility, here we use 2-nitrobenzaldehyde chemical actinometry to measure photon fluxes in ice samples prepared using different freezing techniques in order to put 2-nitrobenzaldehyde into different regions in the ice samples. Overall, the solute location has little effect on photon flux in water ice (purified frozen water) samples, with a maximum observed enhancement of 42 ± 9 % relative to aqueous values. However, the albedo (reflectivity) of the sample container strongly influences the photon flux in water and ice samples: for the same incident irradiance, 2-nitrobenzaldehyde loss is four times higher in a white beaker compared with in a dark-brown beaker. In addition, crushing an ice sample to a 2-mm grain size increases the photon flux in the resulting ice granules by 50 % compared with in an intact ice disc (and by 80 % compared with the corresponding solution). Although photon fluxes are similar in different solute reservoirs in and on ice, our results show that photon fluxes within a frozen (or aqueous) sample cannot be simply determined from incident fluxes, but instead need to be measured using the same sample geometry and container type.