Halogen activation in snow: Laboratory studies on the multiphase reaction of ozone with bromide in aerosol deposits and in aging snow
Abstract
Ozone is one key oxidant involved in activating halogens from their reservoirs in cold and warm areas of the atmosphere. In cold parts of the Globe, snow is a frequent host of halogen activation. In principle, the oxidation of halogens in snow can proceed in aerosol deposits, brine pockets, or with surface adsorbed halides; each of these compartments being a reservoir for chemicals in snow.
Here, we present results from a kinetic laboratory study on the oxidation of bromide in mimics of sea-salt aerosol. The study focuses on the temperature range of 0°C to -25°C where the freeze concentration effect predicts faster reaction rates in snow. A key outcome from this study is that reactivity can be well described by model parameterization of the bulk and surface reactivity. Our finding indicates that changes to diffusivity of reactants and in the solubility of ozone in the aerosol counteracts the freeze concentration effect leading to an overall slower reaction rate. The discussion of these results will then open up to elaborate on the recent results from the Ammann group on a surface-stabilized BrOOO- complex as intermediate of this reaction. Next, we will tackle the question how the reactivity of bromide in snow might change with time as snow ages. The metamorphism of snow goes along with a substantial recrystallisation as it involves the movement of water vapor in direction of temperature gradients from high to low. Change in the structure of snow during metamorphism was assessed using X-ray tomography. The redistribution of bromide ions was assessed via a reactive method at -15°C by exposing the bromide-doped samples to ozone and monitoring ozone loss. Results showed that bromide was absent from the surface of snow, where reaction with ozone occurs, after 12 days of metamorphism. We suggest that the bromide is buried in less accessible reservoirs of the snow during metamorphism. We propose that finding might explain varying reactivity of snow observed in field studies. This presentation deals with how physical processes impact the reactivity in aerosol and snow at cold temperature and might be of general importance, beyond the reaction of bromide with ozone. Edebeli, J. et al.: doi: 10.1039/C8EM00417J, 2019 | Artiglia, L., et al.: doi: 10.1038/s41467-017-00823-x, 2017 | Trachsel JC et al. (2019) doi: 10.3389/feart.2019.00194- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A54A..02B
- Keywords:
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- 0305 Aerosols and particles;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 0738 Ice;
- CRYOSPHERE;
- 0793 Biogeochemistry;
- CRYOSPHERE