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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 Mar 2019

Research article | 28 Mar 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Halogen activation and radical cycling initiated by imidazole-2-carboxaldehyde photochemistry

Pablo Corral Arroyo1,2, Raffael Aellig3, Peter A. Alpert1, Rainer Volkamer4,5, and Markus Ammann1 Pablo Corral Arroyo et al.
  • 1Paul Scherrer Institute, Laboratory of Environmental Chemistry, 5232 Villigen PSI, Switzerland
  • 2Department of Chemistry and Biochemistry, University of Bern, 2012 Bern, Switzerland
  • 3ETH Swiss Federal Institute of Technology Zürich, Institute for Atmospheric and Climate Science, 8006 Zurich, Switzerland
  • 4Department of Chemistry and Biochemistry, 215 UCB, University of Colorado, Boulder, CO 80309, USA
  • 5Cooperative Institute for Research in Environmental Sciences (CIRES), 216 UCB, University of Colorado, Boulder, CO 80309, USA

Abstract. Atmospheric aerosol particles can contain light absorbing organic compounds, also referred to as brown carbon (BrC). In the context of the ocean surface and of sea spray aerosol deriving from the latter, light absorbing organic species are also referred to as chromophoric dissolved organic matter (CDOM). Many BrC or CDOM species (especially carbonyls, dicarbonyls or aromatic carbonyls such as imidazole-2-carboxaldehyde (IC)), referred to as photosensitizers, form triplet excited states upon UV-VIS light absorption. These triplet excited states are strong oxidants and may initiate catalytic radical reaction cycles within atmospheric aerosol particles and at their surface, therefore increasing the reactive oxygen species (ROS) production within atmospheric aerosol particles. Triplet states (or ROS resulting from them) can also react with halides generating halogen radicals and additionally molecular halogens compounds, which can be released into the gas phase and may thus contribute to halogen activation. In this work we study the influence of bromide and iodide on the photosensitized HO2 production and release upon UV irradiation of films in a coated wall flow tube (CWFT) containing IC in a matrix of citric acid (CA). Additionally we measured the iodine release upon irradiation of IC/CA films in the CWFT. We use a kinetic model to interpret our results and to assess radical production and iodine release in sea-spray particles. As indicated by the experimental results and confirmed by the model, significant recycling of halogen species occurs via scavenging reactions with HO2, to prevent the full and immediate release of the molecular halogen (bromine and iodine) produced, while partially shutting down the HOx chemistry. The recycling efficiency is higher and affected by diffusion limitations at high viscosity. Our findings also show that halides can increase substantially the BrC or CDOM photosensitized HO2 production (which in turn promotes radical and ROS production) by reacting with triplet statesin sea-spray particles. The iodine production within sea salt aerosol particles due to iodide oxidation by ozone is estimated at 5.9 × 10−5 M s−1 assuming ozone equilibration in the particle. Under diffusion limitation this activation can drop several orders of magnitude in an aged, organic-rich sea-spray derived aerosol (1.1 × 10−7 M s−1 for an ozone diffusion coefficient of 10−12 cm2 s−1). The estimated iodine production from BrC photochemistry amounts to 2.5 × 10−7 M s−1. This indicates that BrC photochemistry can exceed O3 reactive uptake in controlling the rates of iodine activation from sea spray particles under dry or cold conditions where diffusion is slow within particles.

Pablo Corral Arroyo et al.
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Pablo Corral Arroyo et al.
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Short summary
Oxidation of bromide and iodide is an important process in the troposphere that leads to gas phase halogen compounds that impact the oxidation capacity of the atmosphere. Imidazole-2-carboxaldehyde (IC), an aromatic carbonyl, is a product of multiphase chemistry of glyoxal, an oxidation product of isoprene, a major biogenic volatile organic compound. In this study we demonstrate that IC photochemistry leads to efficient oxidation of bromide and iodide and liberation of halogen compounds.
Oxidation of bromide and iodide is an important process in the troposphere that leads to gas...