Heterogeneous photochemistry of imidazole-2-carboxaldehyde: HO<sub>2</sub> radical formation and aerosol growth

The multiphase chemistry of glyoxal is a source of secondary organic aerosol (SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC). IC is a photosensitizer that can contribute to additional aerosol ageing and growth when its excited triplet state oxidizes hydrocarbons (reac...

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Bibliographic Details
Main Authors: L. González Palacios, P. Corral Arroyo, K. Z. Aregahegn, S. S. Steimer, T. Bartels-Rausch, B. Nozière, C. George, M. Ammann, R. Volkamer
Format: Article
Language:English
Published: Copernicus Publications 2016-09-01
Series:Atmospheric Chemistry and Physics
Online Access:https://www.atmos-chem-phys.net/16/11823/2016/acp-16-11823-2016.pdf
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Summary:The multiphase chemistry of glyoxal is a source of secondary organic aerosol (SOA), including its light-absorbing product imidazole-2-carboxaldehyde (IC). IC is a photosensitizer that can contribute to additional aerosol ageing and growth when its excited triplet state oxidizes hydrocarbons (reactive uptake) via H-transfer chemistry. We have conducted a series of photochemical coated-wall flow tube (CWFT) experiments using films of IC and citric acid (CA), an organic proxy and H donor in the condensed phase. The formation rate of gas-phase HO<sub>2</sub> radicals (<i>P</i><sub>HO<sub>2</sub></sub>) was measured indirectly by converting gas-phase NO into NO<sub>2</sub>. We report on experiments that relied on measurements of NO<sub>2</sub> formation, NO loss and HONO formation. <i>P</i><sub>HO<sub>2</sub></sub> was found to be a linear function of (1) the [IC]  ×  [CA] concentration product and (2) the photon actinic flux. Additionally, (3) a more complex function of relative humidity (25 %  &lt;  RH  &lt;  63 %) and of (4) the O<sub>2</sub> ∕ N<sub>2</sub> ratio (15 %  &lt;  O<sub>2</sub> ∕ N<sub>2</sub>  &lt;  56 %) was observed, most likely indicating competing effects of dilution, HO<sub>2</sub> mobility and losses in the film. The maximum <i>P</i><sub>HO<sub>2</sub></sub> was observed at 25–55 % RH and at ambient O<sub>2</sub> ∕ N<sub>2</sub>. The HO<sub>2</sub> radicals form in the condensed phase when excited IC triplet states are reduced by H transfer from a donor, CA in our system, and subsequently react with O<sub>2</sub> to regenerate IC, leading to a catalytic cycle. OH does not appear to be formed as a primary product but is produced from the reaction of NO with HO<sub>2</sub> in the gas phase. Further, seed aerosols containing IC and ammonium sulfate were exposed to gas-phase limonene and NO<sub><i>x</i></sub> in aerosol flow tube experiments, confirming significant <i>P</i><sub>HO<sub>2</sub></sub> from aerosol surfaces. Our results indicate a potentially relevant contribution of triplet state photochemistry for gas-phase HO<sub>2</sub> production, aerosol growth and ageing in the atmosphere.
ISSN:1680-7316
1680-7324