Heterogeneous photochemistry of imidazole-2-carboxaldehyde: HO2 radical formation and aerosol growth
Item
Title (Dublin Core)
Heterogeneous photochemistry of imidazole-2-carboxaldehyde: HO2 radical
formation and aerosol growth
formation and aerosol growth
Description (Dublin Core)
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 % < RH < 63 %) and of (4) the O<sub>2</sub> ∕ N<sub>2</sub> ratio (15 % < O<sub>2</sub> ∕ N<sub>2</sub> < 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.
Creator (Dublin Core)
González Palacios, Laura
Corral Arroyo, Pablo
Aregahegn, Kifle Z.
Steimer, Sarah S.
Bartels-Rausch, Thorsten
Nozière, Barbara
George, Christian
Ammann, Markus
Volkamer, Rainer
Date (Dublin Core)
2018-09-19
Type (Dublin Core)
info:eu-repo/semantics/Text
Format (Dublin Core)
info:eu-repo/semantics/application/pdf
Identifier (Dublin Core)
10.5194/acp-16-11823-2016
https://acp.copernicus.org/articles/16/11823/2016/
Source (Dublin Core)
eISSN: 1680-7324
Language (Dublin Core)
eng
Relation (Dublin Core)
info:eu-repo/grantAgreement/EC/FP7/265307
Rights (Dublin Core)
info:eu-repo/semantics/openAccess
