Heterogeneous photochemistry of imidazole-2-carboxaldehyde: HO2 radical formation and aerosol growth

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Date
2016Type
- Journal Article
Citations
Cited 33 times in
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Cited 35 times in
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ETH Bibliography
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Abstract
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 HO2 radicals (PHO2) was measured indirectly by converting gas-phase NO into NO2. We report on experiments that relied on measurements of NO2 formation, NO loss and HONO formation. PHO2 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 O2∕N2 ratio (15% < O2∕N2 < 56%) was observed, most likely indicating competing effects of dilution, HO2 mobility and losses in the film. The maximum PHO2 was observed at 25–55%RH and at ambient O2∕N2. The HO2 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 O2 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 HO2 in the gas phase. Further, seed aerosols containing IC and ammonium sulfate were exposed to gas-phase limonene and NOx in aerosol flow tube experiments, confirming significant PHO2 from aerosol surfaces. Our results indicate a potentially relevant contribution of triplet state photochemistry for gas-phase HO2 production, aerosol growth and ageing in the atmosphere. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000120928Publication status
publishedExternal links
Journal / series
Atmospheric Chemistry and PhysicsVolume
Pages / Article No.
Publisher
CopernicusFunding
163074 - Feedbacks between atmospheric aerosol microphysics and photochemical aging (SNF)
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Citations
Cited 33 times in
Web of Science
Cited 35 times in
Scopus
ETH Bibliography
yes
Altmetrics