heterogeneous photochemistry of imidazole-2-carboxaldehyde: ho2 radical formation and aerosol growth

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ID: 188723
2016
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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.
Reference Key
palacios2016atmosphericheterogeneous Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors L. González Palacios, P. Corral Arroyo, K. Z. Aregahegn, S. S. Steimer, T. Bartels-Rausch, B. Nozière, C. George, M. Ammann, R. Volkamer
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Journal Journal of agricultural and food chemistry
Year 2016
DOI
10.5194/acp-16-11823-2016
URL
https://www.atmos-chem-phys.net/16/11823/2016/acp-16-11823-2016.pdf
Keywords
physicschemistry

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