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.
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palacios2016atmosphericheterogeneous Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors ;L. González Palacios;L. González Palacios;P. Corral Arroyo;P. Corral Arroyo;K. Z. Aregahegn;K. Z. Aregahegn;S. S. Steimer;S. S. Steimer;S. S. Steimer;T. Bartels-Rausch;B. Nozière;C. George;M. Ammann;R. Volkamer;R. Volkamer
Journal Journal of agricultural and food chemistry
Year 2016
DOI
10.5194/acp-16-11823-2016
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