Articles | Volume 16, issue 18
Research article
23 Sep 2016
Research article |  | 23 Sep 2016

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

Laura González Palacios, Pablo Corral Arroyo, Kifle Z. Aregahegn, Sarah S. Steimer, Thorsten Bartels-Rausch, Barbara Nozière, Christian George, Markus Ammann, and Rainer Volkamer

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.

Short summary
The sources of radicals at aerosol surfaces are highly uncertain. Here we investigate the HO2 radical production from the UV irradiation of imidazole-2-carboxaldehyde (IC) in bulk aqueous films containing IC and citric acid, as well as IC in ammonium sulfate aerosols. We find that IC is an efficient photosensitizer that forms HO2 radicals from H-donor chemistry. IC is a proxy species for brown carbon in atmospheric aerosols.
Final-revised paper