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 re-generate 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. Atmospheric implications consist in a potentially relevant contribution of triplet state photochemistry for gas-phase HO2 production, aerosol growth and ageing.;
