Abstract
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.
Highlights
The sources and sinks of radicals play an important role in the oxidative capacity of the atmosphere
Three different experimental setups consistently show that HO2 radicals are produced from the photochemistry of IC in a citric acid (CA) + H2O matrix and in seed aerosols containing ammonium sulfate
The linear correlations of phase HO2 radicals (PHO2) yielded maximum PHO2 under atmospherically relevant irradiation, O2 and relative humidity (RH) and revealed a complex role of film viscosity and possibly acidity effects
Summary
The sources and sinks of radicals play an important role in the oxidative capacity of the atmosphere. Our study investigates photosensitizers as an additional HOx source that may be relevant to further modify ROx and NOx reaction cycles in both the condensed and gas phases. It is motivated by the formation of superoxide in terrestrial aqueous photochemistry (Draper and Crosby, 1983; Faust, 1999; Schwarzenbach et al, 2002), by more recent observations that irradiated surfaces containing titanium dioxide generate HOx radicals in the gas phase (Yi et al, 2012) and by the generation of OH from metal oxides acting as photocatalysts in mineral dust (Dupart et al, 2012). We report here on the HO2 radical production from IC in the condensed phase
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