Aromatic Carbonyl Compounds as Aqueous-Phase Photochemical Sources of Hydrogen Peroxide in Acidic Sulfate Aerosols, Fogs, and Clouds. 1. Non-Phenolic Methoxybenzaldehydes and Methoxyacetophenones with Reductants (Phenols)

Abstract

Non-phenolic aromatic carbonyl compounds (i.e., benzaldehydes and acetophenones not containing an -OH group on the aromatic ring) and various phenols are present in the atmosphere from the combustion of wood and other biomass and probably from the entrainment of terrestrial humic/fulvic substances present in wind-blown soil aerosol. Illumination (313 nm) of aqueous solutions containing a non-phenolic aromatic carbonyl and a given phenol (phenol itself or a substituted phenol): produces hydrogen peroxide (HOOH); destroys the phenol; and, in most cases, causes little or no destruction of the carbonyl. Little HOOH is photoformed from these chromophores without added phenol, but the quantum yield for HOOH photoformation (ΦHOOH,313) rapidly increases with increasing phenol concentration. These observations indicate that phenol serves as the terminal electron donor and that the non-phenolic aromatic carbonyl normally acts as a photocatalyst. The HOOH quantum yields of the aromatic carbonyls are strongly dependent upon pH; ΦHOOH,313 increases by up to 20-fold from pH 5.6 to pH 1.6, for non-phenolic methoxybenzaldehydes and methoxyacetophenones. HOOH photoformation is proposed to occur via oxidation of phenol by both protonated and unprotonated excited triplet states of the aromatic aldehyde or ketone, forming a ketyl radical (PhC•(OH)R), which reduces O2, thereby forming HOO• and regenerating the parent carbonyl. Calculated rates of HOOH photoformation in sunlight for methoxy-substituted benzaldehydes and acetophenones are generally greater than those of their unsubstituted analogs. At pH 3.7 calculated rates of HOOH photoproduction in sunlight (midday, equinox, Durham, NC) are typically 3−8 μM h-1, but range up to 34 μM h-1 for aqueous solutions containing 10 μM methoxy-substituted aldehyde or ketone with 30 μM phenol. Calculated rates at pH 1.6 are 2−3 times larger than those at pH 3.7. The combination of high photoreactivity and likely atmospheric prevalence for these chromophores suggests that they are important sources of HOOH in aqueous aerosols, fogs, and clouds.