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.

Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies

“…Photochemical aqueous phase experiments suggest that ring-retaining aromatic oxidation products from phenol and its methoxylated derivates guaiacol and syringol might contribute to aqSOA as they form aromatic dimer products in high yields (near unity, related to the initial aqueous phase concentration) by recombination of phenoxy radicals (Sun et al, 2010). The total aromatic concentrations in these studies (100 µM) resemble those as found from wood smoke markers in winter fog (Sagebiel and Seiber, 1993); however, at other locations much smaller phenol concentrations are generally observed (Anastasio et al, 1997).…”

An overview of snow photochemistry: evidence, mechanisms and impacts

“…These additional oxidants include singlet molecular oxygen ( 1 O 2 *), excited triplet states of organic compounds, and peroxyl radicals (Faust, 1994;Anastasio et al, 1997). Initial work has shown that 1 O 2 * is indeed formed on illuminated Summit snow and that the corresponding steady-state concentrations are high enough to be a significant sink for electron rich species such as polycyclic aromatic hydrocarbons (McKellar et al, 2005).…”

Photochemistry of 1-Nitronaphthalene: A Potential Source of Singlet Oxygen and Radical Species in Atmospheric Waters

“…Taking into account the energy of 3 1NN (E T = 231 kJ mol -1 ) 35 we can argue that, following reaction 4, interaction with molecular oxygen leads to the formation of both the first and the second electronically excited singlet states: O 2 ( 1 ∆ g ) (94 kJ mol -1 ) and O 2 ( 1 Σ g + ) (157 kJ mol -1 ). Quantification of singlet oxygen photoformation via reaction R4 was possible by using FFA as a chemical probe, and we found a formation rate The reaction between 1NN •-and oxygen (R5) resulting in superoxide was proposed by analogy with the corresponding processes that involve species produced upon reduction of the triplet states of aromatic carbonyls 36 The results displayed in figure 4 give clear evidence that the decay of 3 1NN monitored at 620 nm is higher at low pH and in the presence of chloride. At pH 6.5 in aerated solution the 3 1NN lifetime is about 1.8 µs, and it decreases to ~1.0 µs by acidifying the solution to pH 1.1.…”