The chlorine atom (Cl)-initiated oxidation of three polycyclic aromatic hydrocarbons (PAHs; namely, naphthalene, acenaphthylene, and acenaphthene) was investigated. Experiments were performed in an atmospheric simulation chamber using a proton transfer reaction time-of-flight mass spectrometer (TOF-MS) and an aerosol TOF-MS to characterize the oxidation products in the gas and particle phases, respectively. The major products identified from the reaction of Cl atoms with naphthalene were phthalic anhydride and chloronaphthalene, indicating that H atom abstraction and Cl addition reaction pathways are both important. Acenaphthenone was the principal product arising from reaction of Cl with acenaphthene, while 1,8-naphthalic anhydride, acenaphthenone, acenaphthenequinone, and chloroacenaphthenone were all identified as products of acenaphthylene oxidation, confirming that the cylcopenta-fused ring controls the reactivity of these PAHs toward Cl atoms. Possible reaction mechanisms are proposed for the formation of these products, and favored pathways have been suggested. Large yields of secondary organic aerosol (SOA) were also observed in all experiments, and the major products were found to undergo significant partitioning to the particle-phase. This work suggests that Cl-initiated oxidation could play an important role in SOA formation from PAHs under specific atmospheric conditions where the Cl atom concentration is high, such as the marine boundary layer.

The photolysis of o-tolualdehyde by natural sunlight has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photolysis rate coefficient was measured directly under different solar flux levels, with values in the range j(o-tolualdehyde) = (1.62–2.15) × 10–4 s–1 observed, yielding an average value of j(o-tolualdehyde)/j(NO2) = (2.53 ± 0.25) × 10–2. The estimated photolysis lifetime is 1–2 h, confirming that direct photolysis by sunlight is the major atmospheric degradation pathway for o-tolualdehyde. Published UV absorption cross-section data were used to derive an effective quantum yield (290–400 nm) close to unity, within experimental error. Possible reaction pathways for the formation of the major photolysis products, benzocyclobutenol (tentatively identified) and o-phthalaldehyde, are proposed. Appreciable yields (5–13%) of secondary organic aerosol (SOA) were observed at EUPHORE and also during supplementary experiments performed in an indoor chamber using an artificial light source. Off-line analysis by gas chromatography–mass spectrometry allowed identification of o-phthalaldehyde, phthalide, phthalic anhydride, o-toluic acid, and phthalaldehydic acid in the particle phase.

The formation of secondary organic aerosol and gas/particle partitioning of carbonyl products from the photooxidation of p-xylene has been investigated as a function of relative humidity. Experiments were performed in an atmospheric simulation chamber at atmospheric pressure and ambient temperature in the presence of NOx. Aerosol yields increased by a factor of approximately two over the relative humidity range 5−75% and were found to correlate with initial water vapor concentration and hydroxyl radical (OH) concentration. The results indicate that an increase in relative humidity results in higher levels of HONO formation in the chamber which leads to increased OH concentration, a faster p-xylene decay rate, and higher aerosol mass yields. A recently developed denuder-filter sampling technique was used to investigate the gas/particle partitioning behavior of the carbonyl photooxidation products. The identified products accounted for up to 18% of the aerosol mass formed. Dicarbonyls with at least one aldehyde functionality (e.g., glyoxal and methylglyoxal) exhibited gas/particle partitioning coefficients several orders of magnitude higher than expected from absorptive partitioning theory, suggesting that reactive uptake and particle phase reactions are important processes for aerosol formation from these species. Experimental gas/particle partitioning coefficients were also found to be dependent on relative humidity, with every dicarbonyl exhibiting much lower values when the relative humidity was increased from 50% to 75%.

The atmospheric photolysis of E-2-hexenal, Z-3-hexenal and E,E-2,4-hexadienal has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. E-2-Hexenal and E,E-2,4-hexadienal were found to undergo rapid isomerization to produce Z-2-hexenal and a ketene-type compound (probably E-hexa-1,3-dien-1-one), respectively. Both isomerization processes were reversible with formation of the reactant slightly favoured. Values of j(E-2-hexenal)/j(NO2) = (1.80 ± 0.18) × 10−2 and j(E,E-2,4-hexadienal)/j(NO2) = (2.60 ± 0.26) × 10−2 were determined. The gas phase UV absorption cross-sections of E-2-hexenal and E,E-2,4-hexadienal were measured and used to derive effective quantum yields for photoisomerization of 0.36 ± 0.04 for E-2-hexenal and 0.23 ± 0.03 for E,E-2,4-hexadienal. Although photolysis appears to be an important atmospheric degradation pathway for E-2-hexenal and E,E-2,4-hexadienal, the reversible nature of the photolytic process means that gas phase reactions with OH and NO3 radicals are ultimately responsible for the atmospheric removal of these compounds. Atmospheric photolysis of Z-3-hexenal produced CO, with a molar yield of 0.34 ± 0.03, and 2-pentenal via a Norrish type I process. A value of j(Z-3-hexenal)/j(NO2) = (0.4 ± 0.04) × 10−2 was determined. The results suggest that photolysis is likely to be a minor atmospheric removal process for Z-3-hexenal.

The hydroxyl radical initiated oxidation of diisopropyl ether has been studied in the large-volume outdoor European Photoreactor (EUPHORE) and in a small, laboratory-based reactor system. The product distributions determined from the experiments were found to be significantly dependent on the reaction conditions and provide strong evidence for the existence of three distinct regimes within the reaction system. In the presence of NOx, the peroxy radicals react with NO to produce chemically activated (CH3)2CHOC(O)(CH3)2 alkoxy radicals which undergo decomposition by CC bond scission to yield isopropyl acetate and formaldehyde as the major products. Under conditions where the self-reaction of peroxy radicals dominates, thermoneutral (CH3)2CHOC(O)(CH3)2 radicals are produced, which appear to undergo two reaction pathways; CC bond scission to yield isopropyl acetate and formaldehyde and isomerisation to form acetone, acetic acid and formaldehyde. Under conditions where the reaction between peroxy and hydroperoxy radicals dominates, unstable hydroperoxides are produced which decompose to yield acetone as the only major reaction product. The results of our study are used to construct chemical mechanisms for the gas-phase photooxidation of diisopropyl ether under various tropospheric conditions.

Rate coefficients for the reactions of hydroxyl (OH) and nitrate (NO3) radicals with benzaldehyde and the tolualdehydes have been determined at 295 ± 2 K and atmospheric pressure using the relative rate technique. Experiments were performed in atmospheric simulation chambers using gas chromatography for chemical analysis. The rate coefficients (in units of cm3 molecule−1 s−1) are:View Within ArticleThe reactivity of the aromatic aldehydes is compared to other aromatic compounds and it is shown that, for the tolualdehydes, the OH and NO3 rate coefficients do not depend on the position of the CH3 substituent on the aromatic ring. The new data are used to show that the gas-phase reactivity of the aromatic aldehydes towards OH and NO3 radicals follows a linear free energy relationship typical of addition reactions, although the net result is H-atom abstraction. The rate coefficient data are explained in terms of known mechanistic features of the reactions and simple theoretical calculations have been performed in an attempt to understand the observed trends in reactivity. The atmospheric implications are also discussed.