Although it is known that soot particles are emitted in large quantities to the atmosphere, our understanding of their environmental effects is limited by our knowledge of how their composition is subsequently altered through atmospheric processing. Here we present an on-line mass spectrometric study of the changing chemical composition of hydrocarbon soot particles as they are oxidized by gas-phase ozone, and we show that the surface-mediated loss rates of adsorbed polycyclic aromatic hydrocarbons in soot are directly connected to a significant increase in the particle redox cycling abilities. With redox cycling implicated as an oxidative stress mechanism that arises after inhalation of atmospheric particles, this work draws a quantitative connection between the detailed heterogeneous chemistry occurring on atmospheric particles and a potential toxic mechanism attributable to that aerosol.

This paper demonstrates that OH radicals are formed by photolysis of secondary organic aerosol (SOA) material formed by terpene ozonolysis. The SOA is collected on filters, dissolved in water containing a radical trap (benzoic acid), and then exposed to ultraviolet light in a photochemical reactor. The OH formation rates, which are similar for both α-pinene and limonene SOA, are measured from the formation rate of p-hydroxybenzoic acid as measured using offline HPLC analysis. To evaluate whether the OH is formed by photolysis of H₂O₂ or organic hydroperoxides (ROOH), the peroxide content of the SOA was measured using the horseradish peroxidase-dichlorofluorescein (HRP-DCF) assay, which was calibrated using H₂O₂. The OH formation rates from SOA are 5 times faster than from the photolysis of H₂O₂ solutions whose concentrations correspond to the peroxide content of the SOA solutions, assuming that the HRP-DCF signal arises from H₂O₂ alone. The higher rates of OH formation from SOA are likely due to ROOH photolysis, but we cannot rule out a contribution from secondary processes as well. This result is substantiated by photolysis experiments conducted with t-butyl hydroperoxide and cumene hydroperoxide which produce over 3 times more OH than photolysis of equivalent concentrations of H₂O₂. Relative to the peroxide level in the SOA and assuming that the peroxides drive most of the ultraviolet absorption, the quantum yield for OH generation from α-pinene SOA is 0.8 ± 0.4. This is the first demonstration of an efficient photolytic source of OH in SOA, one that may affect both cloud water and aerosol chemistry.

In recent years, ultra-low temperature chemical kinetic experiments have demonstrated that some gas-phase reactions are much faster than was previously thought. One example is the reaction between OH and CH₃OH, which has recently been found to be accelerated at low temperatures yielding CH₃O as its main product. This finding raised the question of whether or not the CH₃O observed in the dense core Barnard 1b could be formed by the gas-phase reaction of CH₃OH and OH. Several chemical models including this reaction and grain-surface processes have been developed to explain the observed abundance of CH₃O, but they have met with little success. Here, we report for the first time the rate coefficients for the gas-phase reaction of OH and CH₃OH down to a temperature of 22 K, which is very close to the temperature in cold interstellar clouds. Two independent experimental set-ups based on the supersonic gas expansion technique coupled to the pulsed laser photolysis laser-induced fluorescence technique were used to determine the rate coefficients in the temperature range 22–64 K. The temperature dependence obtained in this work can be expressed as k(22–64 K) = ( 3.6 ± 0.1 ) × 10 − 12 ( T / 300 K ) − ( 1.0 ± 0.2 ) cm³ molecule⁻¹ s⁻¹. Implementing this expression in a chemical model of a cold, dense cloud results in CH₃O/CH₃OH abundance ratios similar to or slightly lower than the value of ∼3 × 10⁻³ observed in Barnard 1b. This finding confirms that the gas-phase reaction between OH and CH₃OH is an important contributor to the formation of interstellar CH₃O. The role of grain-surface processes in the formation of CH₃O, although it cannot be fully neglected, remains controversial.

New kinetic data and product distributions have been obtained using the experimental CRESU technique combined with a theoretical analysis of the reaction mechanism. The astrophysical implications of fast CH₃O and CH₂OH formation are discussed.