Measurement of hydrogen peroxide and organic hydroperoxide concentrations during autumn in Beijing, China

Gaseous peroxides play important roles in atmospheric chemistry. To understand the pathways of the formation and removal of peroxides, atmospheric peroxide concentrations and their controlling factors were measured from 7:00 to 20:00 in September, October, and November 2013 at a heavily trafficked residential site in Beijing, China, with average concentrations of hydrogen peroxide (H₂O₂) and methyl hydroperoxide (MHP) at 0.55ppb and 0.063ppb, respectively. H₂O₂ concentrations were higher in the afternoon and lower in the morning and evening, while MHP concentrations did not exhibit a regular diurnal pattern. Both H₂O₂ and MHP concentrations increased at dusk in most cases. Both peroxides displayed monthly variations with higher concentrations in September. These results suggested that photochemical activity was the main controlling factor on variations of H₂O₂ concentrations during the measurement period. Increasing concentrations of volatile organic compounds emitted by motor vehicles were important contributors to H₂O₂ and MHP enrichment. High levels of H₂O₂ and MHP concentrations which occurred during the measurement period probably resulted from the transport of a polluted air mass with high water vapor content passing over the Bohai Bay, China.

Water vapour effects on secondary organic aerosol formation in isoprene ozonolysis

A portion of stabilized Criegee intermediates formed in isoprene ozonolysis can be involved in SOA formation even under humid conditions.

Quantum chemical and master equation studies of the methyl vinyl carbonyl oxides formed in isoprene ozonolysis

Methyl vinyl carbonyl oxide is an important intermediate in the reaction of isoprene and ozone and may be responsible for most of the (*)OH formed in isoprene ozonolysis. We use CBS-QB3 calculations and RRKM/master equation simulations to characterize all the pathways leading to the formation of this species, all the interconversions among its four possible conformers, and all of its irreversible isomerizations. Our calculations, like previous studies, predict (*)OH yields consistent with experiment if thermalized syn-methyl carbonyl oxides form (*)OH quantitatively. Natural bond order analysis reveals that the vinyl group weakens the C=O bond of the carbonyl oxide, making rotation about this bond accessible to this chemically activated intermediate. The vinyl group also allows one conformer of the carbonyl oxide to undergo electrocyclization to form a dioxole, a species not previously considered in the literature. Dioxole formation, which has a CBS-QB3 reaction barrier of 13.9 kcal/mol, is predicted to be favored over vinyl hydroperoxide formation, dioxirane formation, and collisional stabilization. Our calculations also predict that two dioxole derivatives, 1,2-epoxy-3-butanone and 3-oxobutanal, should be major products of isoprene ozonolysis.

Cycloalkene Ozonolysis: Collisionally Mediated Mechanistic Branching

Master equation calculations on a computational potential energy surface reveal that collisional stabilization at atmospheric pressure becomes important in the gas-phase ozonolysis of endocyclic alkenes for a carbon number between 8 and 15. Because the reaction products from endocyclic ozonolysis are tethered, this system is ideal for consideration of collisional energy transfer, as chemical activation is confined to a single reaction product. Collisional stabilization of the Criegee intermediate precedes collisional stabilization of the primary ozonide by roughly an order of magnitude in pressure. The stabilization of the Criegee intermediate leads to a dramatic transformation in the dominant oxidation pathway from a radical-forming process at low carbon number to a secondary ozonide-forming process at high carbon number. Secondary ozonide formation is important even for syn-isomer Criegee intermediates, contrary to previous speculation. We use substituted cyclohexenes as analogues for atmospherically important mono- and sesquiterpenes, which are major precursors for secondary organic aerosol formation in the atmosphere. Combining these calculations with literature experimental data, we conclude that the transformation from chemically activated to collisionally stabilized behavior most probably occurs between the mono- and sesquiterpenes, thus causing dramatically different atmospheric behavior.