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.

The mechanism of the OH-initiated oxidation of isoprene in the presence of NO and O 2 has been investigated using a discharge-flow system at and 2 torr total pressure. 298 K OH radical concentration profiles were measured using laser-induced fluorescence as a function of reaction time. The rate constant for the reaction of was measured to OH ϩ isoprene be cm 3 In the presence of NO and O 2 , regeneration of OH(1.10 Ϯ 0.05) ϫ 10 mol s . radicals by the reaction of isoprene-based peroxy radicals with NO was measured and compared to simulations of the kinetics of this system. The results of these experiments are consistent with an overall rate constant of cm 3 (with an uncertainty factor Ϫ12 Ϫ1 Ϫ19 ϫ 10 mol s of 2) for the reaction of isoprene-based hydroxyalkyl peroxy radicals with NO.

A substantial fraction of the total ultrafine particulate mass is comprised of organic compounds. Of this fraction, a significant subfraction is secondary organic aerosol (SOA), meaning that the compounds are a by-product of chemistry in the atmosphere. However, our understanding of the kinetics and mechanisms leading to and following SOA formation is in its infancy. We lack a clear description of critical phenomena; we often don't know the key, rate limiting steps in SOA formation mechanisms. We know almost nothing about aerosol yields past the first generation of oxidation products. Most importantly, we know very little about the derivatives in these mechanisms; we do not understand how changing conditions, be they precursor levels, oxidant concentrations, co-reagent concentrations (i.e., the VOC/NOx ratio) or temperature will influence the yields of SOA. In this paper we explore the connections between fundamental details of physical chemistry and the multitude of steps associated with SOA formation, including the initial gas-phase reaction mechanisms leading to condensible products, the phase partitioning itself, and the continued oxidation of the condensed-phase organic products. We show that SOA yields in the alpha-pinene + ozone are highly sensitive to NOx, and that SOA yields from beta-caryophylene + ozone appear to increase with continued ozone exposure, even as aerosol hygroscopicity increases as well. We suggest that SOA yields are likely to increase substantially through several generations of oxidative processing of the semi-volatile products.

[1] The mechanism of the OH-initiated oxidation of isoprene has been studied at 300 K and 100 and 150 Torr total pressure using a turbulent flow technique coupled with laserinduced fluorescence detection of the OH radical. The rate constant for the reaction of OH with isoprene was measured to be (10.8 ± 0.5) Â 10 À11 cm 3 molecule À1 s À1 at 150 Torr total pressure and independent of pressure between 100 and 150 Torr, in excellent agreement with most previous absolute measurements. In the presence of O 2 and NO, propagation of OH radicals and loss of OH through radical termination resulting from the formation of organic nitrates were measured at 150 Torr total pressure and compared to simulations of the kinetics of this reaction system. The results of these experiments are consistent with an overall rate constant of (1.1 ± 0.8) Â 10 À11 cm 3 molecule À1 s À1 for the reaction of NO with isoprene-based hydroxyalkyl peroxy radicals, with branching ratios of 0.85 ± 0.10 for the bimolecular channel (oxidation of NO to NO 2 ) and 0.15 ± 0.10 for the termolecular channel (formation of organic nitrates). Although the organic nitrate yield reported here is the result of indirect measurements, it suggests that isoprene may be a more significant sink of NO x than previously estimated.

No abstract