Atmospheric ozonolysis of alpha-pinene: Radical generation and aerosol production

The reaction between ozone and alpha-pinene has been studied extensively. Interest in alpha-pinene ozonolysis arises primarily from two phenomena: the alkene-ozone reaction generates OH radicals with high yield, and the products of the ozonolysis of large alkenes such as alpha-pinene results in the production of secondary organic aerosol (SOA).; The pressure dependence of the OH yield in ozone-alkene reactions is both important and controversial; the poor understanding of the pressure dependent OH yield for different ozone-alkene reactions is a major obstacle to developing an accurate simulation of tropospheric chemistry. We have investigated the ozonolysis of a series of alkenes with significant differences in size (C6 vs. C10) and structure (linear vs. cyclic) to elucidate the influence of these competing effects on OH formation. OH yields from linear alkenes decrease with pressure, while OH yields from cyclic alkenes are pressure independent. These observations can be explained by changes in the extent of collisional stabilization of the carbonyl oxide (Criegee) intermediate with increasing pressure.; Studies of SOA formation from alpha-pinene ozonolysis have typically been limited to darkened, low-NOx conditions. However, we believe that the ozonolysis reaction mechanism, and therefore the final product distribution, is affected by factors beyond the oxidant and VOC. We therefore report SOA yields from alpha-pinene ozonolysis under both dark and UV-illuminated conditions, and in the presence of high concentrations of NOx. SOA yield decreases in the presence of both UV light and high concentrations of NOx; in fact, SOA production is completely suppressed in some cases where both UV light and NOx are present. Yield reduction is a result of the formation of a more volatile product distribution as gas-phase conditions change; we propose that the change in the product distribution is driven by changes in gas-phase chemistry.; These results imply that previous experiments may overestimate SOA generation from terpene ozonolysis; ozone concentrations are often highest during the day in urban areas, where NOx concentrations are also large.; We also present a method for measuring secondary organic aerosol yield at low total organic mass concentration (COA) using Proton Transfer Reaction - Mass Spectrometry (PTR-MS). PTR-MS provides high time resolution measurements of gas-phase organic species, and, coupled with particle measurements, allows for the determination of aerosol yield in real time. (Abstract shortened by UMI.)...