Particle nucleation from the reaction of α-pinene and O 3

Abstract

Nucleation precursors from α-pinene+O 3 reactions were investigated with a flow reactor combined with a Teflon bag reactor. Initial particle formation exponentially increased with the reacted amount of α-pinene with O 3, and this, given certain assumptions, is consistent with classical nucleation theory. Initial reaction products indicated products such as cis-pinonaldehyde, cis-pinonic acid, cis-norpinonic acid, and pinalic-4-acid did not initiate particle self-nucleation because of their low concentrations compared to their theoretically estimated saturation concentrations. Although particle formation was observed on the time scale of the flow reactor (⩾0.4–6 s), cis-pinic and cis-norpinic acid were not detected as products, and hence are also not candidates for initiating self-nucleation. Thermally stabilized Criegee intermediates (SCI) were implicated in the nucleation process. Evidence for this comes from the observation of significant reductions in particle formation by scavenging SCIs with low-molecular-weight compounds. On a molecule reaction basis, nucleation was suppressed with SCI scavengers in the following order: water ≪acetaldehyde≈methanol<formic acid. Teflon bag experiments showed that decreased initial particle number concentrations by SCI scavenger (formic acid and acetic acid) did not affect the final total particle mass concentration. Possible nucleation precursors are discussed in the context of estimated vapor pressures and initial concentrations.