Importance of Hydroxyl Radical Chemistry in Isoprene Suppression of Particle Formation from α-Pinene Ozonolysis
Yingqi Wang, Yue Zhao, Ziyue Li, Chenxi Li, Naiqiang Yan, Huayun Xiao
Abstract
Monoterpenes and isoprene, emitted primarily by vegetation, represent a globally dominant source of secondary organic aerosol (SOA) precursors. Previous studies have shown that isoprene suppresses particle formation from the oxidation of monoterpenes in both isoprene-rich regions and laboratory chambers, but a mechanistic understanding of such isoprene effect remains limited. Here we combined flow reactor experiments and kinetic model simulations to understand how isoprene affects particle formation from the ozonolysis of α-pinene. We find that in the O3/OH “two-oxidant” regime (without an OH scavenger), α-pinene particle formation is substantially suppressed by the presence of isoprene, mainly because of the termination of organic peroxy radicals (RO2) arising from α-pinene by those from the OH oxidation of isoprene to form C11–C15 species rather than the less volatile C16–C20 dimers. However, in the “O3-only” regime (with an OH scavenger), such isoprene effect is diminished because of the reduced oxidation of isoprene and inefficient formation of isoprene-derived RO2, and the small reduction in C16–C20 species due to RO2 terminations may be compensated by low-volatility species formed via the reactions of stabilized Criegee intermediates. These results suggest that the OH chemistry plays an important role in the isoprene suppression of particle formation from α-pinene ozonolysis. Model simulations for conditions typical of isoprene-rich forested regions show that the formation of α-pinene-derived C19 and C20 species that play a pivotal role in particle nucleation and early growth, is reduced by 14%–48% in the daytime (high OH) and 2%–9% in the nighttime (low OH) for isoprene/α-pinene concentration ratios of 3 to 15. Therefore, we would expect that the isoprene suppression effect on particle formation from monoterpenes is strongly dependent on, among other factors, the OH level in the atmosphere.