SOA and gas phase organic acid yields from the sequential photooxidation of seven monoterpenes

Abstract

We investigate secondary organic aerosol (SOA) and gas phase organic acid yields from the sequential photooxidation of seven monoterpene isomers (α-pinene, β-pinene, limonene, sabinene, terpinolene, α-terpinene, and γ-terpinene) using an Oxidative Flow Reactor under dry conditions. SOA yields were highest for terpinolene (33% at 5.7 days of aging), followed by sabinene, β-pinene, α-pinene, limonene, γ-terpinene, and α-terpinene. Isomers with exocyclic double bonds (i.e. immediately adjacent to a ring) had higher SOA yields than those with endocyclic double bonds, or with double bonds that are unattached to a ring structure. SOA yields increased with OH exposure, highlighting the limitation of using single values for SOA yield in modeling studies, and the need for isomer-specific SOA parameterizations. SOA yields were adequately fit by a one-product model and in broad agreement with previous studies. SOA yields linearly increased with organic aerosol mass concentration, possibly the result of high OH loading and short residence times in the flow reactor. Gas phase yields of formic, acetic, butyric, and methacrylic acid (or their isomers) followed very different patterns as a function of OH exposure from SOA yields, and were poorly correlated with OH rate constants. These observations suggest that higher volatility (fragmentation) products of monoterpene photooxidation were produced and lost on different timescales from the production and condensation of lower-volatility (functionalization) products. Formic acid yields ranged from 0.06 to 9.3% across all OH exposures. Formic acid yields from γ-terpinene (0.36–3%) monotonically increased with OH exposure, unlike the other monoterpene isomers, which exhibited initial increases in formic acid yields with OH exposure, followed by decreases at higher OH concentrations. This difference in organic acid yield trends is consistent with the distribution of carbon-containing species identified by chemical ionization mass spectrometry.