Formation of secondary organic aerosols from the reaction of γ-terpinene with ozone: yields and morphology

Abstract

Atmospheric secondary organic aerosols (SOAs), formed from condensable oxidation products of anthropogenic and biogenic volatile organic compounds (VOCs), are important constituents of the atmosphere accounting for a significant fraction of ambient tropospheric aerosol. In this study, the formation of SOAs from the gas-phase ozonolysis of γ-terpinene (1-isopropyl-4-methyl-1,4-cyclohexadiene), a monoterpene emitted by many different trees (elm, cypress, water hickory, maple trees …) has been studied in an indoor atmospheric simulation chamber (CHARME), at 294 ± 2 K, atmospheric pressure, under dry conditions (relative humidity, RH < 2%) and in absence of seed particles. The concentration of γ-terpinene was measured by a proton Transfer Reaction - Time of Flight - Mass Spectrometer (PTR-ToF-MS), that of ozone by an UV photometric analyzer and the aerosol mass concentration was monitored using a scanning mobility particle sizer (SMPS). Scanning electronic microscopy (SEM) observations were performed to characterize the physical state and morphology of the formed SOAs. They show the formation of spherical particles in a viscous state.The overall organic aerosol yield (Y) was determined as the ratio of the suspended aerosol mass concentration corrected for wall losses (Mo) to the total reacted γ-terpinene concentration, assuming a particle density of 1 g cm−3. The aerosol formation yield increases as the initial γ-terpinene concentration raises. The presence of an OH radical scavenger (cyclohexane) leads to a decrease in the SOA yields (they varied from 0.11 to 0.54 and from 0.16 to 0.49, without and with, an OH radical scavenger, respectively). Y is a strong function of Mo and the organic aerosol formation can be expressed by a one-product gas/particle partitioning absorption model.