Distribution of Gaseous and Particulate Products from the Oxidation of Isoprene

Isoprene has received considerable attention with regard to gas-phase reaction products and secondary organic aerosol (SOA) formation, with the main focus the identification of tracer compounds (e.g., methyltetrols, methylglyceric acid) that can be used to estimate the contribution of isoprene to ambient organic aerosol. Little is known, however, about the formation and mechanistic pathways leading to these tracer compounds and whether they are formed primarily in the gas phase or in the particle phase. Gaseous secondary organics involved in gas/particle partitioning play a crucial role in understanding the formation of SOA. To date, only very limited studies have investigated the distribution between the gaseous and condensed phases of semi-volatile organic components from isoprene oxidation, and only a few mechanistic pathways have been reported. Nevertheless, isoprene reactions have been incorporated into air quality models even though limited data is available and many uncertainties still exist regarding the mechanisms by which reactions of isoprene form SOA. The National Exposure Research Laboratory of the U.S. Environmental Protection Agency recently undertook an integrated laboratory research effort to better understand the contribution of isoprene reactions to SOA formation. The oxidation of isoprene was investigated in a series of experiments under a variety of conditions conducted in a 14.5-m3 indoor smog chamber. The yield, physical characteristics, and composition of SOA formed in each experiment were analyzed using a suite of instruments including a scanning mobility particle sizer, a Sunset Labs semi-continuous elemental carbon-organic carbon (EC-OC) monitor, a volatility differential mobility analyzer, a direct insertion probe-mass spectrometer, a high-resolution time-of-flight aerosol mass spectrometer (HR-AMS), and a gas chromatograph-mass spectrometer. This study focused on the characterization of partitioning coefficients (Kp) of methyltetrols, methylglyceric acid, and other organic compounds detected in the gas and/or particle phases that might play a role in SOA formation. Time series for large reaction products, including methyltetrols and methylglyceric acid, identified in the gas and/or particle phases are presented for the first time. These results provide new insight into the degradation mechanism of isoprene and its role in the formation of SOA. Partitioning Kps, SOA yields, the effective enthalpy of vaporization, the EC/OC ratio, and tentative reaction schemes leading to some of the reaction products are presented. In addition, SOA mass concentrations, size distributions, and compositions were examined using a number of instruments, including a SMPS, a semi-continuous EC-OC analyzer, and an HR-AMS.