Gas-Particle Partitioning of Semivolatile Organic Compounds When Wildfire Smoke Comes to Town

Wildfires have become an increasingly important source of organic gases and particulate matter in the western United States. A large fraction of organic particulate matter emitted in wildfires is semivolatile, and the oxidation of organic gases in smoke can form lower volatility products that then condense on smoke particulates. In this research, we measured the gas- and particle-phase concentrations of semivolatile organic compounds (SVOCs) during the 2017 Northern California wildfires in a downwind urban area, using the Semivolatile Thermal-Desorption Aerosol Gas Chromatography (SV-TAG), and measured SVOCs in a rural site affected by biomass burning using the comprehensive TAG (cTAG) in Idaho in 2018. Commonly used biomass burning markers such as levoglucosan, mannosan and nitrocatechols were found to stay predominantly in the particle phase, even when the ambient OA was relatively low. The phase partitioning of SVOCs is observed to be dependent on their saturation vapor pressure, while the absorptive equilibrium model underpredicts the particle-phase fraction of most of the compounds measured. Wildfire organic aerosol enhanced the condensation of polar compounds into the particle phase but not some nonpolar compounds, such as polycyclic aromatic hydrocarbons.