Temperature Dependence of Gas-Particle Partitioning of Primary Organic Aerosol Emissions from a Small Diesel Engine

A new experimental technique has been developed to study the gas-particle partitioning behavior of primary organic aerosol (POA) emissions from combustion sources at atmospherically relevant concentrations. The technique involves slowly filling a Teflon chamber with a constant emission source. As aerosol concentrations increase inside the chamber, the gas-particle partitioning of semivolatile organics shifts to the particle phase, thus increasing the fuel-based POA emission factor. The technique allows characterization of partitioning under isothermal conditions and atmospherically relevant concentrations. The technique was evaluated using emissions from a small diesel engine; the measured changes in gas-particle partitioning agreed well with previously published data for this engine measured with a dilution sampler. The temperature dependence of the gas-particle partitioning was investigated by conducting experiments at three different temperatures (15 degrees C, 26 degrees C, and 33 degrees C). Increasing organic aerosol concentration and decreasing temperature increased the fuel-based POA emission factor. The gas-particle partitioning data were fit using absorptive partitioning theory to determine the volatility distribution and enthalpy of vaporization (Delta H(v)) of the emissions. We have derived two fits; one using the volatility basis set approach and a second using a two-product model. Both fits are suitable for use in chemical transport models. These fits were tested using previously published thermodenuder data. Partitioning calculations predict that the gas-particle partitioning from POA emissions from this engine vary by about a factor of 4 across the atmospherically relevant range of temperature and organic aerosol concentrations. This underscores the semivolatile nature of POA emissions.