Gas-particle partitioning of primary organic aerosol emissions: (1) Gasoline vehicle exhaust
The gas-particle partitioning of the primary organic aerosol (POA) emissions from fifty-one light-duty gasoline vehicles (model years 1987–2012) was investigated at the California Air Resources Board Haagen–Smit Laboratory. Each vehicle was operated over the cold-start unified cycle on a chassis dynamometer and its emissions were sampled using a constant volume sampler. Four independent yet complementary approaches were used to investigate POA gas-particle partitioning: sampling artifact correction of quartz filter data, dilution from the constant volume sampler into a portable environmental chamber, heating in a thermodenuder, and thermal desorption/gas chromatography/mass spectrometry analysis of quartz filter samples. This combination of techniques allowed gas-particle partitioning measurements to be made across a wide range of atmospherically relevant conditions – temperatures of 25–100 °C and organic aerosol concentrations of−3. The gas-particle partitioning of the POA emissions varied continuously over this entire range of conditions and essentially none of the POA should be considered non-volatile. Furthermore, for most vehicles, the low levels of dilution used in the constant volume sampler created particle mass concentrations that were greater than a factor of 10 or higher than typical ambient levels. This resulted in large and systematic partitioning biases in the POA emission factors compared to more dilute atmospheric conditions, as the POA emission rates may be over-estimated by nearly a factor of four due to gas-particle partitioning at higher particle mass concentrations. A volatility distribution was derived to quantitatively describe the measured gas-particle partitioning data using absorptive partitioning theory. Although the POA emission factors varied by more than two orders of magnitude across the test fleet, the vehicle-to-vehicle differences in gas-particle partitioning were modest. Therefore, a single volatility distribution can be used to quantitatively describe the gas-particle partitioning of the entire test fleet. This distribution is designed to be applied to quartz filter POA emission factors in order to update emissions inventories for use in chemical transport models.