No abstract

Experiments were conducted to examine the effects of dilution on fine particle mass emissions from a diesel engine and wood stove. Filter measurements were made simultaneously using three dilution sampling systems operating at dilution ratios ranging from 20:1 to 510:1. Denuders and backup filters were used to quantify organic sampling artifacts. For the diesel engine operating at low load and wood combustion, large decreases in fine particle mass emissions were observed with increases in dilution. For example, the PM2.5 mass emission rate from a diesel engine operating at low load decreased by 50% when the dilution ratio was increased from 20:1 to 350:1. Measurements of organic and elemental carbon indicate that the changes in fine particle mass with dilution are caused by changes in partitioning of semivolatile organic compounds. At low levels of dilution semivolatile species largely occur in the particle phase, but increasing dilution reduces the concentration of semivolatile species, shifting this material to the gas phase in order to maintain phase equilibrium. Emissions of elemental carbon do not vary with dilution. Organic sampling artifacts are shown to vary with dilution because of the combination of changes in partitioning coupled with adsorption of gas-phase organics by quartz filters. The fine particle mass emissions from the diesel engine operating at medium load did not vary with dilution because of the lower emissions of semivolatile material and higher emissions of elemental carbon. To measure partitioning of semivolatile materials under atmospheric conditions, partitioning theory indicates that dilution samplers need to be operated such that the diluted exhaust achieves atmospheric levels of dilution. Too little dilution can potentially overestimate the fine particle mass emissions, and too much dilution (with clean air) can underestimate them.

Experimental measurements of gas-particle partitioning and organic aerosol mass in diluted diesel and wood combustion exhaust are interpreted using a two-component absorptive-partitioning model. The model parameters are determined by fitting the experimental data. The changes in partitioning with dilution of both wood smoke and diesel exhaust can be described by two lumped compounds in roughly equal abundance with effective saturation concentrations of approximately 1600 microg m(-3) and approximately 20 microg m(-3). The model is used to investigate gas-particle partitioning of emissions across a wide range of atmospheric conditions. Under the highly dilute conditions found in the atmosphere, the partitioning of the emissions is strongly influenced by the ambient temperature and the background organic aerosol concentration. The model predicts large changes in primary organic aerosol mass with varying atmospheric conditions, indicating that it is not possible to specify a single value for the organic aerosol emissions. Since atmospheric conditions vary in both space and time, air quality models need to treat primary organic aerosol emissions as semivolatile. Dilution samplers provide useful information about organic aerosol emissions; however, the measurements can be biased relative to atmospheric conditions and constraining predictions of absorptive-partitioning models requires emissions data across the entire range of atmospherically relevant concentrations.