Effects of Dilution on Fine Particle Mass and Partitioning of Semivolatile Organics in Diesel Exhaust and Wood Smoke

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.

Organic compounds in PM2.5 emitted from fireplace and woodstove combustion of typical Portuguese wood species

“…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. It should be noted, however, that the dilution ratio used in our study is comparable to the majority of values reported in the literature for biomass burning experiments (Lipsky and Robinson, 2005). The PM 2.5 emission factors for the fireplace and the woodstove are in the range of values reported for identical household combustion appliances in Scandinavia (Bølling et al, 2009; and references therein).…”

“…Fine et al (2001Fine et al ( , 2002Fine et al ( , 2004b obtained PM 2.5 emission factors for the fireplace and woodstove combustion of American tree species lower than those observed in this study; the PM 2.5 emission factors were in the range 0.88e3.4 g kg À1 of wood burned (as fired) in a woodstove, and between 1.6 and 11.4 g kg À1 of wood burned (as fired) in the fireplace. As stated in the literature, particle emission factors from residential biomass combustion may show wide variation due to the type of fuel, the characteristics of combustion facility, dilution techniques used, sampling procedures, burning rate and the moisture content of the fuel (Fine et al, 2004b;Kowalczk et al, 1981;Lipsky and Robinson, 2005;Wardoyo et al, 2006). Burning wood of poor quality (e.g.…”

“…high moisture content), overloading the firebox or insufficient air supply, are examples of conditions that can lead to incomplete combustion, characterised by low temperature (Bølling et al, 2009). Lipsky and Robinson (2005) tested the effects of dilution on fine particle mass concluding that too little dilution can potentially overestimate the fine particle mass emissions, and too much dilution can underestimate them. 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.…”

“…Collection of particulate matter with aerodynamic diameters below 2.5 mm (PM 2.5 ) was performed in a dilution tunnel. Dilution sampling is used to characterise particle emissions from combustion because it simulates the rapid cooling and dilution that occurs as exhaust mixes with the atmosphere (Lipsky and Robinson, 2005). The dilution tunnel consisted of a cylindrical tube with 0.20 m internal diameter and 11 m length.…”

Variability in the fraction of ambient fine particulate matter found indoors and observed heterogeneity in health effect estimates

“…A limitation of this method is that organic PM 2.5 is treated as non-volatile, when in fact it is semivolatile. 46 Sampling studies have demonstrated that phase changes of organics can impact F. Lunden et al 43 concluded that a lower F for OC (F ¼ 0.5) than for EC (F ¼ 0.61) in an unoccupied home was because of evaporation of some particulate OM as organic gases sorbed to indoor surfaces. However, it has also been suggested that ambient OM shifts from the gas phase into the particle phase by sorption into indoor-generated PM in occupied homes.…”