Examination of two assumptions commonly used to determine PM2.5 emission factors for wildland fires

There is a lack of in-depth examination of the two basic assumptions used in calculating particulate matter (PM2.5) emission factors (EFs): 1) that the ambient CO2 concentration is constant whether in a fire plume sample or in the ambient air (the constant ambient CO2 concentration assumption); and 2) there is no significant difference in diffusion rates between the fire-emitted CO2 and PM2.5 during a fire experiment (the equal diffusion assumption). We used carbon isotopic tracer and paired-sampling approaches to examine these assumptions. Our isotopic tracer results showed that the concentrations of ambient CO2 measured in fire plumes were generally greater than those measured in the ambient air, such that use of the constant ambient concentration assumption lead to a general overestimation of the fire-emitted CO2 by 10 ± 2.9% and a general underestimation of PM2.5 EF by 5.9 ± 1.8%. We also found evidence of significant differential diffusion between CO2 and PM2.5 in fire experiments, although increases in PM2.5 concentration through condensation of volatiles may contribute to the decoupling of relative concentrations of PM2.5 and CO2. Further research is warranted for identifying conditions under which the differential diffusion issues are reasonably resolved for accurate estimation of PM2.5 EF.