Biomass burning aerosol emissions from vegetation fires: particle number and mass emission factors and size distributions

Abstract. Aerosol emissions from vegetation fires have a large impact on air quality and climate. In this study, we use published experimental data and different fitting procedures to derive dynamic particle number and mass emission factors (EF_PN, EF_PM) related to the fuel type, burning conditions and the mass of dry fuel burned, as well as characteristic CO-referenced emission ratios (PN/CO, PM/CO). Moreover, we explore and characterize the variability of the particle size distribution of fresh smoke, which is typically dominated by a lognormal accumulation mode with count median diameter around 120 nm (depending on age, fuel and combustion efficiency), and its effect on the relationship between particle number and mass emission factors.

For the particle number emission factor of vegetation fires, we found no dependence on fuel type and obtained the following parameterization as a function of modified combustion efficiency (MCE): EF_PN=34×10¹⁵×(1−MCE) kg⁻¹±10¹⁵ kg⁻¹ with regard to dry fuel mass (d.m.). For the fine particle mass emission factors (EF_PM) we obtained (86–85×MCE) g kg⁻¹±3 g kg⁻¹ as an average for all investigated fires; (93–90×MCE) g kg⁻¹±4 g kg⁻¹ for forest; (67–65×MCE) g kg⁻¹±2 g kg⁻¹ for savanna; (63–62×MCE) g kg⁻¹±1 g kg⁻¹ for grass.

For the PN/CO emission ratio we obtained an average of (34±16) cm⁻³ ppb⁻¹ exhibiting no systematic dependence on fuel type or combustion efficiency. The average PM/CO emission ratios were (0.09±0.04) g g⁻¹ for all investigated fires; (0.13±0.05) g g⁻¹ for forest; (0.08±0.03) g g⁻¹ for savanna; and (0.07±0.03) g g⁻¹ for grass.

The results are consistent with each other, given that particles from forest fires are on average larger than those from savanna and grass fires. This assumption and the above parameterizations represent the current state of knowledge, but they are based on a rather limited amount of experimental data which should be complemented by further measurements. Nevertheless, the presented parameterizations appear sufficiently robust for exploring the influence of vegetation fires on aerosol particle number and mass concentrations in regional and global model studies.