Impact of Combustion Conditions on Physical and Morphological Properties of Biomass Burning Aerosol

Abstract:

Aerosol emission from biomass burning (BB) has been the focus of several studies due to its impacts on climate, air quality, and human health. The physical and morphological properties of BB aerosol are linked to its source, age, microphysical, chemical, and optical properties. The study of particle density can provide information on aerosol aging, new particle formation, transport properties, and serve as an important parameter in aerosol modeling. Density is heavily used in mass closure techniques to estimate the high temporal resolution of particulate mass concentrations. However, the study of particle density from the BB aerosol under a variety of burning conditions is still limited. Here, we present the results from the laboratory measurement of six different sub-Saharan African biomass fuels burned under a range of burning conditions: from pure smoldering to pure flaming conditions. Smoldering-dominated burning (modified combustion efficiency (MCE) < 0.9) aerosol has a very narrow range of effective densities (ρ_eff) (1.03 to 1.21 g cm^-3) and a mass mobility exponent (D_fm) of ~3 (2.97 ± 0.05), indicating that they are homogeneous spherical particles. This is contrast to flaming-dominated burning (MCE>0.95) aerosol, which shows a size-dependent ρ_eff for all six different fuels. In this case, the mean and standard deviation of the ρ_eff decreased with increasing size, from 0.94 ± 0.21 g cm^-3 at a mobility diameter of 80 nm to 0.31 ± 0.07 g cm^-3 at a mobility diameter of 400 nm. The size-dependent ρ_eff of flaming-dominated aerosol suggests the fractal nature of freshly emitted aerosol. The relationship between D_fm and the MCE shows three distinct morphology regimes, which we define as the spherical particle regime, the transition regime, and the fractal regime. Our proposed relationship of D_fm with the MCE can be used as a tool to assess the applicability of Mie theory-based optical closure calculations in the absence of particle morphological information. We acknowledge the support from the National Science Foundation grant number NSF-1831013.