Impact of combustion conditions on physical and morphological properties of biomass burning aerosol
Rudra P. Pokhrel, Janica Gordon, Marc N. Fiddler, Solomon Bililign
Abstract
The study of biomass burning particle density provides information on aging, new particle formation, transport properties, and is an important parameter in aerosol impacts modeling. Density is used in mass closure techniques to estimate the temporal resolution of particulate mass concentrations. However, the study of BB particle density as a function of burning conditions is still limited. Laboratory measurement of six sub-Saharan African biomass fuels burned under a range of conditions, from pure smoldering to pure flaming conditions, is presented. Smoldering-dominated burning (modified combustion efficiency (MCE) < 0.9) particles has a very narrow range of effective density (ρeff) 1.03 g cm−3 to 1.21 g cm−3 and a mass mobility exponent (Dfm) of ∼3 (2.97 ± 0.05), indicating that they are spherical particles. For the flaming-dominated burning (MCE >0.95) particles, show a size dependent ρeff for all six different fuels. In this case, the mean and standard deviation of the ρeff 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 ρeff of flaming-dominated aerosol suggests the fractal nature of freshly emitted particles. The relationship between Dfm and the MCE shows three distinct morphology regimes, which we define as the spherical particle, the transition, and the fractal regime. Our proposed relationship of Dfm with the MCE can be used as a tool to assess the applicability of Mie theory for optical closure calculations in the absence of particle morphological information.