Biomass burning is a major source of atmospheric aerosol, but their role in global climate change is poorly understood. The uncertainty of their climate forcing results from uncertainty of their optical and chemical properties, which are influenced by burning conditions, relative humidity, fuel type, morphology etc. Biomass burning (BB) is also a major source of pollutants that impact local, regional, and global air quality, and public health. However, the influence of burning conditions and fuel type on emission factors of pollutants is still not well understood. We investigated the relationship between morphology (fractal dimensions) and modified combustion efficiency (MCE). We measured emission factors of pollutants from six different sub-Saharan African biomass fuels combusted under a wide range of burning conditions (MCE’s). Enhancement in light extinction coefficient (fRH) was estimated based on the ratio of extinction coefficient measured at relative humidity between 78-92% compared to those below 78% using cavity ring-down spectroscopy (CRDS). Cloud condensation nuclei (CCN) activity was measured at four different supersaturations between 0.4-1.1%.
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. We found that particulate matter (PM) and carbon monoxide (CO) emission factors (EF; g (kg wood)-1 ) are highly sensitive to the burning conditions, with an order of magnitude variation between flaming and smoldering burning conditions. Nitric oxide (NO) EF shows a fuel type dependence, with higher NO EF for fuels with larger nitrogen content. While CO is not generally, a proxy for PM2.5 emissions, in this work a correlation was found between CO and PM emissions generated by combustion of seven wood fuels with 10% moisture content (dry basis). Finally, we discuss preliminary comparison of measurement of f(RH) hygroscopicity and CCN hygroscopicity and factors that can modify agreements in estimated hygroscopicity values. This study adds to the growing knowledge in African fuel sources and their varied impact on climate and air quality.
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