Airborne studies of aerosol emissions from savanna fires in southern Africa: 2. Aerosol chemical composition

We investigated smoke emissions from fires in savanna, forest, and agricult ural ecosystems by airborne sampling of plumes close to prescribed burns an d incidental fires in southern Africa. Aerosol samples were collected on gl ass fiber filters and on stacked filter units, consisting of a Nuclepore pr efilter for particles larger than similar to 1-2 mu m and a Teflon second f ilter stage for the submicron fraction. The samples were analyzed for solub le ionic components, organic carbon, and black carbon. Onboard the research aircraft, particle number and volume distributions as a function of size w ere determined with a laser-optical particle counter and the black carbon c ontent of the aerosol with an aethalometer. We determined the emission rati os (relative to CO2 and CO) and emission factors (relative to the amount of biomass burnt) for the various aerosol constituents. The smoke aerosols we re rich in organic and black carbon, the latter representing 10-30% of the aerosol mass. K+ and NH4+ were the dominant cationic species in the smoke o f most fires, while Cl- and SO42- were the most important anions. The aeros ols were unusually rich in Cl-, probably due to the high Cl content of the semiarid vegetation. Comparison of the element budget of the fuel before an d after the fires shows that the fraction of the elements released during c ombustion is highly variable between elements. In the case of the halogen e lements, almost the entire amount released during the fire is present in th e aerosol phase, while in the case of C, N, and S, only a small proportion ends up as particulate matter. This suggests that the latter elements are p resent predominantly as gaseous species in the fresh fire plumes studied he re.