Contributions of biomass/biofuel burning to organic aerosols and particulate matter in Tanzania, East Africa, based on analyses of ionic species, organic and elemental carbon, levoglucosan and mannosan
Abstract. Atmospheric aerosol samples of PM2.5 and PM10 were collected at a rural site in Tanzania, East Africa, in 2011 during wet and dry seasons and were analysed for car- bonaceous components, levoglucosan, mannosan and water- soluble inorganic ions. The contributions of biomass/biofuel burning to the organic carbon (OC) and particulate mat- ter (PM) mass were estimated to be 46–52% and 8–13 %, respectively. The mean mass concentrations of PM2.5 and PM10 were 28±6 µgm−3 and 47±8 µgm−3 in wet sea- son, and 39±10 µgm−3 and 61±19µgm−3 in dry season, respectively. Total carbon (TC) accounted for 16–19% of the PM2.5 mass and 13–15% of the PM10 mass. On av- erage, 86 to 89% of TC in PM2.5 and 87 to 90% of TC in PM10 was OC, of which 67–72% and 63% was found to be water-soluble organic carbon (WSOC) in PM2.5 and PM10, respectively. We found that concentrations of lev- oglucosan and mannosan (specific organic tracers of pyroly- sis of cellulose) well correlated with non-sea-salt potassium (nss-K+) (r2 =0.56–0.75), OC (r2 =0.75–0.96) and WSOC (r2 =0.52–0.78). The K+ /OC ratios varied from 0.06 to 0.36 in PM2.5 and from 0.03 to 0.36 in PM10 with slightly higher ratios in dry season. Mean percent ratios of levoglu- cosan and mannosan toOCwere found to be 3–4%forPM2.5 andPM10 in both seasons.We found lower levoglucosan /K+ ratios and higher K+ /EC (elemental carbon) ratios in the biomass-burning aerosols from Tanzania than those reported from other regions. This feature is consistent with the high levels of potassium reported in the soils of Morogoro, Tan- zania, suggesting an importance of direct emission of potas- sium by soil resuspension although K+ is present mostly in fine particles. It is also likely that biomass burning of veg- etation of Tanzania emits high levels of potassium that may be enriched in plant tissues. The present study demonstrates that emissions from mixed biomass- and biofuel-burning ac- tivities largely influence the air quality in Tanzania.