Atmospheric Chemistry and Physics, vol. 13, issue 20 (2013) pp. 10325-10338
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.
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