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In this paper, we report for the first time the δ 13 C and δ 15 N data for PM 2.5 and PM 10 aerosols collected in Tanzania during May–August 2011 together with total carbon (TC) and nitrogen (TN) contents. Mean TC concentrations were 6.5±2.1 µg m −3 in PM 2.5 and 9.2±3.5 µg m −3 in PM 10 . δ 13 C of TC ranged from −26.1 to −20.6‰ with a mean of −23.6‰ in PM 2.5 and from −24.4 to −22.4‰ with a mean of −23.6‰ in PM 10 . We found substantially greater δ 13 C values in PM 2.5 samples during dry season as well as strong positive correlation between levoglucosan (and nss-K + ) and TC in PM 2.5 . These results suggest a significant contribution from burning of C 4 plants to fine organic aerosol formation. TN contents showed a mean of 0.7±0.3 µg m −3 in PM 2.5 and 0.8±0.2 µg m −3 in PM 10 . δ 15 N ranged from +13.4 to +22.1‰ with a mean of +16.2±2.7‰ in PM 2.5 and +10.4 to +18.7‰ with a mean of +13.7±2.2‰ in PM 10 . δ 15 N showed higher ratios in fine particles than coarse particles in both wet and dry season. The relatively high δ 15 N values suggest isotopic enrichment of 15 N in aerosols via exchange reaction between NH 3 (gas) and (particle). We found a strong correlation between TN and ( r 2 =0.94 in PM 2.5 and r 2 =0.86 in PM 10 ) and ( r 2 =0.48 in PM 2.5 and r 2 =0.55 in PM 10 ). We also found that organic nitrogen is less significant than inorganic nitrogen in the Morogoro aerosols. Based on stable carbon isotopic composition, contributions of burning C 3 plants to TC were estimated to range from 42 to 74% in PM 2.5 and from 39 to 64% in PM 10, whereas those of C 4 ranged from 26 to 58% in PM 2.5 and from 36 to 61% in PM 10 . These results suggest that burning activities of C 3 and C 4 plants contribute to organic aerosol formation in Tanzania. Keywords: PM 2.5 and PM 10 , aerosols, isotopic composition of total carbon and nitrogen, C 3 and C 4 plants, Tanzania (Published: 15 January 2014) Citation: Tellus B 2014, 66 , 20176, http://dx.doi.org/10.3402/tellusb.v66.20176