Stable Carbon And Nitrogen Isotopic Compositions Of Tropical Atmospheric Aerosols: Sources And Contribution From Burning Of C-3 And C-4 Plants To Organic Aerosols

In this paper, we report for the first time the delta C-13 and delta N-15 data for PM2.5 and PM10 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 mu g m(-3) in PM2.5 and 9.2 +/- 3.5 mu g m(-3) in PM10. delta C-13 of TC ranged from -26.1 to -20.6 parts per thousand with a mean of -23.6 parts per thousand in PM2.5 and from -24.4 to -22.4 parts per thousand with a mean of -23.6 parts per thousand in PM10. We found substantially greater delta C-13 values in PM2.5 samples during dry season as well as strong positive correlation between levoglucosan (and nss-K+) and TC in PM2.5. 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 mu g m(-3) in PM2.5 and 0.8 +/- 0.2 mu g m(-3) in PM10. delta N-15 ranged from +13.4 to +22.1 parts per thousand with a mean of +16.2 +/- 2.7 parts per thousand in PM2.5 and +10.4 to +18.7 parts per thousand with a mean of +13.7 +/- 2.2 parts per thousand in PM10. delta N-15 showed higher ratios in fine particles than coarse particles in both wet and dry season. The relatively high delta N-15 values suggest isotopic enrichment of N-15 in aerosols via exchange reaction between NH3 (gas) and NH4+ (particle). We found a strong correlation between TN and NH4+ (r(2) = 0.94 in PM2.5 and r(2) = 0.86 in PM10) and NO3- (r(2) = 0.48 in PM2.5 and r(2) = 0.55 in PM10). 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 PM2.5 and from 39 to 64% in PM10, whereas those of C-4 ranged from 26 to 58% in PM2.5 and from 36 to 61% in PM10. These results suggest that burning activities of C-3 and C-4 plants contribute to organic aerosol formation in Tanzania.