Stable carbon and nitrogen isotopic compositions of tropical atmospheric aerosols: Sources and contribution from burning of c3 and c4 plants to organic aerosols

Abstract

In this paper, we report for the first time the δ13C and δ15N 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 μg m-3 in PM2.5 and ±.2±3.5 μg m-3 in PM10. δ13C of TC ranged from -26.1 to -20.6-with a mean of -23.6-in PM2.5 and from -24.4 to -22.4-with a mean of -23.6-in PM10.We found substantially greater d13C 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 C4 plants to fine organic aerosol formation. TN contents showed a mean of 0.7±0.3 μg m-3 in PM2.5 and 0.8±0.2 μg m-3 in PM10. δ5N ranged from +13.4 to +22.1-with a mean of +16.2±2.7-in PM2.5 and +10.4 to +18.7-with a mean of +13.7±2.2-in PM10. 15N showed higher ratios in fine particles than coarse particles in both wet and dry season. The relatively high δ15N values suggest isotopic enrichment of 15N in aerosols via exchange reaction between NH3 (gas) and NH+4 (particle). We found a strong correlation between TN and NH+4 (r2=0.94 in PM2.5 and r2=0.86 in PM10) and NO-3 (r2=0.48 in PM2.5 and r2=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 C3 plants to TC were estimated to range from 42 to 74% in PM2.5 and from 3± to 64% in PM10, whereas those of C4 ranged from 26 to 58% in PM2.5 and from 36 to 61% in PM10. These results suggest that burning activities of C3 and C4 plants contribute to organic aerosol formation in Tanzania. © 2014 S. L. Mkoma et al.