Stable carbon and nitrogen isotopic compositions of tropical atmospheric aerosols: sources and contribution from burning of C3 and C4 plants to organic aerosols

In this paper, we report for the first time the δ¹³C and δ¹⁵N data for PM_2.5 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 PM_2.5 and 9.2±3.5 µg m⁻³ in PM₁₀. δ¹³C of TC ranged from −26.1 to −20.6‰ with a mean of −23.6‰ in PM_2.5 and from −24.4 to −22.4‰ with a mean of −23.6‰ in PM₁₀. We found substantially greater δ¹³C 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 C₄ 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 PM₁₀. δ¹⁵N 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 PM₁₀. δ¹⁵N 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 N H 4 + (particle). We found a strong correlation between TN and N H 4 + (r ²=0.94 in PM_2.5 and r ²=0.86 in PM₁₀) and N O 3 - (r ²=0.48 in PM_2.5 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 C₃ 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 C₄ ranged from 26 to 58% in PM_2.5 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.