Compound specific isotope analysis of fatty acids and polycyclic aromatic hydrocarbons in aerosols: implications for biomass burning

Fatty acids and polycyclic aromatic hydrocarbons (PAH) have been investigated as potential tracer species for the products of biomass burning. Fatty acids extracted from unburned sugar cane plants and from particulate aerosols collected during laboratory burns of sugar cane under smoldering and flaming conditions have been chemically and isotopically characterized by gas chromatography-mass spectrometry (GC---MS) and gas chromatography-isotope ratio mass spectrometry (GC---IRMS), respectively. Fatty acids and PAH produced during the burning of a sugar cane field in South Africa were similarly characterized. The fatty acids identified in the aerosols collected above the fire were saturated even-chain species ranging from C12 to C22. The carbon isotopic signatures of the fatty acids ranged between −19.9‰ and −23.6‰, and were more depleted in 13C than the bulk sugar cane plant (−12.9‰) and the total lipid extract (−17.9‰). The isotopic signatures of the individual fatty acids were conserved during the smoldering laboratory burn. However, the fatty acids collected during the flaming burn showed a depletion of 1‰ to 6‰ relative to the fatty acids extracted from the unburned plant. This observed depletion was even greater for the fatty acids obtained from the sugar cane field burn. Low levels of various PAH were identified in aerosols from the laboratory burns. Phenanthrene, fluoranthene and pyrene obtained from the field burn aerosols were isotopically depleted relative to the bulk lipid material, with carbon isotopic signatures ranging from −22.9‰ to −25.4‰. The alterations in the isotopic compositions of fatty acids that occur during combustion provide variables by which burn-derived compounds can be distinguished from those produced from aeolian transport of detrital vegetative matter. The combination of fatty acid isotopic data and PAH data may allow a better understanding of the relative contributions of biogenic and anthropogenic source materials to aerosols.