Source forensics of n-alkanes and n-fatty acids in urban aerosols using compound specific radiocarbon/stable carbon isotopic composition

Abstract

n-Alkanes and fatty acids are important molecular markers for the source apportionment of organic matter in the atmosphere. Traditional approaches to date have mostly relied upon the source-specific differences in their molecular distributions and carbon preference index. Alternatively, we demonstrate here the use of stable carbon and radiocarbon isotopic composition (δ 13C and Δ14C, respectively) of n-alkanes and n-fatty acids in aerosols from two urban receptor sites (Beijing and Tianjin) in Northeast China to assess their sources in autumn. The Δ14C n-alkanes of C19-C24 and C26-C32 even-carbon homologs (-851 to -708‰) indicate their dominance from fossil fuel combustion. In contrast, the Δ14C of most abundant palmitic acid (C16:0) and stearic acid (C18:0) suggest a larger contribution from nonfossil sources (∼91%-94%), mainly due to inputs from cooking, biomass burning and microorganisms. Compared with lower Δ14C of C27 and C31 n-alkanes (-449‰), C29 n-alkane (-241‰) and C20-C30 n-fatty acids (-263‰) showed more contemporary likely due to significant contribution from plant litter and biomass burning that contain more fresh biogenic material. Fossil character of C27-C31 n-alkanes (40%) and C20-C30 n-fatty acids (30%) could be from soil resuspension and/or loess deposits in upwind regions through long-range atmospheric transport.