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

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 (C_16:0) and stearic acid (C_18:0) 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. (letter)