Compound-specific stable carbon isotope ratios of terrestrial biomarkers in urban aerosols from Beijing, China

Molecular compositions and stable carbon isotope ratios (δ13C) of n-alkanes and fatty acids (FA) were investigated in urban aerosols from Beijing, China. Seasonal trends for n-alkanes showed a higher carbon preference index (CPI) and Cmax at C29 in spring indicating the enhanced emission of higher plant waxes. Lower CPIs and Cmax at C23 were found in winter that should be associated with fossil fuel combustion. 13C-depletion in C29n-alkane was observed during an extreme rainfall event, during which the inflow of air masses were originated from lower latitude regions. Higher concentrations of 13C-enriched C23n-alkane in winter together with the air mass trajectories suggested that the Beijing aerosols were affected by the air masses transported from the southern regions. Fatty acids were abundant in fall and showed a bimodal pattern with Cmax at C16:0 and C18:0. Higher C24:0/FA ratios with 13C-enrichment occurred as influenced by southerly air masses, and lower C24:0/FA ratios with 13C-depletion was observed with northerly air masses in fall and winter. These results suggested that C24:0 fatty acid was more abundantly transported from the southwestern air masses and may be a powerful tool to distinguish polluted aerosol source regions. Moreover, the δ13C values of C24:0 fatty acid showed a positive correlation with levoglucosan, implying that the biomass-burning process may contribute 13C-enriched C24:0 fatty acid