Latitudinal distribution of terrestrial lipid biomarkers and n-alkane compound-specific stable carbon isotope ratios in the atmosphere over the western Pacific and Southern Ocean

Abstract

We investigated the latitudinal changes in atmospheric transport of organic matter to the western Pacific and Southern Ocean (27.58°N–64.70°S). Molecular distributions of lipid compound classes (homologous series of C 15 to C 35 n-alkanes, C 8 to C 34 n-alkanoic acids, C 12 to C 30 n-alkanols) and compound-specific stable isotopes (δ 13C of C 29 and C 31 n-alkanes) were measured in marine aerosol filter samples collected during a cruise by the R/V Hakuho Maru. The geographical source areas for each sample were estimated from air-mass back-trajectory computations. Concentrations of TC and lipid compound classes were several orders of magnitude lower than observations from urban sites in Asia. A stronger signature of terrestrial higher plant inputs was apparent in three samples collected under conditions of strong terrestrial winds. Unresolved complex mixtures (UCM) showed increasing values in the North Pacific, highlighting the influence of the plume of polluted air exported from East Asia. n-Alkane average chain length (ACL) distribution had two clusters, with samples showing a relation to latitude between 28°N and 47°S (highest ACL values in the tropics), whilst a subset of southern samples had anomalously high ACL values. Compound-specific carbon isotopic analysis of the C 29 (−25.6‰ to −34.5‰) and C 31 n-alkanes (−28.3‰ to −37‰) revealed heavier δ 13C values in the northern latitudes with a transition to lighter values in the Southern Ocean. By comparing the isotopic measurements with back-trajectory analysis it was generally possible to discriminate between different source areas. The terrestrial vegetation source for a subset of the southernmost Southern Ocean is enigmatic; the back-trajectories indicate eastern Antarctica as the only intercepted terrestrial source area. These samples may represent a southern hemisphere background of well mixed and very long range transported higher plant organic material.