Contribution of dissolved organic matter to submicron water-soluble organic aerosols in the marine boundary layer over the eastern equatorial Pacific

摘要

Stable carbon isotopic compositions of water-solubleorganic carbon (WSOC) and organic molecular markers were measured toinvestigate the relative contributions of the sea surface sources to thewater-soluble fraction of submicron organic aerosols collected over theeastern equatorial Pacific during the Tropical Ocean tRoposphere Exchange ofReactive halogens and Oxygenated VOCs (TORERO)/KA-12-01 cruise. On average,the water-soluble organic fraction of the total carbon (TC) mass insubmicron aerosols was  ∼  30–35 % in the oceans with thelow chlorophyll  (Chl ) concentrations, whereas it was  ∼  60 % inthe high-Chl  regions. The average stable carbon isotope ratio of WSOC(C) was −19.8 ± 2.0 ‰,which was systematically higher than that of TC (C)(−21.8 ± 1.4 ‰). We found that in the oceanswith both high and low Chl  concentrations the Cwas close to the typical values of C for dissolved organiccarbon (DOC), ranging from −22 to−20 ‰ in surface seawater of the tropical Pacific Ocean.This suggests an enrichment of marine biological products in WSOC aerosolsin the study region regardless of the oceanic area. In particular, enhancedlevels of WSOC and biogenic organic marker compounds together with highvalues of WSOC / TC ( ∼  60 %) and Cwere observed over upwelling areas and phytoplankton blooms, which wasattributed to planktonic tissues being more enriched in C.The C analysis estimated that, on average, marine sourcescontribute  ∼  90 ± 25 % of the aerosol carbon,indicating the predominance of marine-derived carbon in the submicron WSOC.This conclusion is supported by Lagrangian trajectory analysis, whichsuggests that the majority of the sampling points on the ship had beenexposed to marine boundary layer (MBL) air for more than 80 % of the time duringthe previous 7 days. The combined analysis of the C andmonosaccharides, such as glucose and fructose, demonstrated that DOCconcentration was closely correlated with the concentration levels ofsubmicron WSOC across the study region regardless of the oceanic area. Theresult implies that DOC may characterize background organic aerosols in theMBL over the study region.