The water-soluble organic component of size-segregated aerosol, cloud water and wet depositions from Jeju Island during ACE-Asia

Abstract

Abstract Water-soluble organic compounds (WSOC) in size-segregated aerosol samples, cloud water and wet depositions collected at Jeju Island during the ACE-Asia experiment were characterized in terms of main functional groups and chemical classes by means of liquid chromatography coupled with total organic carbon (TOC) analysis, and by nuclear magnetic resonance (NMR) spectroscopy. The surface-active character of the WSOC was also deduced by measuring the surface tension (ST) of cloud water, wet depositions and aerosol water-extracts as a function of WSOC concentration. WSOC in the size-segregated aerosol samples at the Kosan coastal site show the occurrence of oxidized species, enriched in the accumulation mode, and a functional group composition characteristic of continental industrialized areas, with a small impact from biomass burning sources. The chemical classes identified and quantified by liquid chromatography accounted for 88% of the water-soluble organic carbon on average. The relatively high proportion of polycarboxylic acids (PA, 33–40% of total characterized WSOC) observed throughout the campaign indicates the persistence of WSOC from continental pollution sources in air masses which spent up to 5 days in the marine boundary layer (MBL) of the Yellow Sea. The analysis of cloud water and wet depositions collected at a mountain site in Jeju shows substantial differences in the organic composition between the cloud/rainwater solutes and the water-soluble fraction of the aerosol at the coastal site, with PA acids contributing to cloud water WSOC in far smaller proportions (6–11%) than in the fine MBL aerosols. It was proposed that such differences resulted from the entrainment of aerosol particles from the lifted layers into the stratiform clouds. Following this hypothesis, the saturated aliphatic compounds determined by NMR analysis in the rainwater samples, which are also responsible for the observed significant ST lowering, were scavenged above the boundary layer and had possibly been transported from the continent in the lifted layers. The hypothesized features of the vertical profiles in the inorganic and organic aerosol chemical composition are in agreement with the results of aircraft-based measurements performed during the same experiment.