Decadal and seasonal distributions of low molecular weight dicarboxylic acids, oxoacids and α-dicarbonyls in the marine aerosols from the western North Pacific

Oxalic, malonic and other dicarboxylic acids comprise an important fraction of water-soluble organic aerosols in various environments. A rapid industrial development in China and East Asian countries for last two decades may have caused a serious change in the air quality and atmospheric composition over the North Pacific. To better understand long-term atmospheric changes in the western North Pacific, we collected marine aerosol samples in 2001-2014 on weekly basis at a remote island, Chichijima (27°04'E; 142°13'N), which is located in the boundary of westerly and easterly wind regimes. Here we present seasonal and decadal distributions of dicarboxylic acids (C2-C11), oxoacids (C2-C9) and α-dicarbonyls (C2-C3), which were determined using gas chromatography (GC) and GC/MS techniques after BF3/n-butanol derivatization. We also measured stable carbon isotopic composition (d13C) of dicarboxylic acids and related compounds using a GC/IR/MS technique. Concentrations of total diacids fluctuated significantly in a range of 10-600 ngm-3 with winter/spring maxima and summer minima. The maximum concentrations in winter/spring can be explained by a combination of enhanced emissions of polluted aerosols and their precursors in the Asian Continent and enhanced atmospheric transport to the North Pacific due to the intensified westerly winds in winter/spring. The concentrations of diacids seemed to increase from 2001 to 2008 and then decrease toward recent years. Recent decline of diacids concentrations may be associated with the changes in the anthropogenic emissions due to the air quality improvement in Asian countries. Stable carbon isotopic compositions were determined for individual water-soluble diacids using GC/combustion/isotope ratio monitoring MS. The results for 2006 showed relatively high d13C values of oxalic acid (-22‰ to -4‰). The isotopic ratios were found to increase from winter to summer. Significant enrichment of 13C in oxalic acid should be associated with photochemical aging of organic aerosols in the marine atmosphere. It may be caused by kinetic isotopic fractionation during the photo degradation of oxalic acid in the presence of Fe (III), and/or the gas/particle exchange reaction of glyoxal and glyoxylic acid, potential sources of oxalic acid. We will discuss the decadal changes in the stable carbon composition of oxalic acid in relation to the atmospheric oxidation capability in the western North Pacific.