Origins of free and combined amino acids in the aerosols at an inland urban site in Japan

Abstract

A year-round observation of the free amino acids (FAA) and combined amino acids (CAA) in coarse and fine aerosols were conducted at an urban site in Japan. The averaged concentrations of the total FAA and total CAA measured in this study were 149.9 and 237.6 pmolN m−3, respectively, in coarse aerosols, and 177.9 and 562.2 pmolN m−3, respectively, in fine aerosols. The concentrations of the total CAA were higher than those of the total FAA in both mode ranges. The CAA were mainly partitioned into the fine-mode range, whereas the FAA were contained in both mode ranges with a similar concentration level. Only 8.1% of the water-soluble organic nitrogen (WSON) in fine aerosols was explained by these amino acids, although a significant contribution (26.4%) of them to the WSON was found in coarse aerosols. Correlation and factor analyses including these amino acids and other chemical species in the aerosols and gas phase were conducted to discuss potential sources of these amino acids. Plant debris and their associated materials, sea-salt, and Asian dust particles would be important potential sources of the coarse-mode FAA. The most dominant FAA in coarse aerosols was serine that is considered to be derived from sea-salt and Asian dust particles, whereas the other major FAA in coarse aerosols would be from plant debris and associated materials. The coarse-mode CAA can be also considered to be mainly from plant debris and associated materials. Glutamic acid and aspartic acid were dominant in the coarse-mode CAA, and estimated to be from these sources. Significant parts of the fine-mode FAA would be derived from the photochemical aging of the fine-mode CAA in polluted air masses from the Asian continent. Glycine and arginine were dominant in the fine-mode FAA, and estimated to be from the photochemical aging process. The fine-mode CAA is considered to be mainly from both the fossil fuel and biomass burning particles from both the Asian continental and local sources. Glutamic acid and glycine were dominant in the fine-mode CAA, and would be mainly from these sources, especially the Asian continental sources. Serine that is the most dominant compound in the fine-mode CAA, however, is considered to be derived from other unidentified sources.