Water-soluble organic nitrogen in the ambient aerosols and its contribution to the dry deposition of fixed nitrogen species in Japan

Measurements of the water-soluble organic nitrogen (WSON) in the aerosols were conducted over three years at two sites; an urban site in Kofu and a forested site in Fujiyoshida, Japan. Our preliminary experiment demonstrated that the adsorption of organic gases on the filter had little effect on the measurement of the WSON. The mean concentration of the WSON in the aerosols at the urban site was 0.221 μg m−3, which was higher than that of 0.101 μg m−3 at the forested site. Large portions of the WSON were found in the fine-mode range; 90.3% at the urban site and 86.4% at the forested site. The WSON constituted a significant fraction of the water-soluble total nitrogen (WSTN) in the aerosols; 11.1% and 16.2% of the WSTN in the coarse and fine particles, respectively, at the urban site, and 11.5% and 13.1% in the coarse and fine particles, respectively, at the forested site. The fine-mode WSON would be derived from the reaction of its basic precursor gases with particulate or gaseous acidic species. Photochemical reactions and combustion emissions could also be important sources of the fine-mode WSON. In the coarse-mode range, on the other hand, vegetation sources could be an important source of the WSON. The mean concentration of urea in the aerosols at the urban site was 1.7 ngN m−3. A large portion (87.2%) of the urea was partitioned into the fine-mode range. Urea is a minor compound in the particulate WSON in this region with contributions to about 1% of the WSON. Approximately 26% of the dry deposition of the WSTN was attributed to the WSON in the cold season. The significant part (96.5%) of the WSON was deposited by coarse aerosols. Coarse-mode WSON is important for discussing the dry deposition fluxes of the WSON and fixed nitrogen.