Simultaneous measurement of the water-soluble organic nitrogen in the gas phase and aerosols at a forested site in Japan

Abstract

A simultaneous observation of the water-soluble organic nitrogen (WSON) in the gaseous and particulate phases was conducted at a forested site in Japan for two years to clarify the generation mechanism and possible sources of the gaseous and particulate WSON. The concentration of the WSON in the aerosols was 0.070 μg m−3 on average, ranging from the concentration below the detection limit (DL) to 0.302 μg m−3. A large portion (about 73%) of the particulate WSON was distributed in the fine-mode range (d < 2.0 μm). The concentration of the gaseous WSON was 0.041 μg m−3 on average, ranging from the concentration below the DL to 0.195 μg m−3. About 37% of the atmospheric WSON was distributed in the gas phase, and about 59% of the gaseous WSON was basic. The coarse-mode (d = 2.0–10 μm) WSON showed an unclear seasonal trend, but increased in the summer. Its concentration showed a significant correlation only with the WSOC concentration among the measured species in the coarse aerosols. Suspended organic materials from primary sources, such as plant debris, would be a possible source for the coarse-mode WSON. The WSON in the fine-mode range showed a clear seasonality with enhanced concentrations from the spring to summer. The fine-mode WSON concentration showed a strong correlation with the acidity of the fine particles and the concentration of the gaseous basic WSON, suggesting the secondary production of the WSON in the acid fine particles from the gaseous basic WSON. The basic WSON in the gas phase showed a similar seasonal trend with the gaseous ammonia and strongly correlated with the temperature, which suggests the temperature dependent-volatilization of the gaseous basic WSON species from terrestrial vegetation and/or soil. The seasonal trend of the gaseous acid WSON was unclear, and its concentration did not show significant correlations with any parameters. The possible source and chemical forms of the gaseous acid WSON cannot be deduced at the present time.