[Atmospheric Ammonia/Ammonium-nitrogen Concentrations and Wet and Dry Deposition Rates in a Double Rice Region in Subtropical China]

Abstract

Ammonia (NH₃) is the most abundant alkaline gas in the ambient air, and it is also one of the important precursors for the ammonium salts in aerosol and rainwater. Though the emission intensities of NH₃ and acidic gases are high, the concentrations and deposition rates of atmospheric ammonia-nitrogen (NH₃-N), particulate ammonium-nitrogen (NH₄⁺-N_p) and rainwater ammonium-nitrogen (NH₄⁺-N_r) in double rice regions in subtropical China are still less known. In this study, atmospheric concentrations of NH₃-N, NH₄⁺-N_p in PM₁₀ and NH₄⁺-N_r and related meteorological parameters were observed simultaneously in a typical double rice region in the subtropical hilly region of China, with the aim to clarify the characteristics and influencing factors of atmospheric NH₃/NH₄⁺-N concentrations and to quantify the wet and dry deposition rates of atmospheric NH₃/NH₄⁺-N. The results showed that the annual mean concentrations of nitrogen in NH₃-N, NH₄⁺-N_p and NH₄⁺-N_r were 5.7 μg·m^-3, 12.8 μg·m^-3 and 0.8 mg·L^-1, respectively, and their deposition rates were 8.38, 5.61 and 9.07 kg·(hm²·a)^-1, respectively. The NH₃-N concentration was significantly increased after application of nitrogen fertilizer in the paddy field, and had a significant positive correlation with the air temperature. The NH₄⁺-N_p concentration did not show significant correlation with NH₃-N concentration, indicating that atmospheric NH₃-N concentration was not the main limiting factor for the NH₄⁺-N_p pollution in the studied region. The NH₄⁺-N_r concentration was positively correlated with the NH₄⁺-N_p concentration but negatively correlated with precipitation. The high concentrations and deposition rates of atmospheric NH₃-N/NH₄⁺-N in the studied region indicated that the atmospheric NH₃/NH₄⁺-N pollution was serious, and atmospheric deposition of NH₃/NH₄⁺-N was an important nitrogen source in paddy fields, which should be considered in cropland nitrogen nutrient management.

Keywords: aerosol; ammonia; atmospheric deposition; reactive nitrogen; rice.