Nitrogen isotope composition of ammonium in PM2.5 in the Xiamen, China: impact of non-agricultural ammonia
2019
Wu, Shui-Ping | Zhu, Heng | Liu, Zhe | Dai, Lu-Hong | Zhang, Ning | Schwab, James J. | Yuan, Chung-Shin | Yan, Jin-Pei
Since NH₃ is a significant precursor to ammonium in PM₂.₅ and contributes significantly to atmospheric nitrogen deposition but largely remains unregulated in China, the insight into the source of NH₃ emissions by the isotopic investigation is important in controlling NH₃ emissions. In this study, atmospheric concentrations of NH₃ and water-soluble ion composition in PM₂.₅ as well as nitrogen isotope ratios in NH₄⁺ (δ¹⁵N-NH₄⁺) in Xiamen, China, were measured. Results showed that average NH₃ concentration for the five sites in Xiamen was 7.9 μg m⁻³ with distinct higher values in the warm season and lower values in the cold season, and PM₂.₅ concentration for the two sites (urban and suburban) was 59.2 μg m⁻³ with lowest values in summer. In the PM₂.₅, NH₄⁺ concentrations were much lower than NH₃ and showed a stronger positive correlation with NO₃⁻ than that with SO₄²⁻ suggesting the formation of NH₄NO₃ and equilibrium between NH₃ and NH₄⁺. Although the concentrations of NH₃ at the urban site were significantly higher than those at the suburban site, no significant spatial difference in NH₄⁺ and δ¹⁵N-NH₄⁺ was obtained. The distinct heavier δ¹⁵N-NH₄⁺ values in summer than in other seasons correlated well with the equilibrium isotopic effects between NH₃ and NH₄⁺ which depend on temperature. The initial δ¹⁵N-NH₃ values were in the range of waste treatment (− 25.42‰) and fossil fuel combustion (− 2.5‰) after accounting for the isotope fractionation. The stable isotope mixing model showed that fossil fuel–related NH₃ emissions (fossil fuel combustion and NH₃ slip) contributed more than 70% to aerosol NH₄⁺. This finding suggested that the reduction of NH₃ emissions from urban transportation and coal combustion should be a priority in the abatement of PM₂.₅ pollution in Xiamen.
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