Nitrogen isotope composition of ammonium in PM2.5 in the Xiamen, China: impact of non-agricultural ammonia

Since NH 3 is a significant precursor to ammonium in PM 2.5 and contributes significantly to atmospheric nitrogen deposition but largely remains unregulated in China, the insight into the source of NH 3 emissions by the isotopic investigation is important in controlling NH 3 emissions. In this study, atmospheric concentrations of NH 3 and water-soluble ion composition in PM 2.5 as well as nitrogen isotope ratios in NH 4 + (δ 15 N-NH 4 + ) in Xiamen, China, were measured. Results showed that average NH 3 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 PM 2.5 concentration for the two sites (urban and suburban) was 59.2 μg m −3 with lowest values in summer. In the PM 2.5 , NH 4 + concentrations were much lower than NH 3 and showed a stronger positive correlation with NO 3 − than that with SO 4 2− suggesting the formation of NH 4 NO 3 and equilibrium between NH 3 and NH 4 + . Although the concentrations of NH 3 at the urban site were significantly higher than those at the suburban site, no significant spatial difference in NH 4 + and δ 15 N-NH 4 + was obtained. The distinct heavier δ 15 N-NH 4 + values in summer than in other seasons correlated well with the equilibrium isotopic effects between NH 3 and NH 4 + which depend on temperature. The initial δ 15 N-NH 3 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 NH 3 emissions (fossil fuel combustion and NH 3 slip) contributed more than 70% to aerosol NH 4 + . This finding suggested that the reduction of NH 3 emissions from urban transportation and coal combustion should be a priority in the abatement of PM 2.5 pollution in Xiamen.