Sources identification of ammonium in PM2.5 during monsoon season in Dhaka, Bangladesh

Ammonia (NH₃) is taken up by fine particulate matter (PM_2.5), and there are concerns about its impact on the environment and health. The source of NH₃, which was thought to be of agricultural sources, has recently been suspected to be non-agricultural sources in urban areas. Here, we collected PM_2.5 during the monsoon season in Dhaka, Bangladesh, the most polluted city in the world, and analyzed the δ¹⁵N-NH₄⁺ in PM_2.5. As the result, the δ¹⁵N-NH₄⁺ ranged from 9.2 ‰ to 34.4 ‰ (average: 20.7 ± 4.8 ‰), the highest of any of the averaged values annual reported in previous researches. In order to perform source analysis, the NH₃ concentrations were estimated using the thermodynamic model ISORROPIA-II. The estimated concentration of NH₃ gas averaged 40.8 μg/m³ (3.0-154.6 μg/m³). The contributions calculated with the mixing model to the δ¹⁵N-NH₄⁺ values in PM_2.5 in Dhaka, Bangladesh averaged 25.3 ± 14 %, 22.8 ± 10 %, 26.5 ± 15 %, and 25.4 ± 10 % for waste, fertilizer, NH₃ slip, and fossil fuel combustion, respectively. Non-agricultural sources (NH₃ slip, and fossil fuel combustion) accounted for almost half (51.9 %) of the contributions. In addition, the several validation tests of the isotope mixing model were also performed. For validating the uncorrected and corrected source data for δ¹⁵N-NH₃, the contribution of non-agricultural sources with uncorrected source data would have been very high (>80 %), much higher than the corrected source data.

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

Since NH₃ 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 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 PM_2.5 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 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, NH₄⁺ concentrations were much lower than NH₃ and showed a stronger positive correlation with NO₃^- than that with SO₄^2- 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 PM_2.5 pollution in Xiamen.