Long-Term Source Apportionment of Ammonium in PM2.5 at a Suburban and a Rural Site Using Stable Nitrogen Isotopes.

Ammonia gas (NH3) is an important alkaline air pollutant and a precursor to particulate matter, and its source has been thought to be agricultural, but in recent years, nonagricultural sources have been suspected. In this study, stable nitrogen isotope ratios of ammonium (δ15N-NH4+) in fine particulate matter (PM2.5) were measured at a suburban site and a rural site in Japan. Then, the long-term sources of NH4+ were identified using the δ15N-NH3 and an isotopic mixing model. The results showed that the averaged contribution from nonagricultural sources was 67% at the suburban site and 78% at the rural site. We also reanalyzed NH3 data collected at the same location. The result showed that the averaged contribution of nonagricultural sources to NH3 was 39%. This result is reasonable because bottom-up estimates are close to the contribution, and the NH3 emissions are affected by warm season activities in the rural site. It was first found that the sources vary greatly, depending on the gas and particles. Back-trajectory results suggested that PM2.5 measured at the rural site was derived from the Asian continent. We inferred that the NH4+ had been formed on the continent and that these particles thus represent transboundary pollution.

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

Ammonia (NH3) is taken up by fine particulate matter (PM2.5), 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 PM2.5 during the monsoon season in Dhaka, Bangladesh, the most polluted city in the world, and analyzed the δ15N-NH4+ in PM2.5. 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 PM2.5 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.

Ion-exchange resin and denitrification pretreatment for determining δ15 N-NH4 + , δ15 N-NO3 - , and δ18 O-NO3 - values.

RATIONALE: There has never been a highly sensitive method for simultaneously measuring the δ15 N and δ18 O values of nitrate ions (NO3 - ) and the δ15 N values of ammonium ions (NH4 + ) in particulate matter using denitrifying bacteria. In this study, we explored a method that combines use of an anion-exchange resin and denitrifying bacteria to make such measurements. METHODS: The δ15 N-NH4 + values of samples obtained using the hypobromite and denitrifying bacteria method were measured by isotope ratio mass spectrometry. Tests (effect of flow rate, breakthrough, and acid concentration) were conducted to verify the removal of NO3 - using an AG1-X8 anion-exchange resin for NH4 + measurements and the enrichment of NO3 - . For aerosol samples, the optimized method was used to measure the δ15 N-NO3 - , δ18 O-NO3 - , and δ15 N-NH4 + values of atmospheric particulate matter (PM2.5 , aerodynamic diameter < 2.5 µm). RESULTS: The δ15 N-NO3 - and δ18 O-NO3 - values measured following extraction with 1-6 mol/L HCl, at sample flow rates of 1-2 mL/min, with total anion amounts of less than 2.2 mmol, and in concentration tests were found to be in very close agreement with reagent values. The precisions and the accuracies of the δ15 N-NH4 + and δ15 N-NO3 - values were in all cases less than 1‰. In addition, the accuracies for the δ18 O-NO3 - values were less than 1.4‰ and generally acceptable. The δ15 N-NH4 + , δ15 N-NO3 - , and δ18 O-NO3 - values in six PM2.5 samples were similar to those reported in previous studies. CONCLUSIONS: Our proposed method for removing anions using AG1-X8 resin, for isotopic analysis using denitrifying bacteria, and for concentrating samples containing low concentrations of NO3 - will make it possible to perform high-precision and accurate analyses easily and inexpensively. These methods are applicable not only to aerosols, but also to samples from diverse locations such as rivers, oceans, and Antarctica.