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Secondary inorganic components significantly contribute to the modification of ambient aerosol properties by forming haze or reducing visibility. This study characterizes the water-soluble components in PM2.5 and explores secondary inorganic aerosol (SIA) over the air in Dhaka, Bangladesh, throughout 2019. PM2.5 samples were collected on a 24 h basis using a high-volume air sampler, and water-soluble inorganic compositions were measured using an ion chromatograph (IC). The observed PM2.5 may pose potential health risks given that their 24 h mean exceeds the ambient air quality guidelines proposed by the World Health Organization (WHO) and the Department of Environment (DoE) of Bangladesh. Among the ions, SO4 2–, Ca2+, and NO3 – were identified as the predominant species that account for 51, 20, and 11% of all soluble components, respectively. The soluble ions in PM2.5 were relatively higher in the summer monsoon (13.26 ± 6.12 μg/m3), possibly due to a combination of rampant anthropogenic activities and the pre-monsoonal meteorology. Humid summer plays a significant role in increasing the amount of SIA through the liquid-phase oxidation of precursor gases. Therefore, scavenging of ions may potentially occur (23% from the overall mean of ions) during the long rainy monsoon season over Dhaka. The anthropogenic origins of PM2.5, such as transportation, industry, and construction dust, are widely present in natural sources all over Dhaka. Dust was more sensitive to enriched PM2.5 than ions from a seawater origin. Excluding winter data, K+ may significantly resuspend from urban dust over Dhaka. The elements and molecular tracer technique reveal that the potential reactive ions (e.g., Cl–, SO4 2–, and NO3 –) were more sensitive to anthropogenic human activities in Dhaka air than to seawater and terrestrial soil. The influence of converting vehicle fleets into compressed natural gas (CNG) run and upgrading kiln technology on increasing SO4 2– aerosol in Dhaka is yet to be investigated.

Rationale There has never been a highly sensitive method for simultaneously measuring the δ15N and δ18O values of nitrate ions (NO3−) and the δ15N 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 δ15N‐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 δ15N‐NO3−, δ18O‐NO3−, and δ15N‐NH4+ values of atmospheric particulate matter (PM2.5, aerodynamic diameter < 2.5 μm). Results The δ15N‐NO3− and δ18O‐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 δ15N‐NH4+ and δ15N‐NO3− values were in all cases less than 1‰. In addition, the accuracies for the δ18O‐NO3− values were less than 1.4‰ and generally acceptable. The δ15N‐NH4+, δ15N‐NO3−, and δ18O‐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.

Ammonia gas (NH 3 ) 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 (δ 15 N−NH 4 + ) in fine particulate matter (PM 2.5 ) were measured at a suburban site and a rural site in Japan. Then, the long-term sources of NH 4 + were identified using the δ 15 N−NH 3 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 NH 3 data collected at the same location. The result showed that the averaged contribution of nonagricultural sources to NH 3 was 39%. This result is reasonable because bottom-up estimates are close to the contribution, and the NH 3 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 PM 2.5 measured at the rural site was derived from the Asian continent. We inferred that the NH 4 + had been formed on the continent and that these particles thus represent transboundary pollution.

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