Long-Term Source Apportionment of Ammonium in PM_2.5 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 (PM_2.5) 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 PM_2.5 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.