Long-Term Source Apportionment of Ammonium in PM 2.5 at a Suburban and a Rural Site Using Stable Nitrogen Isotopes.

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.