Persistent Nonagricultural and Periodic Agricultural Emissions Dominate Sources of Ammonia in Urban Beijing: Evidence from N Stable Isotope in Vertical Profiles.

Ammonia (NH) emission reduction is key to limiting the deadly PM pollution globally. However, studies of long-term source apportionment of vertical NH are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of N-NH at 5 heights (2, 15, 102, 180, and 320 m) on a 325-m meteorological tower in urban Beijing, we found that vertical profiles of NH concentrations generally remained stable with height. N-NH increased obviously as a function of height in cold seasons (with heating) and decreased in warm seasons (with fertilization), indicating a stronger human-induced seasonal variation via regional transport at higher altitudes. Relatively stable N-NH near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured N-NH only would overestimate the contribution of agricultural emissions to NH. By using an estimation of initial N-NH, we found that nonagricultural sources contributed ∼72% of NH on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured N-NH only likely underestimates fossil fuel source contribution, if the combined NH isotope effects are not considered.