Persistent Nonagricultural and Periodic Agricultural Emissions Dominate Sources of Ammonia in Urban Beijing: Evidence from ¹⁵N Stable Isotope in Vertical Profiles

Abstract

Ammonia (NH₃) emission reduction is key to limiting the deadly PM_2.5 pollution globally. However, studies of long-term source apportionment of vertical NH₃ are relatively limited. On the basis of the one-year measurements of weekly vertical profiles of <i>δ</i>¹⁵N-NH₃ 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 NH₃ concentrations generally remained stable with height. <i>δ</i>¹⁵N-NH₃ 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 <i>δ</i>¹⁵N-NH₃ near the ground surface suggested the strong local emission. The results of isotopic mixing model (SIAR) indicate that source apportionment using measured <i>δ</i>¹⁵N-NH₃ only would overestimate the contribution of agricultural emissions to NH₃. By using an estimation of initial <i>δ</i>¹⁵N-NH₃, we found that nonagricultural sources contributed ∼72% of NH₃ on average. Our study suggests that (i) both persistent nonagricultural and periodic agricultural emissions drive atmospheric NH₃ concentration and its vertical distribution in urban Beijing; and (ii) source apportionment based on measured <i>δ</i>¹⁵N-NH₃ only likely underestimates fossil fuel source contribution, if the combined NH_<i>x</i> isotope effects are not considered.