Nitrogen isotope variations of ammonium across rain events: Implications for different scavenging between ammonia and...

Enhanced ammonia (NH₃) emissions and deposition caused negative effects on air quality and ecosystems. Precipitation is an efficient pathway to remove NH₃ and particulate ammonium (p-NH₄⁺) from the atmosphere into ecosystems. However, precipitation scavenging of p-NH₄⁺ in chemical transport models has often considered fine p-NH₄⁺, with inadequate constraints on NH₃ and coarse p-NH₄⁺. Based on distinct δ¹⁵N values between NH₃ and NH₄⁺ in PM_2.5 (particulate matters with aerodynamic diameters ≤ 2.5 μm) or TSP (total suspended particulates), this paper interpreted intra-event variations of precipitation NH₄⁺ concentrations and δ¹⁵N values (δ¹⁵N-NH₄⁺ values) at Guiyang (Xiao et al., 2015). Generally decreased NH₄⁺ concentrations across rain events reflected decreasing scavenging of NH₃ and p-NH₄⁺. Using a Bayesian isotope mixing model, we found that differing contributions between ¹⁵N-depleted NH₃ and ¹⁵N-enriched p-NH₄⁺ were responsible for the three-stage variations of intra-event δ¹⁵N-NH₄⁺ values. The decreases of δ¹⁵N-NH₄⁺ values across the first and third stages indicated more decreases in scavenging p-NH₄⁺ than NH₃, while the increases of δ¹⁵N-NH₄⁺ values across the second stage were resulted primarily from more increases in scavenging p-NH₄⁺ (particularly fine p-NH₄⁺) than NH₃. These results stressed influences of differing scavenging between NH₃ and p-NH₄⁺ on precipitation δ¹⁵N-NH₄⁺ values, which should be considered in modeling precipitation scavenging of atmospheric p-NH₄⁺.