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

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