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

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 delta N-15 values between NH3 and NH4+ in PM2.5 (particulate matters with aerodynamic diameters <= 2.5 mu m) or TSP (total suspended particulates), this paper interpreted intra-event variations of precipitation NH4+ concentrations and delta N-15 values (delta N-15-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 N-15-depleted NH3 and N-15-enriched p-NH4+ were responsible for the three-stage variations of intra-event delta N-15-NH4+ values. The decreases of delta N-15-NH4+ values across the first and third stages indicated more decreases in scavenging p-NHZ than NH3, while the increases of delta N-15-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-NI-14- on precipitation delta N-15-NH4+ values, which should be considered in modeling precipitation scavenging of atmospheric p-NH4+. (C) 2018 Elsevier Ltd. All rights reserved.