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

Xu-Dong Zheng,Xue‐Yan Liu,Wei Song,Xinchao Sun,Cong‐Qiang Liu

Published 2018 in Environmental Pollution

ABSTRACT

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 PM2.5 (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+.

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