Abstract
ABSTRACT Nitrogen isotope of ammonium (δ15N-NH4+) in the wet deposition in Zhanjiang, a typical tropical city in the southernmost region of mainland China, were analyzed from October 2015 to November 2018 in order to examine the monthly variations and identify the sources of ammonia. The NH4+ exhibited higher concentrations during the dry season than the wet one, whereas the δ15N-NH4+ displayed the opposite trend of higher values during the wet season. Comparing the δ15N-NH4+ and the weather parameters (e.g., rainfall, temperature and duration of sunshine), we found the change in the duration of sunshine to be primarily responsible for the observed temporal isotopic variation. During the wet season, a significantly longer duration increased the opportunities for photooxidation and enhanced the formation of free radicals, which resulted in larger amounts of sulfur dioxide (SO2) and nitrogen oxide (NOx) being transformed into sulfuric acid (H2SO4) and HNO3. This process accelerated the unidirectional conversion of NH3 into (NH4)2SO4- and NH4NO3-enriched 14N particles, which can be deposited by aerosols, and led to 15N-enriched residual NH3 being present in the atmosphere; this NH3 was then scavenged by precipitation and released as NH4+ during rainfall . By contrast, less isotopic fractionation occurred during the shorter sunshine duration of the dry season, suggesting that δ15N-NH4+ in the precipitation should be similar to those of NH3 in the atmosphere and can therefore be applied in source apportionment. A Bayesian isotope mixing model demonstrated that volatilization contributed less (18 ± 21% and 19 ± 20% from animal waste and fertilizer, respectively) than combustion (28 ± 26%, 24 ± 26% and 11 ± 5% from coal combustion, vehicle exhaust and biomass burning) to the concentration of NH4+ in Zhanjiang’s precipitation.
Highlights
Nitrogen (N) is an important element in the ecosystem (Galloway et al, 2008; Xiao et al, 2012)
The lack of NH4+ from (NH4)+ concentrations and δ15N-NH4+ during February 16, December 16, January 17 and December 17 is because very little (< 0.5 mm) or no rainfall occurred during these periods
Considering only the annual averages, we find that there is no significant trend in the variation of NH4+ concentration
Summary
Nitrogen (N) is an important element in the ecosystem (Galloway et al, 2008; Xiao et al, 2012). Sunshine duration influences δ15N-NH4+ in precipitation because free radicals and photooxidation can produce sulfur dioxide (SO2) and nitrogen oxide (NOx) to acidic matter (H2SO4 and HNO3) in the atmosphere, which accelerates the transition of NH3 (gas phase) into (NH4)2SO4 and NH4NO3 (particulate phase) by unidirectional reactions for isotopically enriched 14N (Xiao et al, 2012). This process results in 15N-enriched residual NH3 (NH4+) in wet deposition. We strived to identify the main factors that control seasonal variations of δ15N-NH4+ and to elucidate the different sources of NH4+ in wet deposition in such tropical areas
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