Monthly Variations of the Nitrogen Isotope of Ammonium in Wet Deposition in a Tropical City of South China

Nitrogen isotope of ammonium (delta N-15-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 delta N-15-NH4+ displayed the opposite trend of higher values during the wet season. Comparing the delta N-15-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)(2)SO4- and NH4NO3-enriched N-14 particles, which can be deposited by aerosols, and led to N-15-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 delta N-15-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.