环境地球化学国家重点实验室知识库

Nitrogen isotopes of ammonium (δ¹⁵N–NH₄⁺) in precipitation have been analyzed, to study their monthly variations, from Oct. 2008 to Sep. 2009 at Guiyang, Southwest China. δ¹⁵N–NH₄⁺ values were higher in winter and lower in summer showing a strong sine curve. In summer (rainy season), raindrop may remove more light ammonia by washout process, because raindrop incorporated ¹⁵NH₃ preferentially while left the ¹⁴N in the atmosphere. At the same time, longer sunshine times imply more hv for producing H₂SO₄ of SO₂ to H₂SO₄, which accelerates gaseous to particle conversion of NH₃ to (NH₄)₂SO₄ by unidirectional reactions for isotopic enriched ¹⁴N. The above two aspects can somewhat cause the seasonal variation of nitrogen isotopic composition, but are not the main or direct reasons. The temperature has an opposite trend with seasonal variation of δ¹⁵N values. The temperature not only causes seasonal variation of δ¹⁵N values, but also increases the volatilization rate of NH₃ and microbial activities. And there is a robust linear relationship between temperature and δ¹⁵N, showing that the temperature is the main factor to decide the monthly variation of δ¹⁵N–NH₄⁺.