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Abstract: 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 +. [Copyright &y& Elsevier]