Sources of nitrogen and sulfur in wet deposition at Guiyang, southwest China

Whilst limited information on nitrogen and sulfur isotopic composition in wet deposition in cities is available, very few data on the sources of sulfur and nitrogen in both light rainfalls (<5 mm in precipitation and <4 h in duration) and heavy rainfalls (>100 mm in precipitation and <24 h in duration, caused by typhoon from the Pacific Ocean) have been accumulated. For both species a significant difference in mean sulfur and nitrogen isotopic composition was reported in this study between samples collected from light rainfalls (−4.9±2.8‰ for sulfate and +2.0±4.4‰ for nitrate) and heavy rainfalls (+4.6±5.0‰ for sulfate and +4.1±4.0‰ for nitrate), supporting that sulfur and nitrogen in the two types of rainfalls are of different sources. For sulfate, the negative δ34S values in light rainfalls are associated with local sources (coal combustion and biogenic sulfur) while sulfur in heavy rainfalls, with positive δ34S values, is of maritime origin. For nitrate, there is no definite information on the sources of nitrogen in light rainfalls because of a wide range of δ15N values (−3.8‰ to +10.7‰), but the higher δ15N values (>+6.0‰) are probably ascribed to dry deposition or coal-fired power plants; the mean δ15N value of +4.1‰ in heavy rainfalls may also be a reflection of maritime origin. Measurements of δ15N values for ammonium in light rainfalls show that δ15N values are closely related with ammonium concentrations (R2=0.92). The lower values in the range of δ15N values (−1.7‰ to −22.0‰) for ammonium are due to less incorporation of 15N by cloud water, which probably has a δ15N value of about −22.0‰. But the higher concentrations (averaging 1.25 mg l−1) and lower δ15N values (averaging −12.2±6.7‰) indicated that ammonium steps from excretory wastes are extensively applied in agriculture and NH3 released from soils.