Determination of Stable Nitrogen and Oxygen Isotope Ratios in Atmospheric Aerosol Nitrates

Several conditions of the two-step chemical reduction method for simultaneous determination of both the stable nitrogen and oxygen isotope compositions in atmospheric nitrate ( NO3- ) particles, such as the pH, reaction time, type of cadmium powder and concentration of NaCl were optimized. The nitrite in the samples was reduced to nitrous oxide ( N2O) through the improved method and the produced N2O was measured for their isotope compositions (delta N-15 and delta O-18) by coupling an autosampler and a pre-concentration device with the MAT253 isotope ratio mass spectrometer. The standard curves of nitrogen ( R-2 = 0. 9999) and oxygen ( R-2 = 0. 9996) isotope ratios of N2O and NO3- were established by measuring the international standards with known isotope ratios, and the nitrogen and oxygen isotope ratios of nitrates of the sample could be therefore converted. The principle of this method was clear, and only required very small amounts of samples (i. e., 0. 8 mu g N). The standard deviations of delta N-15 and delta O-18 measurements from the present method were 0. 08 parts per thousand and 0. 24 parts per thousand, respectively. This research method was not only suitable for atmospheric aerosol samples, but also for the other environmental samples such as rainwater and snow samples with low concentrations. This study provided a novel method for better understand emission sources and transformation pathways of atmospheric nitrate.