Determination of Stable Nitrogen and Oxygen Isotope Ratios in Atmospheric Aerosol Nitrates
Abstract Several conditions of two-step chemical reduction method for simultaneously determination of 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 (NO3−) in the samples was reduced to nitrous oxide (N2O) through the improved method and the produced N2O was measured for their isotope compositions (δ15N and δ18O) by coupling an autosampler and a pre-concentration device with the MAT253 isotope ratio mass spectrometer. The standard curves of the nitrogen (R2 = 0.9999) and oxygen (R2 = 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 ratio 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 μg of N). The standard deviations of δ15N and δ18O were 0.08‰ and 0.24‰, respectively. This 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 understanding emission sources and transformation pathways of atmospheric nitrate.