Low blank sampling method for measurement of the nitrogen isotopic composition of atmospheric NOx.

The nitrogen isotopic composition of nitrogen oxide (NO_x) is useful for estimating its sources and sinks. Several methods have been developed to convert atmospheric nitric oxide (NO) and/or nitrogen dioxide (NO₂) to nitrites and/or nitrates for collection. However, the collection efficiency and blanks are poorly evaluated for many collection methods. Here, we present a method for collecting ambient NO_x (NO and NO₂ simultaneously) with over 90% efficiency collection of NO_x and low blank (approximately 0.5 μM) using a 3 wt% hydrogen peroxide (H₂O₂) and 0.5 M sodium hydride (NaOH) solution. The 1σ uncertainty of the nitrogen isotopic composition was ± 1.2 ‰. The advantages of this method include its portability, simplicity, and the ability to collect the required amount of sample to analyze the nitrogen isotopic composition of ambient NO_x in a short period of time. Using this method, we observed the nitrogen isotopic compositions of NO_x at the Tsukuba and Yoyogi sites in Japan. The averaged δ¹⁵N(NO_x) value and standard deviation (1σ) in the Yoyogi site was (−2.7 ± 1.8) ‰ and in the Tsukuba site was (−1.7 ± 0.9) ‰ during the sampling period. The main NO_x source appears to be the vehicle exhaust in the two sites. [ABSTRACT FROM AUTHOR]