Preparation and Calibration of ¹⁷ O-Enriched Nitrite Isotope Standards

The oxygen-17 isotope anomaly (Δ17O) serves as a powerful tool to elucidate the chemical transformation mechanisms of atmospheric reactive nitrogen species such as NO2 and HONO. Current studies employ denuder collection methods to convert atmospheric NO2 and HONO into nitrite for isotopic analysis. However, accurate Δ17O measurement of atmospheric NO2 and HONO is hampered by the lack of internationally recognized nitrite isotope reference materials with applicable Δ17O signals. In this study, we prepared new nitrite isotope standards with nonzero Δ17O signals through oxygen isotope exchange between high-purity nitrite reagents and 17O-enriched water. Using a developed ozone oxidation calibration method, the Δ17O values of a newly prepared nitrite standard (i.e., N-Δ17O-1) and the international nitrite reference material RSIL-N10219 were determined as (69.7 ± 1.0) ‰ (n = 10, 1σ) and (−8.7 ± 0.3) ‰ (n = 11, 1σ), respectively. The two additional O-17-enriched nitrite standards were then measured and calibrated against RSIL-N10219 and N-Δ17O-1, yielding Δ17O values of (34.5 ± 0.3) ‰ (n = 6, 1σ) and (6.4 ± 0.1) ‰ (n = 8, 1σ), respectively. The δ15N and δ18O values of the three homemade nitrite isotope standards were also calibrated against international nitrite reference materials. This study introduces a new and reliable method to obtain the Δ17O values of nitrite, and the establishment of Δ17O values of nitrite standards provides a foundation for accurately assessing Δ17O variations of atmospheric NO2 and HONO. The latter will facilitate the application of the Δ17O tracer in investigating atmospheric cycling of reactive nitrogen and radicals.