Preparation and calibration of O-17 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 the 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 home-made 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 atmospheric NO₂ and HONO. The latter will facilitate the application of the Δ¹⁷O tracer in investigating atmospheric cycling of reactive nitrogen and radicals.