~(17)O excess transfer during the NO_2 + O_3 → NO_3 + O_2 reaction

摘要

The ozone molecule possesses a unique and distinctive ~(17)O excess (Δ~(17)O), which can be transferred to some of the atmospheric molecules via oxidation. This isotopic signal can be used to trace oxidation reactions in the atmosphere. However, such an approach depends on a robust and quantitative understanding of the oxygen transfer mechanism, which is currently lacking for the gas-phase NO_2 + O_3 reaction, an important step in the nocturnal production of atmospheric nitrate. In the present study, the transfer of Δ~(17)O from ozone to nitrate radical (NO_3) during the gas-phase NO_2 + O_3 → NO_3 + O_2 reaction was investigated in a series of laboratory experiments. The isotopic composition (δ~(17)O, δ~(18)O) of the bulk ozone and the oxygen gas produced in the reaction was determined via isotope ratio mass spectrometry. The Δ~(17)O transfer function for the NO_2 + O_3 reaction was determined to be: Δ~(17)O(O_3*) = (1.23 ± 0.19) × Δ~(17)O(O_3)bulk + (9.02 ± 0.99). The intramolecular oxygen isotope distribution of ozone was evaluated and results suggest that the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The results obtained in this study will be useful in the interpretation of high Δ~(17)O values measured for atmospheric nitrate, thus leading to a better understanding of the natural cycling of atmospheric reactive nitrogen.