Oxygen-Isotope Exchange between CO₂ and NO₂ with Implications for Atmospheric Chemistry

Elucidating isotope exchange between atmospheric trace molecular species is important for environment monitoring, climate control studies, and a fundamental understanding of atmospheric chemistry. Here, we provide direct experimental evidence of oxygen-isotopic exchange between carbon dioxide (CO₂) and nitrogen dioxide (NO₂), which are simultaneously emitted into the atmosphere from common sources. A combined near-infrared and UV-vis optical cavity-enhanced experimental investigation along with quantum-chemical calculations followed by a reaction modeling study revealed that CO₂ and NO₂ can communicate isotopically by near-ultraviolet-driven NO₂ photolysis. Our results found evidence for a near-barrierless (1.67 kcal/mol) nitrate-containing complex having a very short lifetime (∼13 ns) which facilitates the transfer of ¹⁸O-isotopes from ¹⁸O¹²C¹⁶O to N¹⁶O¹⁶O, leading to isotopic depletion of ¹⁸O in ¹⁸O¹²C¹⁶O, thus opening a new gas-phase isotope-selective chemical transformation mechanism in the lower atmosphere. This isotope exchange study may serve as a new window into the fundamental understanding of isotopic photochemistry, oxygen isotopic fractionations, and climate modeling.