Removal of the potent greenhouse gas NF3 by reactions with the atmospheric oxidants O(1D), OH and O3

Abstract

Nitrogen trifluoride, NF₃, a trace gas of purely anthropogenic origin with a large global warming potential is accumulating in the Earth's atmosphere. Large uncertainties are however associated with its atmospheric removal rate. In this work, experimental and theoretical kinetic tools were used to study the reactions of NF₃ with three of the principal gas-phase atmospheric oxidants: O(¹D), OH and O₃. For reaction (R2) with O(¹D), rate coefficients of k₂(212–356 K) = (2.0 ± 0.3) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ were obtained in direct competitive kinetics experiments, and experimental and theoretical evidence was obtained for F-atom product formation. These results indicate that whilst photolysis in the stratosphere remains the principal fate of NF₃, reaction with O(¹D) is significant and was previously underestimated in atmospheric lifetime calculations. Experimental evidence of F-atom production from 248 nm photolysis of NF₃ was also obtained, indicating that quantum yields for NF₃ destruction remain significant throughout the UV. No evidence was found for reaction (R3) of NF₃ with OH indicating that this process makes little or no contribution to NF₃ removal from the atmosphere. An upper-limit of k₃(298 K) < 4 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ was obtained experimentally; theoretical analysis suggests that the true rate coefficient is more than ten orders of magnitude smaller. An upper-limit of k₄(296 K) < 3 × 10⁻²⁵ cm³ molecule⁻¹ s⁻¹ was obtained in experiments to investigate O₃ + NF₃ (R4). Altogether these results underpin calculations of a long (several hundred year) lifetime for NF₃. In the course of this work rate coefficients (in units of 10⁻¹¹ cm³ molecule⁻¹ s⁻¹) for removal of O(¹D) by n-C₅H₁₂, k₆ = (50 ± 5) and by N₂, k₇ = (3.1 ± 0.2) were obtained. Uncertainties quoted throughout are 2σ precision only.