A study of the dissociative recombination of CaO+ with electrons: Implications for Ca chemistry in the upper atmosphere. Issue 24 (29th December 2016) - Bones, D. L.
Abstract: The dissociative recombination of CaO + ions with electrons has been studied in a flowing afterglow reactor. CaO + was generated by the pulsed laser ablation of a Ca target, followed by entrainment in an Ar + ion/electron plasma. A kinetic model describing the gas‐phase chemistry and diffusion to the reactor walls was fitted to the experimental data, yielding a rate coefficient of (3.0 ± 1.0) × 10 −7 cm 3 molecule −1 s −1 at 295 K. This result has two atmospheric implications. First, the surprising observation that the Ca + /Fe + ratio is ~8 times larger than Ca/Fe between 90 and 100 km in the atmosphere can now be explained quantitatively by the known ion‐molecule chemistry of these two metals. Second, the rate of neutralization of Ca + ions in a descending sporadic E layer is fast enough to explain the often explosive growth of sporadic neutral Ca layers. Plain Language Summary: Cosmic dust particles enter the atmosphere at speeds of up to 155, 000 mph, which causes many of them to flash heat and evaporate. The resulting metallic elements such as calcium and iron then exist as global layers of atoms at an altitude of around 90 km in the atmosphere, with a layer of metal ions about 10 km higher. In this study, the fast reaction between the calcium oxide ion and an electron was measured in the laboratory for the first time. This reaction is a crucial step in the ion‐neutral chemistry of calcium. The results are used to explain why the ratio of Ca to Fe ions in the upper atmosphere is so much larger than the ratio of the neutral atoms. The laboratory result is also used to demonstrate how thin layers of Ca atoms ‐ known as sporadic layers ‐ form explosively around 95 km. Key Points: The rate constant for the recombination of CaO + ions with electrons, which is important in the lower thermosphere, has been measured The eightfold larger ratio of Ca + /Fe + ions compared with Ca/Fe atoms between 90 and 100 km can now be explained Ca + ions in a descending sporadic E layer neutralize sufficiently rapidly to explain the sudden growth of sporadic neutral Ca layers