Influences of ice particles on the ion chemistry of the polar summer mesosphere

Abstract

In the polar summer mesosphere, charge is distributed over a wide range of constituents closely connected to phenomena like noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSEs). In this paper, we study how the presence of ice particles influences mesospheric ion chemistry, and how this may feed back on the particle population. To this end, we present an ion‐chemical model that for the first time features close coupling with cluster growth and ice particle charging. Starting out from molecular ion reactions, the H+(H2O)n proton hydrate chain is described using the Thomson model and Natanson's recombination scheme. Under most mesospheric conditions, electron capture by particles is expected to enhance the lifetimes and concentrations of positive ions and clusters. This has important consequences for the total charge density and mobility in the environment of particle layers. Extending the proton hydrate chain to large cluster sizes, we also quantify the efficiency of ionic nucleation of mesospheric ice particles. While ionic nucleation is not feasible as a major mesospheric nucleation process, it can become efficient given moderate atmospheric variations as induced by gravity waves. This leads to a scenario of rapid generation of populations with many small particles in local temperature minima. We show that electron capture to existing particles can significantly enhance the ionic nucleation of new particles. In summary, there are many potential connections between ion chemistry and layered phenomena in the mesosphere that should be included in comprehensive models of NLC/PMSE. Unfortunately, uncertainties in ionic reaction rates are a persistent problem and in great need of laboratory measurements representative for cold summer mesopause conditions.