Reply on RC1

Abstract

Many different atmospheric, physical and chemical processes are affected by ions. An important sink for atmospheric ions is the reaction and mutual neutralisation of a positive and a negative ion, also called ion-ion recombination. While the value for the ion-ion recombination coefficient α is well known for standard conditions (namely 1.7 · 10–6 cm3 s–1), it needs to be calculated for deviating temperature and pressure conditions, especially for applications in higher altitudes of the atmosphere. In this work, we review the history of theories and parameterisations of the ion-ion recombination coefficient, focussing on the temperature and pressure dependencies and on the altitude range between 0 and 20 km. Starting from theories based on J. J. Thomson’s work, we describe important semi-empirical adjustments as well as field, model and laboratory data sets, followed by a short review of physical theories that take the microscopic processes during recombination into account, including a molecular dynamics approach. We present a comparison between all theories, parameterisations, field, model, and laboratory data sets to conclude on a favourable parameterisation. While many theories agree well with field data above approx. 10 km altitude, the nature of the recombination coefficient is still widely unknown between Earth’s surface and an altitude of 10 km. According to the current state of knowledge, it appears most reasonable to assume a constant value for the recombination coefficient for this region, while we recommend using a parameterisation for altitudes above 10 km. Overall, the parameterisation of Brasseur and Chatel (1983) shows the most convincing results. The need for future research, be it in the laboratory or by means of modelling, is identified.