This work provides the theoretical background and necessary practical tools to study one of the most spectacular natural features: vigorous evaporation from active crater lakes. We will give qualitative insights (lake water chemical –Cl content, and isotopical composition) rather than quantify evaporation fluxes from lakes. A major problem is that, with the current methods, we are only able to sample the lake water, while the input fluid rising into the lake (sublacustrine) and evaporation plume coming off the lake remain "untouchable". This means that the lake behaves as a "black box", being the result of in- and outgoing fluids of unknown chemical and isotopic composition. As visually demonstrated at many active crater lakes, evaporation is a major process. Strong evaporation from the lake surface will affect the isotopic composition of the remnant lake water, and the "steam devils" (evaporation plume) swirling over the lake. It is found that the kinetic (diffusion) isotope fractionation overwhelms the equilibrium isotope fractionation effect, as it is intuitively hard to imagine a dynamic crater lake as an equilibrated system. Besides a hot water mass in evaporation, water of active crater lakes are generally hyper-saline (total salinity >100,000 mg l-1) and hyper-acidic brines (pH as low as -1). Although "small scale" equilibrium fractionation effects, the "isotope salt effect" and "isotope acid effect" lead to isotopically heavier evaporation plumes, with respect to vapor coming off pure neutral water. Besides isotope fractionation of the water itself under such extreme lake conditions, HClgas (and HF) will partition between the liquid and vapor phase. HCl degassing is enhanced when pH is continuously lowered by the input of acidic gases (SO2, HCl, HF), lake temperature is higher, and evaporation is physically favored by wind or lake convection. It is empirically deduced that HCl partitioning into the vapor phase is chemically controlled by the lake water temperature and density, rather than the Cl content or pH. A better quantification of the chemical and isotopic composition of evaporative gas plumes from active crater lakes will be of importance for volcano monitoring when we aim to deduce the flux and composition of the "hot magmatic end member", through chemical and isotope budget analyses.
A major challenge for the future is to develop field methods to enable to sample the evaporation plume coming off lake surfaces, so we can directly determine its chemical and isotopic composition and compare them with the theoretical approach of this review.
We should elaborate experiments, mimicking the natural situation of a crater lake, with controllable factors to better understand how evaporation and HCl degassing occurs at crater lakes, and how they relate to the underlying processes of magmatic input and fluid recycling. The here reviewed literature of similar experiments provide the theoretical base for such future work. Evaporating real lake samples under various conditions is probably most suitable, as crater lake brines are extremely complex solutions very hard to copy in the laboratory.

1. acidic crater lakes
2. evaporative degassing
3. HCl degassing
4. isotopic fractionation