We are looking at ion concentrations in the atmosphere at increasing altitudes.  Much of the planet's radioactivity arises from within the Earth's surface.  But radioactive decay does not necessarily release ions.  It releases energy, atomic particles, and smaller elements.  As you increase in elevation away from the Earth's surface into the upper atmosphere it seems that you get an increase in ion concentration.  This is due to one major factor, the ultraviolet radiation produced from the Sun.  In fact, the upper atmosphere of the Earth is called the ionosphere.  UV radiation from the sunlight reacts with gas molecules in the air to produce ions and radicals.  These radicals are highly reactive and can produce additional ions in the atmosphere.  There is a direct correlation between solar activity and the levels of ions in the upper atmosphere.