All the natural water contains dissolved ions (positive valency anions and negative valency captions) within them.
Rain water which is formed due to natural distillation process (i.e.) evaporation, condensation and precipitation. It is supposed to be in a very dilute solution with very less dissolved solids generally between 10-20mg/lit.
All the dissolved ions present in them are in the range of 1-3 mg/lit. So it is considered to be pure and useful for drinking purposes. Moreover their PH is between 6.7 and 7.2.for consumption purposes. But the concentration varies from region to region.
Near the coastal area due to sea spray the ions is higher, if more suspended particles in the air, they react with water molecules to give more dissolved ions.
If the air is full of Carbon dioxide, Nitrogen dioxide and Sulphur dioxide, then they all dissolve in water to form an acid rain, which will cause more harm to vegetation an its PH is reduced.
Pure rain water does not contain enough dissolved minerals and ions needed for the body. So after suitable modification and alteration the rain water can be used for consumption.