This is called the hydrological, or water cycle. Water evaporates from the Earth and rises into the atmosphere where it forms clouds. In nature this is where water is in its purest form. However, it does not stay that way for very long. Its stay in the air is short.
Water droplets forming in clouds absorb particles and impurities found floating in the air.
Water is sometimes known as the universal solvent. It has a tendency to dissolve a little bit of everything it touches. Dust, smoke from industry, carbon dioxide, spores and smog may be absorbed by water droplets. For example, if it dissolves sulfur from industrial smokestacks, it can form acid rain. This increases its capacity to dissolve other substances.