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HNO3, this is a corrosive, non-volatile, and inorganic acid. It is a strong acid (dissociates completely in aqueous solution) and is also an oxidizer. In the atmosphere it is formed by the conversion of nitrogen monoxide into nitrogen dioxide, and ultimately into nitric acid:

2NO + O2 --> 2NO2
3NO2 + H2O --> 2HNO3 + NO

Nitrogen monoxide in this process most often come from (fossil fuel-based) combustion processes that use atmospheric air (containing 78.1% N2) which combines with atmospheric oxygen in those high temperature combustion process (see nitrogen monoxide and nitrogen dioxide entries below). Nitric acid is highly water soluble. This solubility with water allows easy removal of nitric acid from the troposphere by atmospheric precipitation. Commonly, this is referred to as acid rain or snow. Nitric acid has a relatively low concentration in the atmosphere but provides an important role in the production of sulfuric acid. It acts as a catalyst in the conversion of sulfur dioxide to sulfuric acid. Sulfuric acid is usually formed within cloud droplets by oxidation of dissolved sulfur dioxide again most often released by fossil fuel combustion.

[T.E Graedel and Paul J. Crutzen. Atmospheric Change: An Earth System Perspective. WH Freeman and Co, NY.1993.]

Source: Atmospheric Chemistry Glossary