Co-condensation of ammonia and nitric acid to particles is an excellent model system to consider a simple multi-phase phase process involving condensation of volatile reactive species followed by a reaction. In this case, acid-base salt formation of ammonium nitrate forms an effectively non-volatile but thermally unstable product. Traditionally modeled as an equilibrium process for atmospheric aerosols in the plantary boundary layer, below 0 degrees C this condensation moves progressively toward an irreversible process and thus becomes progressively more capable of driving growth of even extremely small particles. Ultimately it can even drive new particle formation, either as a binary systsem or including trace sulfuric acid. Consequently, ammonium nitrate condenstaion is important to new particle formation (via rapid growth) in wintertime megacities such as Beijing and (via nucleation itself) in the Upper Troposphere. The microphysics of this process as temperature rises from 223 K toward 273 K reveals fascinating aspects of aerosol microphysics and chemistry in the atmosphere.

Ammonium nitrate is semi volatile, right?

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