A wide-ranging subject that consists of chemistry involving two phases, usually one or more gaseous reactants, and a condensed phase substrate where the reaction occurs, either liquid or solid. Generally the substrate facilitates the reaction and many heterogeneous reactions do not proceed in the gas phase. Reactants include free radicals as well as closed-shell molecules. Often, a reactant will hydrolyze (react with H2O) heterogeneously; in this case a major constituent of the substrate is a reactant. Common substrates are fog and rain droplets as well as aerosol in the lower atmosphere and sulfuric acid aerosol (see stratospheric sulfate layer) and water ice particles in the upper atmosphere. See polar stratospheric clouds.