Atmospheric equilibrium number densities are estimated for the van der Waals molecules N2⋅N2, N2⋅O2, O2⋅O2, N2⋅CO2, O2⋅CO2, N2⋅H2O, O2⋅H2O, CO2⋅CO2, H2O⋅H2O, A⋅A, A←CO2 and A⋅H2O for the altitude range 5-90 km. It is concluded that number densities of atmospheric dimer species are appropriately given by equilibrium estimates, and that some are of the same order as minor, rather than trace constituents. Possible roles of van der Waals molecules in neutral and ionic1 termolecular4 reactions, as well as infrared absorption and emission in the natural atmosphere, are proposed and examined.