The condensation of water vapor onto a cooled surface can be used as method of sampling atmospheric components; the method is investigated theoretically and experimentally. From solutions to the heat and mass flow within the convective boundary layer, it is shown that the method has greatest collection efficiency for highly soluble gases. Discrimination against particulates and relatively insoluble gases should be excellent. The method is illustrated by some measurements of NH3, HNO3, HNO2, SO2, HCl, H2O2, HCHO, HCOOH, and CH3COOH in the vicinity of Tuscon, Arizona. Concentrations are in the part-per-billion range or below. Diurnal variations in these concentrations are discussed. Explanations are offered for a discrepancy between theoretical and actual rate of collection of water vapor.