The concentration of ozone in the stratosphere is influenced by a variety of photochemical, gas phase, and condensed phase reactions. Species such as OH, NO, Cl, and Br can destroy ozone via catalytic processes at rates often exceeding those of ozone production. Fortunately, these cycles are interrupted when the catalysts are converted into relatively inert "reservoir" species such as HCl and ClONO₂. However, these stable gas phase molecules can be converted back into reactive species through heterogeneous reactions with stratospheric aerosol particles. In the mid-latitude regions of the stratosphere, these aerosols are composed primarily of dilute, supercooled sulfuric acid at concentrations fr om 60 to 80 weight percent. In order to study the dynamics of these heterogeneous processes, we are performing molecular beam scattering experiments to probe collisions and reactions of stratospherically important molecules with the surface of liquid sulf uric acid.The identities, translational energies, and solvation times of the scattered products are monitored by time-of-flight velocity analysis and gas uptake (sticking probability) measurements using a mass spectrometer. The experiments reveal the competition between the direct scattering and adsorption of gases impinging on the acid and between the subsequent desorption and reaction of the accommodated molecules.