A flow cell built to simulate heterogeneous reactions in the stratosphere is described. Small particles (~1 &mgr;m diameter) have been made from nitric acid solutions and distilled water. These have many of the important physical properties of types I and II polar stratospheric clouds (PSCs), respectively. The FTIR spectra of the ice particles are compared with calculated extinction (scattering plus absorption) spectra to obtain an estimate of their size distributions. The average particle size can be controlled by varying the flow conditions. The nitric acid particles have not been as well characterized, because the required optical constants are not yet available. Their spectra, however, have the characteristics expected for type I PSCs, and resemble those previously reported for thin-film nitric acid trihydrate.