A LABORATORY STUDY OF THE UPTAKE OF HNO3 AND HCL VAPOR BY SNOW CRYSTALS AND ICE SPHERES AT TEMPERATURES BETWEEN 0-DEGREES-C AND -40-DEGREES-C

A laboratory experiment is described during which the uptake of HNO3 and HCl vapor by dendritic snow crystals and by single crystalline and polycrystalline small ice spheres was studied at ppbv and ppmv gas levels and at temperatures between 0 and -40 degrees C. In one experimental investigation the vapor was allowed to be adsorbed onto the surface of the ice particles. During another experimental investigation the ice particles were allowed to grow from water vapor on fine fibers in the presence of the HNO3 and HCl vapor. The results of our experiments show that under both conditions significant amounts of HNO3 and HCl became scavenged by the ice particles. Scavenging by adsorption was maximum for both vapors at temperatures near 0 degrees C where a quasi-liquid layer exists on the surface of ice. With decreasing temperature the uptake of HNO3 and HCl vapor decreased and kept on decreasing for HCl with further decrease in temperature. In contrast, the uptake of HNO3 reached a minimum near -18 degrees C to increase again strongly with further decrease in temperature. The temperature-dependent uptake of both vapors were explained on the basis of surface melting caused by the vapors. For the case that the ice crystals were growing in an atmosphere supersaturated with respect to ice while simultaneously being exposed to HNO3 or HCl vapor we noted that gas scavenging was less than during simple adsorption by a nongrowing crystal. Our experiments further showed that HNO3 once taken up by an ice particle would not desorb if the ice particle remained at ice saturation. Our experiments also indicated that some of the adsorbed HNO3 and HCl diffuses into the ice particle.