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Sodium chloride has been postulated to be. formed in the stratosphere by the reaction ofNaOH with HCI, and an analogous reaction should occur also for KOH, with the formation of KCI. Photodissociation rates have been calculated for both NaCl and KCI using the published ultraviolet absorption cross sections below 300 nm. Both molecules absorb strongly and have photodissociation lifetimes ofonly a few minutes in the upper stratosphere. Neither molecule is an effective sink for chlorine above an altitude of about 35. km.The presence of significant amounts of sodium in the upper atmosphere has been known since 1938, with the identification of the yellow radiation of the twilight air glow as the 589.3-nm emission from excited Na(2P) atoms (1, 2). In the next few years after. this discovery, several possible physicochemical mechanisms were suggested as the source of these Na(2P) atoms, including the possibility that they were released by reaction 1, NaCl + UV-Na+ Cl, [1] during the solar UV photolysis of NaCl (3, 4). This explanation was never generally accepted and has long since been superseded by current explanations that (i) the more intense daylight and twilight emission from Na(2P) arises from the resonant scattering ofsunlight, and (ii) the nighttime emissions occur after reactions with oxygen-or hydrogen-containing compounds (5). More accurate knowledge of the altitude and amounts of free sodium atoms in the atmosphere has become available with the introduction of lidar techniques for nighttime detection of the ground state Na(2S) atoms (6). The same techniques are. also applicable to the less abundant K(2S) atoms (7) and have furnished comparable information on its abundance and altitude distribution. These direct observations demonstrate the existence in the mesosphere ofa steady influx ofthese alkali metals, mostly of meteoric origin (7), but provide no concentration iniformation below about 80 km, as the abundance of the free atoms during nighttime declines below the limits of lidar detection.The chemical interactions of sodium in the mesosphere have been described through reactions of Na and NaO, modified recently by the addition of cluster ion chemistry (8). Modeling of the behavior of sodium into the lower mesosphere and stratosphere was stimulated by the postulate of Ferguson (9, 10) that the neutral entity, X, of mass 41 ± 2 observed in the 35-km region in H+Xn(H20)m ions (11) duce the harmful effects of halocarbons on ozone for several years" because "it would require several years for the diffusion cycle to reestablish those concentrations (Cl, CIO) if the release occurred above 30 km" (17).The laboratory experiments of Perry et al. (18) have demonstrated the existence of rapid ion-switching mechanisms that lead to the formation of Na(H2O)n ions from reactions ofneutral sodium-containing molecules with the proton hydrate ions. With the higher resolution ion measurements now available in the.midstratosphere, the absence of ions such as Na(H20)n then can be used to place upper limits on the ...