Thursday, 6 June 2002
An observational study on the scavenging of HNO3 and HCl vapor by snow crystals
Until 1980s, it has been accepted that a snow crystal does not significantly scavenge gases, while falling in the atmosphere. Diehl et al. (1995, 1996) experimentally showed that a snow crystal is capable of scavenging significant amounts of HNO3
and HCl vapor from ambient air. However, they suggested that scavenging of gas by a snow crystal might be neglected against that by a cloud droplet. Takahashi et al. (1996) collected snow particles at intervals of approximately 1 hr at a site in a sparsely populated suburban area of Sapporo City, Japan and found that the samples even in vapor depositional growth were acidic. NO3-
contributed to the acidification of a snow crystal. It was inferred that these ions originated not in aerosol but HNO3
and HCl gases because the concentration of NH4+
was much lower than those of NO3-
. This observational result raised an unsettled question how these gases scavenged by a snow crystal were formed.
Then, further observation was carried out at the same site. The concentrations of HCl and HNO3 vapors in air were measured on the ground. These vapors contributed to the acidification not only in vapor depositional growth but also in riming growth, although the concentrations of the vapors in air on the ground were extremely low. The same result was obtained in snow samples collected at another site in a remote area in northern Japan. The HNO3 and HCl gases scavenged by snow particles did not originate in the local urban polluted air. It was inferred that these vapors dissolved in cloud droplets and were transported with the droplets.