POLLUTANT CONCENTRATIONS AND BELOW-CLOUD SCAVENGING OF SELECTED N(-III) SPECIES ALONG A MOUNTAIN SLOPE

Gas and aerosol measurements were performed at 3 ground based measurin g sites at Mt. Rigi in central Switzerland during 2 winter seasons. Bo th NH3 and NH4+ show a strong vertical concentration gradient between the top station (1620 masl) and the bottom station (430 masl). High co ncentrations Of NH3 with values up to 29 ppbv, were found at the botto m station. HNO3 Concentrations were usually below 1 ppbv, with lower v alues at the bottom station than at the top station that presumably re flect particulate NH4NO3 formation due to high NH3 concentrations at t he lower site. No vertical concentration gradient was found for SO2. S imple Models have been used to estimate below-cloud scavenging of gase ous NH3 and particulate NH4+ by rain between two sites with a vertical separation of 600 m. The calculations used measurements from three ca se studies. Below-cloud scavenging Of NH3 by rain was found to be more important than below-cloud NH4+ scavenging. From 58 to 88 % of the in crease of [NH4+] in precipitation between the two sampling sites was c alculated to result from gas scavenging. Both observations and scaveng ing calculations were in relatively good agreement for three events. O bservations from the present study and tests using different aerosol a nd raindrop diameters in the calculations point to the importance of u sing real data in below-cloud scavenging studies considering the relat ive importance of aerosol and gas scavenging.