Pollutant concentrations and below-cloud scavenging of selected N (-III) species along a mountain slope

Abstract

Gas and aerosol measurements were performed at 3 ground based measuring sites at Mt. Rigi in central Switzerland during 2 winter seasons. Both NH3 and NH4 + show a strong vertical concentration gradient between the top station (1620 masl) and the bottom station (430 masl). High concentrations of NH3 with values up to 29 ppbv, were found at the bottom station. HNO3 concentrations were usually below 1 ppbv, with lower values at the bottom station than at the top station that presumably reflect particulate NH4NO3 formation due to high NH3 concentrations at the lower site. No vertical concentration gradient was found for SO2. Simple models have been used to estimate below-cloud scavenging of gaseous NH3 and particulate NH4 + by rain between two sites with a vertical separation of 600 m. The calculations used measurements from three case 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 increase of [NH4 +] in precipitation between the two sampling sites was calculated to result from gas scavenging. Both observations and scavenging calculations were in relatively good agreement for three events. Observations from the present study and tests using different aerosol and raindrop diameters in the calculations point to the importance of using real data in below-cloud scavenging studies considering the relative importance of aerosol and gas scavenging.