Scavenging ratios for sulfate, ammonium and nitrate determined at Mt. Sonnblick (3106 m a.s.l.)

For a time period of two years scavenging ratios were calculated for the high alpine site Mt. Sonnblick. Annual average values are 2.6×106, 1.4×106 and 1.8×106 for total nitrate, sulfate and ammonium, respectively, when the scavenging ratios are calculated based on overall monthly averages. Two other calculation methods (based on ‘paired’ monthly averages and on single days) are discussed in the paper. Higher scavenging ratios are observed for the beginning of the cold season (October–January) than for the summer months (June–September) the respective ratios being in the range of 3 to 5. Precipitation sulfate was found to be formed predominately by particulate sulfate (89–96%) while reactive gas-phase scavenging of sulfur dioxide is only of minor importance (4–11%). Precipitation nitrate on the other hand is predominantly formed by gas-phase scavenging of nitric acid (88–96%) and particulate nitrate contributes to a lesser extent (4–12%). The first estimate for ammonium in precipitation indicates that particulate ammonium accounts for 49–79%, while ammonia forms the remaining 51–21%. An equation was derived to interrelate the scavenging ratio of sulfate to the scavenging efficiency of particulate sulfate into cloud droplets. The comparison showed very satisfactory agreement in respect to the absolute values as well as to the annual variations