The VOTALP Mesolcina Valley Campaign 1996 – concept, background and some highlights

Abstract The Mesolcina Valley campaign was an important part of the VOTALP project. Its main goals were the study of the effects of thermal wind systems on horizontal and vertical ozone transport over various distances, and on local production of ozone in an Alpine valley. The field measurements took place in the Mesolcina Valley in southern Switzerland in July and August 1996. The Mesolcina Valley is typical for the Alps for its size and its rural character, and contains an important traffic route. Ground-based and airborne instruments were deployed to obtain a four-dimensional chemical and physical picture of the valley atmosphere and the surroundings. Field measurements were complemented by numerical modeling studies. An overview of the campaign, its geographic background, and the measurement plan is given together with selected highlights of the results obtained so far. A synoptic-climatological approach tries to evaluate the representativeness of the observation days. It was found that during the campaign representative data for typical ozone days were collected, while extreme events did not occur. The main results of the campaign are that very high pollutant concentrations at the Alpine crests are the effect of advective, larger-scale transport, mainly from the Alpine forelands. The valley is very effective in pumping air into elevated layers during the day, with the slope winds probably carrying the bulk of the air volume to higher levels. Up to five times the valley volume may be exported during one upwind phase. Net local production is estimated to be approximately 3–8 ppb h −1 of ozone, but seems to be compensated by dry deposition. This indicates that the Alpine valleys may be important net sinks of air pollutants, which may lead to negative impacts on the Alpine ecosystems.