Nocturnal trans-alpine transport of ozone and its effects on air quality on the Swiss Plateau

Trace gas measurements in the Leventina valley in southern Switzerland were performed in August and September 2001. Highest concentrations of secondary air pollutants (ozone and formaldehyde) were measured at an elevated site during nights with southerly winds. The analysis of ozone data at an elevated site in southern Switzerland from 1991 to 1998 corroborated that enhanced pollutant levels originating from the residual layer of the Po Basin led to an increase of ozone during spring and summer nocturnal southerly advection. Average differences of up to more than 20 ppb were found when comparing the nighttime ozone mixing ratios during persistent southerly and northerly advection. The impact of the nocturnal trans-Alpine transport on the residual layer north of the Alps could be shown by means of a case study and an analysis of a perennial time series of ozone for an elevated station north of the Alpine crest. The afternoon spring and summer ozone concentrations in the boundary layer north of the Alps were on average 2.5 ppb higher after persistent southerly advection and fair weather conditions in the previous night. This concentration increase due to nocturnal trans-Alpine transport was estimated to be responsible for additional 5% of hours with ozone levels above the Swiss national air quality standard.