Diurnal variations of volatile organic compounds and local circulation systems in an Alpine valley

Volatile organic compounds (VOCs) were measured during one summer month on the floor of the deep Swiss Alpine Mesolcina valley, accompanied by aircraft-based VOC measurements during six intensive observation days. In the late morning, a strong decrease of VOC concentrations despite an increasing traffic intensity was observed on the valley floor. This occurred when clean air masses were advected during the preceding night from high altitudes north of the Alps. During these nights the average VOC concentration was 40% lower in the nocturnal surface layer than in cases of southerly advection. Nights with southerly advection led to high VOC concentrations in the valley on the following morning and no decrease of VOC concentrations between 9 and 12 h. Only during these nights high ozone concentrations produced in the Po Basin of northern Italy could reach the Alpine crest and yielded ozone concentrations above 100 ppb there. The diurnal cycle of VOCs, influenced by the various meteorological features characteristic for deep Alpine valleys, is discussed in detail.