Persistent inversion dynamics and wintertime PM10 air pollution in Alpine valleys

Persistent inversions occur systematically during a high-pressure regime, which first triggers a synoptic scale elevated inversion due to the advection of warm air masses in the mid-troposphere. In valleys, the sheltered boundary layer becomes decoupled from the free troposphere, which allows a ground-based inversion to intensify in the following days. An inversion layer of quasi-constant temperature gradient, greater than 5 K km−1, then forms up to an altitude of about 1600 m, close to the average elevation of the summits. If the episode is sufficiently long, a stagnation stage is reached during which daytime insolation produces a shallow convective surface layer which does not destroy the persistent inversion. The inversion break-up occurs rapidly, in less than a day, because of synoptic scale advection of cold air masses in the mid-troposphere when a low-pressure regime approaches. The end-of-episode decrease in PM10 concentration is due to precipitation combined with an increased ventilation inside the boundary-layer.