The influence of the North-Föhn on tracer organic compounds in ambient air PM10 at a pre-alpine site in Northern Italy

The ambient air in Northern Italy is characterised by relatively high PAH and PM10 concentrations in relation to calm wind and intensive temperature inversions, especially during cold periods. These stagnant conditions are occasionally interrupted by North-Föhn events, which cause a drop in relative humidity and an increase in O3 levels, indicating the mixing of local and free troposphere air flows. In this study the influence of the North-Föhn on concentrations of tracer organic compounds, including polycyclic aromatic hydrocarbons (PAHs), PM10, O3 and black carbon (BC) was studied during a North-Föhn event in November 2007. Large fluctuations in the concentration of these compounds, as well as changes in PM10 composition due to daytime and night-time variations of local source emissions were observed. Although these events occur at low frequencies (6–10%) they can for short periods, strongly affect the regional air quality by quickly decreasing the concentrations of these tracer organic compounds.