Transport of ozone towards the Alps – results from trajectory analyses and photochemical model studies

We present results of statistical trajectory source analyses applied on ozone concentrations measured at high mountain peaks within and at the fringes of the Alps supported by Lagrangian photochemical box model calculations. These analyses yielded coherent pictures of transport processes causing elevated ozone concentrations in the Alps, and of the amount of ozone produced during transport over high-emission areas. Using measurement data, specific emission areas like the Po Basin, southern Germany, the “Black Triangle” region and some areas in eastern Europe were identified as important source regions, causing elevated ozone concentrations in the Alps. These statistics were supported by model calculations of transport and formation of ozone, giving similar results. Mesoscale transport processes and ozone formation in the boundary layer along the pathways were found to play an important role in determining Alpine ozone concentration levels. Ozone concentration tendencies along transport pathways were quantified climatologically using the box model. During the last 24 h of transport, concentration increases of 6–13 ppb, on the average, were found along 60–80% of all trajectories reaching the Alps, depending on the specific location. These estimates were confirmed by a measurement-based analysis of ozone formation during transport over the Po Basin, obtaining values of similar order of magnitude.