THE MILAN PHOTOOXIDANT PLUME

In Switzerland, measurement campaigns including aircraft measurements were carried out in the summers of 1992 and 1993 as part of the Pollut ion and Meteorology (POLLUMET) study. Ozone (O-3) concentrations, up t o 185 ppb, with a large spatial variability were found south of the Al ps in the afternoon. Comparison to measurements north of the Alps show s that these concentration levels are extraordinarily high for central Europe. Backward trajectories reveal that the highest O-3 levels were found 4-5 hours downwind of Milan, Italy. The measurements suggest a reactive organic gas (ROG) sensitive O-3 production regime 1-3 hours d ownwind in the plume, and a NOx (sum of nitrogen oxide (NO) and nitrog en dioxide (NO2)) limitation in air masses not affected by the Milan p lume. Air masses originating north of Milan are probably close to the transition zone between the two photochemical regimes. This was found by using measurements of total odd nitrogen (NOy), NO, NO2, formaldehy de (HCHO), and hydrogen peroxide (H2O2) yielding indicators for ROG an d NOx sensitive O-3 production. The slope of ozone versus NOz (=NOy-NO x: photochemical products of NOx) were markedly higher in NOx limited conditions (Delta O-3/Delta NOz = 13.6) than in air masses close to th e transition zone (Delta O-3/Delta NOz = 4.2).