On the vorticity patterns at the 300 hPa level during NO2 episode days in Athens

Summary The vertical thermal structure of the troposphere and the vorticity patterns at the 300 hPa level were examined for days with high concentrations of NO₂ in Athens. Twenty days with maximum daily concentrations exceeding 300 µg/m³ were selected at two stations near the center of the city from the years 1989, 1991 and 1992. It was found from soundings of 02.00 LST that the vertical thermal structure was warmer than normal on these days and that at the 850 hPa level the temperature exceeded the normal values by at least 5 °C on half of the days. A nocturnal surface inversion also existed on all days and the increase in potential temperature () between the surface and 1000 hPa was at least 4°C. One or two days before the occurrence of the episodes, one or more negative vorticity centers were found at the 300 hPa level, approaching Greece from the north or northwest. The negative vorticity advection caused by the approach of these centers produced the warming of the troposphere by subsidence.With 10 Figures