Nocturnal depletion of ozone in the Upper Rhine Valley

Abstract

During the TRACT’92 (Transport of Air Pollutants over Complex Terrain) Field Measurement Campaign a total of 42 vertical profiles of ozone and meteorological variables (temperature, pressure, humidity, wind speed, and wind direction) were measured near Sasbach in the Upper-Rhine Valley on four days using a tethered balloon. A novel light-weight and fast-response ozone sensor proved to be very effective for measuring vertical profiles in the atmospheric boundary layer up to 1000 m altitude. The vertical profiles of ozone, temperature and humidity show an excellent agreement in their fine structures allowing the nocturnal inversion height to be precisely determined. In the evening of sunny days, the formation of a nocturnal inversion layer is favoured by radiative cooling at the surface. Taking into account the height of the nocturnal inversion layer, the ozone concentration measured at the surface and at the top of the nocturnal inversion layer as well as an exponential decay of the ozone concentration from the onset of formation of the nocturnal inversion, the dry deposition velocity of ozone within the nocturnal inversion layer in the evening hours can be calculated with the help of a simple model. The results of the dry deposition velocity obtained by this balloon-borne determination method are compared with ground-based measurements of the dry deposition velocity simultaneously measured in the same area by use of the eddy correlation technique. As the ground-based measurements of the dry deposition velocity of ozone cover only 20–50% of measurements under the balloon-borne determination method, it is concluded that in addition to the dry deposition process also chemical reactions with the nitrogen oxides are responsible for the ozone depletion within the nocturnal inversion layer in the Upper-Rhine Valley.