Boundary-layer turbulent transport and production/destruction of ozone during summertime smog episodes over the Swiss Plateau

Turbulent fluxes of sensible and latent heat and ozone were measured aboard a German Aerospace Research Establishment motorglider over the Swiss Plateau during the first field experiment of the Swiss air pollution experiment POLLUMET. In the lower part of the boundary layer, the ozone fluxes are negative (downward) throughout the day as a result of deposition and photochemical destruction at the surface. In the upper part of the boundary layer, the ozone fluxes tend to be negative until mid-afternoon and then become positive. The change in sign occurs after the ozone concentration in the boundary layer exceeds that in the reservoir above the inversion. Downdrafts bringing air parcels with ozone deficits across the inversion then become major contributors to the flux. The positive fluxes at upper levels result in an increase in flux divergence in mid-afternoon that is balanced by a relatively large source term in the ozone concentration budget.