Transport of trace gases from the atmospheric boundary layer (ABL) to the free troposphere (FT) is negligible under cloudless conditions over uniform terrain. However, over complex topography, an instantaneous exchange between the structured and discontinuous ABL and the FT may be possible. Aircraft measurements at the ABL - FT transition zone reveal pollutant fluxes larger than could be expected from an one-way entrainment. Those fluxes significantly influence regional pollutant budgets. Above populated areas and under high insolation, flux of O₃ and NO₂ is usually directed from the ABL to the free troposphere, where the overall tropospheric ozone production is altered.
The exchange with the free troposphere is examined for two mesoscale areas: The Swiss Plateau in Switzerland and the Hong Kong area in Asia. The Swiss Plateau exhibits mainly hilly terrain while the Hong Kong area displays a very complex terrain with sea surfaces as well as steep mountains. The flux across the top of the ABL is obtained from high resolution airborne measurements taken by the NCAR King Air.
A flux partitioning is performed to identify the most important atmospheric scales contributing to this vertical transport. Turbulent transport is important for the Swiss Plateau as well as Hong Kong. Over Hong Kong, larger scales also contribute significantly to vertical exchange. This larger scale transport might indicate that local thermotopographic circulations are involved.
A statistical model based upon the measurements over the Swiss Plateau allows a rough estimation of the vertical flux densities from the concentration, potential temperature and windshear profiles for cases where high resolution airborne measurements are not available.