Vertical exchange and regional budgets of air pollutants over densely populated areas

Process parameters of photochemical smog formation are derived and vertical exchange between the boundary layer and the free troposphere is identified as an important transport contribution. The analysis is based on three-dimensional observations with aircraft measurements as a key contribution. The information is needed to supplement and validate numerical model results. The wind and concentration fields are reconstructed from the measurements and the fluxes into and out of a closed box are calculated. The flux budgets give net export rates for the box. In addition, a net accumulation rate and a lower limit to the fraction of locally produced pollution are obtained. The accumulation rate comprises net chemical production plus deposition for the secondary pollutant ozone. In case of primary pollutants such as NOx, local emissions also contribute to this accumulation rate. Six diverse areas are compared: Swiss Plateau, Vienna, Berlin, Heilbronn, Ticino Region and Hong Kong. Typical net production rates for ozone are between 2.5 and 3.5 ppb h-1 in the afternoon. Fresh local production contributes between 5 and 30% to the observed ozone, while the remainder is advected or is already present. Berlin, as a fairly isolated urban area in a rural environment, has the highest fraction of “homemade” pollution. Over Hong Kong, strong local NOx emissions hinder significant net ozone production in the vicinity of the urban center, but enhanced ozone production occurs further downwind. Budgets of the primary pollutant NO2 are much more dependent on the regional emission situation. The corresponding fractions of “homemade” NO2 pollution are higher than for O3 and typically between 30 and 100%. An exception is the Ticino Region which imports large amounts of pollutants from highly industrialized northern Italy. Vertical exchange between the atmospheric boundary layer and the free troposphere contributes substantially to the regional pollutant budget. It dominates the budget when horizontal pollutant gradients across the box are absent or weak, as over the heterogeneous Swiss Plateau. The vertical flux can be calculated directly from the high-resolution aircraft measurements over Switzerland and Hong Kong using the eddy correlation method. A statistical model is developed to describe the fluxes as a function of the concentration gradients, the potential temperature and the wind profiles. This model is then used for the cases without high-resolution aircraft measurements (Vienna, Berlin and Heilbronn).