The regional pollutant budget of the atmospheric boundary layer: Concept, interpretations and observational results

Regional budgets of pollutants in the atmospheric boundary layer (ABL) provide valuable information on transport and chemical processes, especially over complex terrain where local assessments are not representative. Horizontal transport, vertical transport (advective and non-advective), deposition, emissions and chemical production/destruction constitute the regional budget. Partial budgets using reduced sets of budget terms also have meaningful interpretations: If only horizontal and vertical advective transport is included, the corresponding budget describes the accumulation of the pollutant within the ABL. The transport terms can be estimated from simultaneous observations of wind and concentrations. Observational budget results from the areas Swiss Plateau, Heilbronn, Berlin, Vienna and Hong Kong are analyzed: On sunny summer afternoons, typical accumulation rates from larger urban areas in Europe are 1.5 ppb/h for O-3 and 0.1 ppb/h for NO2. If estimates of non-advective vertical transport and deposition are available, the budget provides net chemical production rates of secondary pollutants. Typical values for O-3 are between 2 and 5 ppb/h. In case of primary pollutants such as NO2, this budget estimates the sum of net chemical production and emission. Observational results range around 0.15 ppb/h. From the budgets, also an estimation of the fraction of locally produced or "homemade" pollution is possible and typical values from 10 to 30 % and 20 to 70 % for O-3 and NO2, respectively, are found. It is shown that all individual terms achieve non-negligible contributions to the regional budget and that the overall budget as well as the relative importance of the individual terms depend on size and extent of the budget box.