engleski

Parameterization of vertical diffusion in an atmospheric chemical model

Based on gradient transport theory or K-theory, turbulent transport in the atmosphere has been parametrized using the eddy diffusivity. Due to its simplicity, this approach has often been applied in many numerical and air quality models but it is rarely verified on observations. Here, the widely used O’ Brien cubic polynomial approach has been validated together with an exponential approach against eddy diffusivity profiles determined from measurements and from LES data in stable conditions. It is shown based on Large Eddy Simulation (LES) that the Grisogono method performs better than the O’ Brien’ s polynomial, especially in the stable conditions. Verification is completed by analyzing the variability effects on pollutant concentrations of two different vertical diffusion, K(z), schemes incorporated in an atmospheric chemical model, i.e. Unified EMEP (European Monitoring and Evaluation Programme) model. The operational and proposed new parameterization for eddy diffusivity K(z) have been validated against observed daily surface nitrogen dioxide (NO2), sulphur dioxide (SO2) and sulphate (SO4) concentrations at different EMEP stations during year 2001. Moderate improvement in the correlation coefficient and bias for NO2 and SO2 and slight improvement for sulphate is found for most of the analyzed stations with the Grisogono K(z) scheme, which is recommended henceforth for further application due to its scientific and technical advantages. Special emphasis is given to the representation of the atmospheric boundary layer (ABL) in order to capture vertical transport and dispersion of atmospheric air pollution. Furthermore, two different ABL schemes are evaluated against radiosounding data in January and July 2001, and against data from the Cabauw tower, the Netherlands, in the same year. Based on validation of the ABL parameterizations, it is found that the EMEP model is able to reproduce spatial and temporal mixing height variability. Improvements are identified especially in stable conditions with the new ABL scheme based on the bulk Richardson number (RiB). Simulations of 222Rn are performed during year 2005 and 2006 and compared to available 222Rn measurements in Europe in order to validate vertical mixing. In addition to the non-local schemes, the O’ Brien and Grisogono, a new scheme which is a local and based on total turbulent energy (TTE) closure (e.g., Mauritsen, 2007) has been implemented in the EMEP model and evaluated. This work has been conducted within the EMEP4HR project which main purpose is to develop and test an operative framework for environmental control of air pollution problems in a broader region of Croatia.

atmospheric boundary layer; turbulence; air quality modelling

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