Influence of the Grid Resolutions on the Computer Simulated Transport and Transformation Atmospheric Composition Processes over the Territory of Bulgaria

It should be expected that the computational grid resolution will have a significant impact on the computer simulation results. Therefore, a numerical experiment is made in order to evaluate the horizontal grid resolution impact on the simulated transport and transformation atmospheric composition processes over the territory of Bulgaria. The computer simulations are performed with a set of models used worldwide - US EPA Models 3 System. Using the “nesting” capabilities of the models, a resolution of 9 km was achieved for the territory of Bulgaria, by sequentially solving the task in several consecutive, nested areas. Three cases are considered in this paper: – The computer simulations results from the domain with a horizontal resolution (both of the emission source description and the grid) of 27 km; – The computer simulations results from the domain with a horizontal resolution (both of the emission source description and the grid) of 9 km; – Hybrid case with the computer simulations performed with a grid resolution of 9 km, but with emissions like in the 27  \(\times \)  27 km domain. The simulations were performed, for all the three cases, for the period 2007–2014 year, thus creating an ensemble large and comprehensive enough, as to reflect the most typical atmospheric conditions with their typical recurrence. The CMAQ “Integrated process rate analysis” option is applied to obtain the spatial/temporal distribution of the different transport and transformation atmospheric composition processes. Comparing them for the above 3 cases makes it possible to evaluate the grid resolution impact.