## 4 Conclusions
However, it should be pointed out that the calculations made for a grid with the resolution of 250 m do not guarantee achieving the highest values of maximum 1-h concentrations possible. To determine these maxima, it is therefore recommended to conduct additional calculations for a denser grid (e.g., of a 100 m spacing), which may be limited only to short episodes related to their occurrence. It is possible, since the highest values of maximum 1-h concentrations, in the case of grids with the resolution of 100 and 250 m, are likely to occur at the same time and at the same weather conditions. The conducted calculations and analyses also enable taking a position on the problem of the use of computational grids with the resolution of 500 or 1000 m in the CALMET/CALPUFF modeling system for near-field applications. Considering the better proportion of obtainable overestimated values to the underestimated ones, the results obtained for the negative and positive FB components, as well as such statistical indicator values as NMSE, VG, FB, and MG, using a computational grid with the resolution of 1000 m is a slightly better solution than using one with the resolution of 500 m. However, an important argument, indicating the disadvantage of a grid with the resolution of 1000 m is the occurrence of high underestimations of the highest values of maximum 1-h and 24-h and average annual concentrations, respectively, at the level of ca. 64%, 31% and 48%, as compared to the results obtained for a grid with the resolution of 100 m. In addition, in the case of a grid with the resolution of 1000 m, the standard underestimations of the highest values of these concentrations occur several times. For comparison, in the case of a grid with the resolution of 500 m, the underestimation of the highest values of maximum 1-h and 24-h and average annual concentrations, as compared to the variant with the resolution of 100 m, amounted to ca. 20-26%. The results yielded in the study correspond well with general guidelines regarding the selection of the grid resolution introduced in the manual describing the optimal settings of the analyzed modeling system [59]. According to these guidelines, it is necessary to find the optimum balance between the desire to make the grid size as large as feasible to reduce the run time and files size, and the need to make the grid size small enough to optimize the terrain effects on the wind field. Smaller domains for near-field applications may require a grid spacing of about 250 m, but if the dominant terrain features are not resolved it is recommended to change the grid resolution to about 150 m [59]. The present study provides an area for discussion with regard to the regulations concerning the use of the appropriate resolution for close range calculations with the use of the CALMET/CALPUFF modeling system. However, it does not exhaust the subject, and requires additional assessment and validation, based on the measurement data. It is recommended to take into account the problem of selecting the optimal horizontal grid resolution for a given case in further efficiency assessments of this modeling system at a local scale, especially in the case of terrain characterized by more complex topography and land use. Conceptualization, R.O. and M.R. ; Data curation, M.R. ; Formal analysis, R.O. and M.R. ; Funding acquisition, R.O. ; Investigation, R.O. and M.R. ; Methodology, R.O. and M.R. ; Project administration, R.O. ; Resources, R.O. and M.R. ; Software, M.R. ; Supervision, R.O. ; Visualization, R.O. and M.R. ; Writing--original draft, R.O. and M.R. ; and Writing--review and editing, R.O. This research was funded by AGH UST, statutory research No. 11.11.150.008. The authors would like to thank [PERSON] for support and assistance with this project and [PERSON] for editorial comments. The authors would like to express their gratitude towards the CYFRONET AGH Academic Computer Centre for putting the computational resources at their disposal. The authors declare no conflict of interest.