## 3 Results and Discussion
For the examined Cases V1-V2, V1-V3 and V1-V4, respectively, 57%, 64% and 61% of discrete calculation receptors showed overestimation of the results, and due to the highest values of maximum 1-h concentrations, as much as a two times the standard overestimation of these results (FB \(<-2/3\)) was observed for,  
Figure 6: Comparison of spatial distributions of average annual NO\({}_{2}\) concentrations in the air near the land surface (\(\mu\)g/m\({}^{3}\)), received in the CALMET/CALPUFF modeling system, in the case of the computational grids with the resolution of: (**a**) 100 m; (**b**) 250 m; (**c**) 500 m; (**d**) 1000 m.  
respectively, 1%, 4% and 3% of the analyzed receptors. It should be pointed out that the area of good compliance (1 FB! 0.3) includes, respectively, 75%, 68% and 58% of the aforementioned calculation results obtained in the discrete receptors for those cases. The lowest deviation of the obtained values was observed in Case V1-V2, the highest deviation of the results for FB! 0 was observed in Case V1-V4, and the highest deviation with respect to FB! 0--in Case V1-V3. As it results from the fractional bias (FB) variability presented in Figure 7 for the calculations of maximum 24-h air concentrations, in every considered case for most of the analyzed calculation receptors, a value overestimation of these concentrations was obtained, in the case of Variants V2, V3 and V4, as compared to Variant V1. Depending on the case, 63-82% of the analyzed results were subject to overestimation. The highest and the lowest FB values were recorded for Cases V1-V3 and V1-V4, for which the values amounted to, respectively, \(-1.07\) and \(0.78\). It should be pointed out that twofold underestimation of the results was recorded only in Case V1-V4, and twofold overestimation in Cases V1-V3 and V1-V4. The FB values, presented in Figure 7, indicate that for Variant V4 (Case V1-V4), for as much as 5.5% of the analyzed receptors, the twofold underestimation or overestimation was recorded as compared to the reference Variant V1. The range of the good compliance for Variant V2 (Case V1-V2, 1 FB! 0.3) includes as much as 88% of the receptors subject to analysis. The FB ratio variability for the calculation results of average annual concentrations presented in Figure 7, has similar characteristics to the variability of this ratio obtained for the maximum 24-h concentrations. The average annual concentrations received for Variants V2, V3 and V4 are characterized mainly by overestimation as compared to Variant V1. The most distinctive is Case V1-V3, in which 98% of the analyzed receptors showed overestimation of the calculation results. On the other hand, the range of 1 FB! 0.3 in Case V1-V2, includes as much as 99% of calculation receptors. The results obtained for the average annual concentrations have smaller deviation with respect to the  
Figure 7: Graphic presentation of the variability of the systematic error value (FB) in the computational discrete receptors for the analyzed cases. reference value, as compared to other averaging periods. It is also worth mentioning that twofold overestimation of the concentrations results was not recorded, and only one receptor in the case of V1-V4 showed at least a twofold underestimation. Figure 8 presents the spatial differentiation of the correlation coefficients (R) for the analyzed cases (compared variants) with the discrete receptors considered and grouped depending on the distance from the emission source. The lowest values of the correlation coefficients were obtained between the calculation results of the maximum 1-h concentrations. Moreover, along with increase of the computational grid resolution, the correlation coefficient values decrease on general. Especially noticeable are the differences of the correlation coefficients between the groups of receptors located at different distance ranges from the emission source, which represent distinct terrain characteristics as well.