## 3 Results
### Evaluation of ERA5 Boundary Layer Heights  
During winter (Figure 10a,b), the observed boundary layer heights reveal a nocturnal stable layer typically below 500 m, with a daytime mixed boundary layer ranging between 800 and 1500 m. Throughout most of the observation period, ERA5 estimates of the maximum daytime boundary layer height are higher than the MWR-derived values. ERA5 overestimated the mixed-layer height in 43% of the cases when observational data were available. In Figure 10b, the scatter plots overlaid with surface temperature differences indicate that ERA5 surface temperatures are lower than observed surface temperatures during polluted periods in winter over Baotou. From January to March 2021, multiple pollution events occurred, including haze, blowing dust, and sandstorms. These pollution episodes caused a dome effect, where aerosols suppressed surface heating during the daytime, leading to a more stable thermal stratification [45]. The dome effect caused ERA5 to underestimate surface temperatures, while insufficient heating of the atmosphere above the observation site resulted in lower observed daytime thermal boundary layer heights compared to ERA5 estimates. Conversely, when ERA5 overestimated surface temperatures, it also overestimated surface-heating effects. This overestimated heating enhanced buoyancy-driven turbulence, increased vertical mixing intensity, and strengthened boundary layer turbulence, ultimately leading to an overestimation of boundary layer heights.  
During summer (Figure 10c,d), the boundary layer height exhibits a nocturnal stable layer mainly ranging between 10 and 350 m, and a daytime mixed layer ranging between 500 and 2100 m. Similarly to winter, ERA5 generally overestimated the maximum daytime boundary layer heights; however, the discrepancies were more pronounced in summer, with maximum daytime overestimations reaching up to 2000 m. ERA5 overestimated the  
Figure 10: Time series and scatter plots comparing boundary layer heights observed by the microwave radiometer (MWR) with those from ERA5 reanalysis data. Panels (**a,b**) illustrate comparisons for January, February, and March (winter), with scatter plots overlaid by the temperature difference between surface ERA5 data and observations, while panels (**c,d**) present the corresponding comparisons for July and August (summer). In (**b**), the blue points circled in red represent the scatter points under heavy pollution conditions. In (**d**), the blue points circled in red represent the scatter points under strong convective weather conditions. The red fitted line represents the fit for temperature differences greater than 0, while the blue fitted line represents the fit for temperature differences less than 0.  
mixed-layer height in 59% of the cases when observational data were available. During July and August, ERA5 overestimated surface temperatures, which further led to an overestimation of the surface-heating effect, ultimately resulting in overestimated boundary layer heights. Additionally, multiple strong convective weather events occurred during this period, such as the intense convection on August 29. Vertical transport induced by mountain airflow and radiative forcing likely contributed to variations in mixed boundary layer heights [21, 46, 47, 48].