In what season does smog often occur in North China?

The smog in North China is mostly in autumn and winter. Fog and haze are actually two different weather phenomena. Fog is a large number of tiny water droplets or ice crystals floating in the air, and haze refers to the turbidity phenomenon formed by a large number of suspended particles such as smoke and dust. Large-scale haze weather mainly appears in the situation of large-scale atmospheric circulation with weak cold air and good water vapor conditions. One of the necessary conditions is that there is still wind or breeze at low altitude near the ground. Autumn and winter are the seasons with low temperature and weak ground pressure field. The wind near the ground is weak, there is no cold air activity, the atmospheric stratification is relatively stable, and there is an inversion layer near the ground, which is not conducive to the dilution and diffusion of pollutants. At this time, if the humidity is high, it is easy to form haze weather.