What causes the smog weather in Harbin?

The main reason for the recent smog weather in Harbin is meteorological conditions.

The pollutants over Harbin were not accumulated in one day. There were signs of air pollution in Harbin as early as four days ago. At that time, Harbin continued to be controlled by high-pressure cold air, just like wearing a "hat" to the city to prevent the spread of pollutants. Then, the warm air from the south enters, which has brought some pollution to North China and cast a layer of dust on the air quality in Harbin. Until the calm wind yesterday morning, the horizontal direction was not conducive to diffusion, resulting in more and more pollution like "snowballing", which was held in the air and eventually formed smog.