Is it correct to say that there is smog only in winter?

It is wrong to have smog only in winter.

Haze occurs all the year round, but it is not obvious and naturally invisible in summer because there is no fog. Haze is a combination of fog and haze. Smog is very common in cities. In many areas of China, fog and haze are combined as a disastrous weather phenomenon for early warning and forecasting, which is collectively called haze weather.

Haze is the result of interaction between specific climatic conditions and human activities. The economic and social activities of high-density population will inevitably emit a large number of fine particles (PM 2.5). Once the emission exceeds the atmospheric circulation capacity and carrying capacity, the concentration of fine particles will continue to accumulate. At this time, if it is affected by calm weather, it is easy to have a wide range of smog.

The smog in winter is mainly caused by inversion, and there are many reasons for the emission of heating pollutants in winter. In winter, due to the obvious ground radiation cooling at night, the "inversion layer" is easy to appear in the low altitude of the atmosphere, and the exchange and circulation ability of air in the horizontal and vertical directions becomes weak. The pollutants discharged into the air are confined in the shallow atmosphere and gradually gather into smog, leading to air pollution.

At the same time, the frequency of foggy days in winter is higher, because the winter nights are longer, there are more opportunities for sunny days and small winds, the ground heat dissipation is faster, and the temperature drops more significantly.

When the temperature drops to the lowest in the morning, the water vapor in the air is easy to reach saturation and condense into small water droplets. When more and more water droplets accumulate to form fog, it often means that the atmosphere is relatively stable, and the stable atmosphere is easy to accumulate pollutants, so foggy days are usually accompanied by mild or severe pollution.

In many cases, low visibility weather is actually caused by the mixture of fog and haze. When the relative humidity is high in the morning, it is mostly fog, and it turns into haze after the temperature rises and the humidity drops during the day. Especially under the influence of heating, more pollutants are discharged in winter, which is also an important cause of winter smog.