Why is there smog in summer?

The smog in summer contains a lot of photochemical smog, which is a gaseous pollutant produced by photochemical reaction in the air. Photochemical smog mostly appears in the afternoon and is light blue, which can crack rubber, discolor leaves and irritate people's eyes. Photochemical smog increases the concentration of PM2.5 and is one of the important sources of smog in summer. Air conditioning has become "the biggest producer of pollution sources". As the temperature rises, everyone starts to use air conditioners, doors and windows are closed, indoor air exchange rate is relatively low, and air circulation is not smooth. In the case of high temperature and high humidity in summer, the release of some toxic and harmful gases increases. It is related to the sharp increase in the number of private cars owned by urban families.

Although small passenger cars directly emit a small amount of particulate matter, they emit a lot of hydrocarbons and nitrogen oxides. These pollutants can generate secondary particles (especially secondary organic aerosols) through a series of physical and chemical reactions, and the contribution rate of secondary particles to smog events exceeds 50%.