Why did the average data of air quality test drop significantly in summer?

Air quality is closely related to meteorological conditions.

Years of investigation and study show that urban air quality is closely related to seasons and meteorological conditions. During the heating period in winter, the main culprit of air pollution in Beijing and many cities in the north is coal smoke, followed by automobile exhaust, which makes air pollution more serious. In the non-heating period, it is mainly polluted by a large number of motor vehicle exhaust and suspended particles. Compared with every week or every day, when there is no big change in pollution source emissions, meteorological conditions such as wind, rain, air pressure and temperature directly affect air quality, making the air pollution index very different. For example, in late September of 1998, many pedestrians in Beijing felt dry eyes and sore throats, and the air in the street was suffocating. This is closely related to the calm and sultry weather at that time.

First of all, the phenomenon of atmospheric inversion directly affects the diffusion of atmospheric pollutants. Inversion is a vertical stratification phenomenon, and the temperature increases with height. Generally speaking, the inversion layer is strong and thick in winter and lasts for a long time; Summer is relatively weak. Usually, there will be inversion in the clear and breezy night, which makes the lower atmosphere relatively stable and is very unfavorable to the diffusion of pollutants. After the sun comes out, with the increase of surface temperature, the inversion layer gradually disappears, and the atmospheric turbulent mixing and vertical convection are strengthened, which is helpful to the diffusion of pollutants.

Second, it is related to the size of the wind. Usually, the greater the wind speed, the more conducive to the dilution and diffusion of pollutants in the air. However, long-term breeze or calm wind will inhibit the diffusion of pollutants and multiply the pollutants in the near-surface layer. But there will be exceptions. In the dry season of winter and spring in China, almost every year, a strong northwest wind sweeps across the whole northern and even southern regions, bringing a large amount of topsoil and sand into the air in Inner Mongolia and the Loess Plateau, forming strong winds and floating dust, blowing sand or severe sandstorm weather, making the sky yellow or dark. For example, in mid-April last year, a typical sandstorm occurred in China, which was mixed with yellow mud rain when passing over North China. At this time, the monitored suspended particulate matter seriously exceeded the standard.

Third, it is related to whether there is rain or snow. Natural rainfall and snowfall can remove and wash away air pollutants. Under the action of rain and snow, some polluted gases in the atmosphere can be dissolved in water, which reduces the air.

Pollution gas concentration, heavy rain and heavy snow on the air.

Pollutant dust particles also play an effective role in removing pollutants. However, it should be pointed out that the rain in the air is not very clean at present. Precipitation mixed with sulfur dioxide and other gases in the air will form acid rain, which is another manifestation of poor air quality.

From the seasonal point of view, there are many factors that are not conducive to the diffusion of pollutants in the air, such as less precipitation in winter in the north, dry climate, less windy weather, weak sunshine, short sunshine time, most capture in inversion layer, low temperature and inactive atmospheric convection. In summer, due to strong solar radiation and strong atmospheric convection, the generation and existence time of inversion layer is shortened, and there are more rainy days and heavy rainfall, which has obvious removal effect on pollutants and relatively reduces the degree of air pollution. Therefore, the weekly air quality reports issued by environmental protection departments in Beijing and other places are accompanied by a week's meteorological conditions.