When is the worst air quality in foggy weather?

It usually forms in the early hours of the morning, so the concentration is the highest and the harm is the greatest in the morning.

The concentration level of PM2.5 is influenced by pollution sources and meteorological conditions, and it has obvious seasonal and time-varying characteristics. Although there are differences in specific values between the north and the south due to the daily and seasonal changes of humidity and temperature, the overall trend is roughly the same, that is, high in autumn and winter, low in summer, high in the morning and evening, and low in the afternoon.

When is the highest density in a day?

Figure: Daily variation of PM2.5 mass concentration

Except for special weather, the concentration of PM2.5 is generally higher in the morning and evening, and the lowest in the afternoon. Generally speaking, 8:00- 10 every morning and 6:00-8:00 every night are the peak hours of commuting, and the massive emission of automobile exhaust is one of the reasons for the highest PM2.5 value in the air during these two periods.

In addition, it depends on the weather and diffusion conditions of the day. Inversion is very important in it. What is inversion? In general, the temperature decreases with the elevation, and the low-level air is hot and the high-level air is cold. The cold air will sink again, and the hot air will float slightly, thus forming convection.

However, in some cases, such as autumn and winter nights, the temperature of the lower atmosphere near the ground is very low, and the temperature of the upper air is not so fast, but the temperature is higher than that of the lower atmosphere, resulting in the phenomenon of "temperature inversion". Once this inversion stratification is formed, the air can't convection up and down, and it is difficult for pollutants to spread.

Inversion usually occurs at night and in the morning. With the rising of the sun, the ground temperature gradually rises, the inversion stratification is broken, and pollutants begin to spread, and it will start again and again after night. Therefore, the concentration of particulate matter in the surface layer in the second half of the night is generally higher than that in the first half of the night, and the period between 2 1-23:00 is also affected by the warm layer. The low value of PM2.5 often appears in the afternoon, because the temperature conditions at this time are not easy to form an inversion layer.

What season is the highest concentration?

Figure: Seasonal variation of PM2.5 mass concentration

Some experts, such as China Institute of Meteorological Sciences and China Meteorological Bureau, analyzed the composition characteristics and sources of PM2.5 in Beijing. The results show that the concentration of PM2.5 is higher in autumn and winter and lower in summer due to the influence of atmospheric diffusion conditions such as pollution source discharge, precipitation and wind speed in different seasons. The highest monthly concentration appears in autumn and winter seasons of 165438+ 10, 65438+February and 65438+ 10, and the low value appears in July and August in summer.

Taking Beijing as an example, the average concentration of PM2.5 is the highest in winter, followed by autumn and spring, and the lowest in summer. There are two main reasons for the highest winter value in Beijing:

First, the concentration and intensity of local pollutants are high, including the obvious increase of coal consumption during heating period, and the decrease of temperature leads to the increase of vehicle exhaust emissions, which leads to the increase of PM2.5 and its precursors (sulfur dioxide, nitrogen oxides, VOCs, etc.). );

Second, meteorological conditions are not conducive to the diffusion of atmospheric pollutants. The increase of ground inversion frequency in winter makes pollutants accumulate in the near-surface layer, which leads to the gradual accumulation of PM2.5 in the near-surface layer atmosphere, reaching a high concentration level.

In spring, PM2.5 mainly comes from the contribution of dust in the north and straw burning in the surrounding areas. In autumn, due to strong solar radiation and enhanced atmospheric oxidation, photochemical smog often appears. At the same time, poor atmospheric diffusion conditions make pollutants gather, which increases the chance of gaseous pollutants transforming into secondary particles, thus leading to serious PM2.5 pollution.

The temperature conditions in summer are not easy to cause inversion, and frequent rainfall and windy weather are conducive to the diffusion and removal of PM2.5, so the concentration of PM2.5 in summer is the lowest.

Protecting the environment is everyone's responsibility. Puwechat Tmall imported from the United States can effectively prevent smog.

Haze is harmful to human respiratory system and cardiovascular system.