Beijing launches orange alert for heavy air pollution. How to protect it scientifically?

Beijing launched an orange alert for heavy air pollution at 0:00 on the 11th. According to the air quality forecast, affected by adverse meteorological conditions, a regional air pollution process will occur in the Beijing-Tianjin-Hebei and surrounding areas. It is expected that from the 11th to the 14th, the air quality in Beijing may reach level 4 moderate pollution, and reach level 5 severe pollution in some areas and during certain periods.

At the same time, the Central Meteorological Observatory issued an environmental meteorological bulletin at 6:00 on the 11th. From the 11th to the 14th, atmospheric diffusion conditions gradually deteriorated in central and southern North China, western Huanghuai and other places, with light to moderate haze in some areas and severe local haze.

Zhou Lixian, a senior engineer at the China Meteorological Administration, said that from the perspective of meteorological causes, the worsening of diffusion conditions in this round is the result of the superposition of three factors: circulation, wind field, and water vapor.

First of all, in terms of circulation situation, North China and Huanghuai are at the rear of high pressure, with southerly winds prevailing and accompanied by downdrafts. On the one hand, the downdraft suppresses the upward movement, and pollutants lose the channel for upward diffusion; on the other hand, the warm air at high altitude sinks to the lower levels, forming a temperature inversion layer near the ground with cold below and warm above. The temperature inversion layer is like a lid, sealing pollutants in a shallow space of several hundred meters to 1,000 meters, making it difficult for them to spread outward.

At the same time, due to the small pressure gradient, the wind speed near the ground is very weak, and it is often close to calm wind at night. Southerly winds continue to transport pollutants from central and eastern Henan and southern Hebei to the north, and pollutants accumulate in the central and southern parts of the North China Plain; southerly winds and northerly winds converge in the middle of the region, wind speeds further decrease, and pollutant concentrations also jump significantly in a short period of time.

In addition, water vapor is also "disadvantaging". The southerly wind transports water vapor, coupled with the moisture on the ground after the previous rain and snow, the particles in the air absorb more moisture, become stronger in blocking light, and the visibility and PM2.5 deteriorate simultaneously. The low visibility at this time is not just dry haze, but also fog droplets formed by moisture absorption.

"Once this state of calmness, high humidity, and temperature inversion is established, pollution will strengthen itself. Therefore, it is difficult to break it by the power of nature alone. You must wait for a strong enough cold air to come over to truly clean up the fog and haze." Zhou Lixian said.

Zhou Lixian said that in order to break the calm weather and dispel the haze, the core is to restore the ventilation and dilution capabilities of the atmosphere, mainly relying on cold air. Cold air mainly does three things: first, it is advection dilution. The northerly wind brought by the cold air brings in clean, dry and cold air to replace the polluted air masses; second, it is dynamic mixing. The strong wind brought by the cold air creates strong turbulence, raises the boundary layer, and pollutants spread from shallow layers to thicker spaces; finally, it is dehumidification. The dry and cold air causes the humidity to drop rapidly. When particulate matter loses the conditions for hygroscopic growth, visibility often improves faster than the concentration decreases.

However, the cold air also has a "threshold", and it needs to reach a certain intensity and last for a sufficient period of time before it can be completely cleared. Zhou Lixian said that if the wind is too light, the path is eastward, or the duration is short, it will only push the pollutants southward, resulting in a situation where "the north becomes better and the south becomes worse." In addition, if the cold air quickly warms up and the wind direction turns back to the south, pollutants can easily accumulate again, causing a cycle of "clearing for two days and returning for one day".

Under such weather conditions, Zhou Lixian also reminded that children, the elderly, pregnant women, and patients with chronic diseases such as asthma, coronary heart disease, and hypertension should stay indoors as much as possible and close doors and windows; ordinary people should reduce outdoor activities and avoid strenuous outdoor sports such as long-distance running and cycling. Because haze is heavier at night and early morning and lighter in the afternoon, if you must go out, try to arrange it in the afternoon and wear KN95 or N95 masks when going out.

If you are driving in low visibility, you should also turn on your fog lights, slow down, increase the distance between you and other vehicles, and give priority to public transportation. In case of persistent cough, chest tightness, shortness of breath, heart palpitations and other discomforts, seek medical attention promptly.