Why is the fog and haze in Beijing, Tianjin and Hebei coming again?

What is the cause of the current fog and haze weather? Will we encounter fog and haze weather frequently this spring?

Written by reporter Zhao Tianyu, Photo and text editor Chen Yongjie

Interview experts:

Sun Yang (Researcher and Senior Engineer, Institute of Atmospheric Physics, Chinese Academy of Sciences)

At the beginning of March, along with the rising temperature, there also came lingering fog and haze.

▲Affected by unfavorable diffusion conditions and emission sources, heavy pollution weather occurred in Beijing, Tianjin and Hebei. The real picture shows that the visibility in a certain area in southeastern Beijing is poor and the air pollution is serious (Photo: Zhao Tianyu)

Why did the fog and haze weather that disappeared for a long time "come back"? On this issue, the reporter interviewed Sun Yang, a researcher and senior engineer at the Institute of Atmospheric Physics of the Chinese Academy of Sciences.

In fact, the frequency of blue skies in the Beijing-Tianjin-Hebei region has been low since the Spring Festival this year, and Beijing also experienced local heavy pollution weather at the end of February. Therefore, Sun Yang pointed out that it should be noted that the fog and haze weather this time does not only refer to Beijing, but the entire Beijing-Tianjin-Hebei region along the Taihang Mountains and Yanshan Mountains. Pollution in places like Baoding and Shijiazhuang is also relatively serious.

▲Changes in national PM2.5 concentration from March 3 to March 5, 2023 (Source: China National Environmental Monitoring Center, Henan Provincial Ecological Environment Monitoring and Safety Center)

Analyzing the causes of the fog and haze weather, Sun Yang said that the National Joint Center for Air Pollution Control has been paying close attention to it and has organized experts to conduct analysis and judgment.

First, the high level of industrial production leads to high emissions: since the Spring Festival, industrial electricity consumption in Beijing-Tianjin-Hebei and surrounding areas has risen rapidly, and the steel, coking, and glass industries in the region have been running at high levels; cement, lime, bricks, tiles, ceramics and other industries have also started work one after another, which will inevitably lead to increased emissions. In addition, the level of mobile source activity in the region has also grown rapidly. At present, indicators such as diesel truck traffic flow, diesel sales, and construction machinery start-up time are at their highest level since 2023.

Secondly, unfavorable meteorological conditions are an important factor in the occurrence of fog and haze. According to the Central Meteorological Observatory, the Beijing-Tianjin-Hebei region has experienced a sharp rise in temperature since March this year, with the daily maximum temperature in most areas reaching 22-25°C. More than 20 national meteorological observation stations in Beijing, Langfang and other places have broken the historical maximum temperature extremes in early March. The rapid temperature rise has led to high humidity and strong inversion in the Beijing-Tianjin-Hebei region at night, and poor diffusion conditions. In particular, since March 3, an inversion layer pollution belt with "warm upper layer and cold lower layer" has appeared at a height of 1 km near the ground, and pollutants cannot diffuse vertically.

▲CMAQ-ISAM source analysis of PM2.5 concentration time series and percentage pie chart in Beijing and surrounding areas (Source: Beijing Ecological Environment WeChat)

The third is that pollution presents a "superposition" effect: monitoring data analysis shows that the concentrations of secondary components such as nitrates in PM2.5 in the upwind areas of Beijing are relatively high, indicating that NOx emitted by key industries, gas boilers, motor vehicles, and construction machinery and its secondary conversion contribute significantly to PM2.5 pollution.

In addition, relevant monitoring data showed that Beijing was significantly affected by regional transmission during this round of pollution.

▲Schematic diagram of the atmospheric movement paths (red and blue) in Beijing-Tianjin-Hebei and surrounding areas affected by the "arc-shaped mountain range" (black) terrain. (Image source: China Meteorological Administration)

Does this mean that the Beijing-Tianjin-Hebei region will frequently encounter fog and haze this spring? Sun Yang believes that the possibility is not high. This is related to the climate characteristics of the Beijing-Tianjin-Hebei region. The four seasons are distinct and there are few climate events that last for a long time, such as the plum rain in the south of the Yangtze River and the autumn rain in the southwest of China. After the polluted weather lasts for 2-3 days, it often spreads automatically with the arrival of cold air or atmospheric movement . "In fact, at the turn of winter and spring, the changes in pollution sources are not as great as those in meteorological conditions. In other words, as long as the meteorological conditions are not good, it is easy to cause pollution." Sun Yang said.

Further reading >>>

An important factor driving fog and haze weather: calm and stable weather

▲Stable weather causes fog and haze (Photo source: Dahe.com)

Calm weather, as the name suggests, refers to calm and stable weather. It usually refers to a low-level atmospheric dynamic and thermal feature with low wind speed near the ground and stable atmosphere (accompanied by temperature inversion). If the atmosphere is continuously calm and stable, fog and haze are likely to form.

Stable weather mainly occurs in autumn and winter or at the turn of winter and spring. Since the lower atmosphere is less stable and has less unstable energy, it is easy to form a stable atmospheric stratification, and the atmospheric mixing layer is relatively low, resulting in a high frequency of stable weather.

Stable weather is also an important condition for the formation of fog and haze, but there are also differences between fog and haze. First, the visibility range is different. The horizontal visibility of fog is less than 1 km, and the horizontal visibility of haze is less than 10 km. Second, the relative humidity is different. When fog appears, the relative humidity is supersaturated, while when haze appears, the relative humidity is low. Third, the time of appearance is different. Fog is most likely to appear from midnight to early morning, and dissipates after the sun rises. The daily variation characteristics of haze are not obvious, and it is more likely to appear when the air mass is relatively stable.

From a meteorological point of view, fog often means that the atmosphere is relatively stable, and a stable atmosphere tends to attract pollutants, so fog and haze can easily transform into each other.

It is worth noting that heavy pollution caused by calm weather not only seriously affects the city's visibility and interferes with normal production and life, but also easily causes respiratory diseases. The long-term impact of air particulate pollutants may also cause acute and chronic bronchial asthma, pneumonia and other respiratory diseases and cardiovascular diseases. The public must pay attention to heavy pollution and take precautions.

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Produced by: Beijing Science and Technology News | Beijing Science and Technology Media

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