Why did the smog in Beijing disappear without cooling down?

On February 22nd last year (12) and this year (12), the smog in Beijing suddenly disappeared without wind or cooling. The reason is explained by experts from a three-dimensional perspective.

Waiting for the wind is people's anxious expectation in the fog. However, unlike the strong wind at the end of the smog on June 8, Beijing residents are generally puzzled: without strong wind or cooling, the smog disappeared. Then, can we get inspiration from the smog "disappearing in place" incident and formulate effective measures to control smog? Related speculations are widely forwarded in the circle of friends.

In this regard, Jiang Dahe, a blogger of Science Network and a professor at Tongji University School of Environmental Science and Engineering, published a blog post and questioned that the smog dissipation process should be explained in three dimensions, and this complex frontal process played a role.

When reporters visited environmental and meteorological experts, they also got the answer that "smog will not disappear on the spot", but in the three-dimensional dynamics of air flow, there has been a concentration change that is difficult for the human body on the ground to detect.

The smog just disappeared?

There is no strong wind, sunny and warm, where did the smog go?

Hui Jin, a member of Friends of Nature, published an article about the "disappearance" of smog on February 22nd, which gained a high reading volume and reprint rate.

He proposed: "According to data observation, the smog index of many stations in Beijing 14 dropped by as many as 400 points in an hour or two, and no diffusion and transfer of pollutants were found. There is no sign of strong wind blowing. Where did the smog go? "

By comparing and analyzing the specific real-time indexes of several monitoring stations in Beijing, Hui Jin believes that for this severe smog that lasted from February 6 last year 16 to the early morning of the 22nd, exceeding 120 hours, "in the final stage, no matter you look at each station or the whole of Beijing, suspended pollution particles in the atmosphere basically do not exist, or have not spread to the outside world at all."

So is the smog "disappearing on the spot", or are there any meteorological phenomena that we have ignored, or are there other reasons?

Cold air blowing haze is a frontal process.

With full of doubts, the reporter of China Science News contacted Jiang Dahe. He responded: "Does the pollution index in the relevant areas have to go through the process of rising and then falling for several hours? This is an idea of plane propulsion, which explains the phenomena discussed in two-dimensional space. "

Jiang Dahe observed that after February 16 last year, the meteorological cause of smog pollution in Beijing, Tianjin and Hebei was that warm and humid air currents prevailed and lasted for many days. It was not until the early morning of February 22nd that a strong cold air entered Beijing from north-northwest to south-southeast direction.

"This is the meeting of two different air masses: one is wet, warm and polluted, and the other is dry, cold and clean." There is a frontal process near the interface where the cold air mass pushes the warm air mass south, and different air masses will basically maintain their own characteristics, but only mix near the interface.

The distribution of temperature, humidity, air pressure and wind force during the frontal process is very complicated, and the specific situation is related to the strength, wind speed, wind direction and topography of the air masses on both sides. The mixed layer will be destroyed and produce vertical movement, which may be accompanied by precipitation. It is easy to imagine that it is a three-dimensional dynamic. Moreover, "pollutants are easily blown away in the sky, and the ground monitoring results cannot explain all the problems." Jiang Dahe further explained, "On February 22 last year, the cold air of Beijing 65438 went south, which seemed to belong to a weak cold front process in winter."

At the same time, although the transmission, diffusion and accumulation of haze in Beijing were not obvious from 20 16 to 12-22, Jiang Dahe thought that the haze in Beijing-Tianjin-Hebei region was actually transported to the south by consulting the national haze distribution map.

However, since the cold wind blows away the smog, why can't it feel cool? "The cold air mass is too weak." Jiang Dahe answered the reporter: "This is also the reason why the smog has gone and hurried back."

The horizontal wind is very strong, and pollutants spread upstairs.

Meteorologists also explained the "disappearance" of smog from a three-dimensional perspective.

Taking the smog that disappeared on June 2, 65438 as an example, Zhang Hengde, deputy director of the Environmental Meteorological Center of China Meteorological Bureau, said that since the early morning of the 2 nd, although the ground wind speed was very low, the human body did not feel obvious, but at a height of 200~3000 meters from the ground, the northerly wind was very obvious, and the northerly wind came earlier than the ground wind.

The pollutants in the air are distributed vertically, and when the horizontal wind is strong, the pollutants will also spread vertically. "When the cold air arrived over Beijing, the smog thickness had reached 800 meters. At that time, the ground wind was very small, but the northerly wind at a height of 200 meters was very obvious, so the emission reduction effect was obvious. " Zhang Hengde told the reporter of China Science Journal. However, unlike the process of 65438+February 22 last year, due to the weak cold air mass on June 2, "the whole air layer turned southerly again at noon on the 2 nd, and the haze came back soon."

As Hui Jin once mentioned: "Severe smog has always been the cause: a large number of pollutants are the root cause, and meteorological conditions that are not conducive to diffusion are the direct cause." It is true that when the smog dissipates, we still need to wait for the change of meteorological conditions.

Liu Hongli, an associate researcher at China Institute of Meteorological Sciences, said in response to a reporter's question from 65438+ on17/0 that unless there is human intervention, such as closing factories, the pollution emission level will not fluctuate greatly. Meteorological conditions promote smog changes, and high pollution levels and bad meteorological conditions form smog events. If it rains or is windy, it can clear the smoke. Under the same emission conditions, the role of meteorological factors exceeds 70%.