沙尘暴是沙暴和尘暴两者兼有的总称，是指强风把地面大量沙尘物质吹起并卷入空中，使空气特别混浊。春天刚刚到来，人们还来不及欣赏美景，沙尘暴就袭来了。

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There is an old saying that a year's plan starts with spring. However, for Beijing, a year's bad weather also starts in that season. Sandstorm could bring tons of mud to cover the city overnight.

How does it form and why does it always come and visit at the best time of year? Chai Fahe is vice director of the Chinese Research Academy of Environmental Sciences.

"The formation of a sandstorm requires two conditions. One is strong wind; the other is that there should be large areas of bare ground. The strong wind will blow the sand on the ground into the air and make the atmosphere turbid. When the visibility drops below one kilometer, we call it a sandstorm."

Chai Fahe says the dry and cold weather in winter make sand stay on the ground. And sand is made of silicate. When the temperature rises in spring, the hydrated silica will dehydrate and carry negative charge to push themselves away from each other. So at the time, even a small breeze can easily blow the sand into the air. That's why sandstorms are mostly seen in spring.

Chai Fahe notes that sandstorms do make Beijing's air quality worse, but it's not the sand in the storms we should fear.

"Sandstorms transport not only sand and dust, but also air pollutants. What we see in Beijing today is a mixture of sand and harmful pollutants. "

The major air pollutants are airborne particles, sulfur dioxide and nitrogen dioxide. And airborne particles PM10 pose the most serious pollution problem.

Professor Li Yujun works at the Chinese Academy of Social Sciences. She explains that PM10 refers to airborne particles with an aerodynamic diameter smaller than 10 micrometers and it does affect our health.

"PM10 is very small. It can be inhaled into people's lungs and deposited there. It will affect the respiratory system. For those who have respiratory illnesses, exposure to PM10 will worsen the illness. It will greatly impact those with chronic lung disease, heart trouble and asthma. Elderly people and children are especially vulnerable."

Li Yujun suggests that people should avoid going outside in a sandstorm or at least wear a mask on such days.

When looking at all the negative effects that a sandstorm brings, Professor Chai reminds us that that sometimes, it does benefit our environment. One example is that sandstorms can reduce the incidence of acid rain in Northern China. He says the silicate in the sandstorm will neutralize hydrogen ion's in acid rain, raising the PH value of rain by 0.8 to 2.5.

Nevertheless, Professor Chai says it's still necessary to prevent sandstorms.

"Both acid rain and sandstorms are not what we want. But when weighing their harm, we can see that acid rain affects plants, while sandstorm directly harms people's health. Sandstorm is a much more serious problem. It's very necessary for us to prevent it."

Professor Chai says the only effective way to curb sandstorms is to keep the sand on the ground. Thus we should plant more trees.

For CRI, I'm Wang Wei.

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