Four women struggle across a street in Beijing during the sandstorm on Friday. [Photo/China Daily]

BEIJING - Most parts of North and Northwest China have witnessed sandstorms since March 12, with Beijing wrapped in sand and dust on Friday.

The air quality in the capital is classified as slightly polluted in most areas, but severely polluted in Badaling in Yanqing county, according to the latest statistics from the Beijing environmental protection bureau.

The bureau said several of its downtown surveillance stations had detected higher than normal levels of particulate matter - a major indicator of air pollution, at an average 500 milligrams per cubic meter.

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In the outer Changping and Daxing districts, the level topped 800 mg per cu m, the bureau said.

The sandstorm is likely to last until Monday, and will cover Xinjiang, Qinghai, Gansu, Inner Mongolia and Ningxia, the China Meteorological Administration (CMA) said on its website on Friday.

"Generally speaking, about 80 percent of the country's sandstorms happen in spring, especially from March to May. We are entering the sandstorm season," said Li Xiaoquan, a meteorologist with the CMA.

The monitoring results of the National Climate Center show there will be slightly more sandstorms this spring than in 2010, but below the average level in North China.

Besides the wind, the movement of cold air will also bring a dramatic drop in temperature, with 10 C decreases in parts of North China, the CMA said.

Hu Yinghua, a senior weather engineer in the Inner Mongolia autonomous region, told Xinhua News Agency on Friday the sandstorm in Inner Mongolia this year is "normal".

Hu added it would pass through the area before 8 pm on Friday, but fresh cold air will hit China over the weekend.

Residents in Golmud city, Northwest China's Qinghai province, witnessed a violent gale with a top speed of 26.3 meters a second, the fastest in 30 years, said Zhang Jinong, chief weather forecaster at the Qinghai weather bureau.

The sandstorm in Qinghai's Nuomuhong county, lasted nearly 3 hours on Friday, Zhang said.

Due to the decreasing air quality in the capital, Beijing environmental protection bureau suggested local residents close doors and windows, and cover their faces with masks or scarves when going out.

Recently, sandstorms have been a big problem in North China after years of overgrazing, deforestation, urbanization and drought, with deserts making up about 16 percent of the country.

Responding to this situation, the government had invested about 4 billion yuan ($588 million) in a sandstorm control project in Beijing and Tianjin from 2000 to 2009.