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UPDATE 1-Beijing chokes in heavy dust as sandstorm blows in

* Chinese capital's air quality "hazardous" on Wednesday

* Area of 1.5 million km in northern China affected

* Dusty conditions set to linger into Thursday, improve gradually (Updates air quality levels, adds detail)

BEIJING, March 28 (Reuters) - Beijing's air quality index (AQI) soared to hazardous levels on Wednesday as a sandstorm blew in from Inner Mongolia and smothered the Chinese capital and other parts of northern China in dust.

China's National Meteorological Center (NMC) issued a 'blue' sandstorm warning late on Tuesday. It is the lowest in China's four-tier alert system, telling citizens in some northern regions to close doors and windows, wear scarves and dust masks and drive slowly due to poor visibility. The alert would remain in place until 1200 GMT on Wednesday, the agency said.

In an update at 0500 GMT on Wednesday, it said dust and sand had covered an area of 1.5 million square kilometers, including parts of Beijing, Tianjin, Hebei, Shanxi, Jilin, Liaoning, Heilongjiang, Inner Mongolia and Xinjiang.

Because atmospheric diffusion conditions in Beijing had not improved, the city will still be affected by dust on Thursday, although the impact will gradually weaken, the NMC said.

The AQI reading for Beijing provided by the U.S. State Department, based on the concentration of breathable particles known as PM2.5, was 164 as of 0900 GMT, rated as "unhealthy", down from a "very unhealthy" 244 at 0100 GMT.

The notoriously polluted Chinese capital was one of 34 cities in northern China to issue major smog alerts over the weekend, as industrial activity starts to ramp up again after winter curbs.

China's own official AQI reading for Beijing - calculated using measurements of six different pollutants including PM2.5 - was 871 by 0800 GMT, rated as "hazardous". That was down from 999 earlier in the day, when readings provided by some mobile phone applications gave figures as high as 2000. Sandstorms register on the index as larger particles known as PM10.

The NMC said the sandstorm had originated in China's Inner Mongolia region and been blown eastward.

China normally blames Mongolia's Gobi desert for its annual sandstorms. Delegates from China's arid, desertified Gansu region said in a proposal to China's parliament earlier this month that more than half of the dust storms that descend on China each year come from abroad, mainly from southern Mongolia.

Beijing has been planting millions of trees along its border in order to block out sandstorms, part of a project known as the "Great Green Wall".

The number of dust events recorded in northern China stands at four in 2018, the NMC said, noting that this was slightly below the average for the period of 4.4 in the last 10 years. (Additional reporting by David Stanway in SHANGHAI Editing by Kenneth Maxwell and Matthew Mpoke Bigg)