Dust Masks Acid Rain 'Time Bomb' in China

March 31, 2009 -- Powerful dust storms that whip across China's north and central deserts are infamous for blotting out the skies over Beijing. They wreak havoc with transportation and industry, and pose a serious health risk to the 17 million people who live there.

But they may be a blessing in disguise. According to a new study, the dust is protecting the city from a horrible case of acid rain.

And government reforestation and farmland management programs may be backfiring, inviting corrosive precipitation into the country's capital region.

Acid rain is a known scourge in China's heavily industrialized southern and northeastern reaches, threatening soil quality, forests and food supplies.

But for all its smog-ridden reputation, Beijing remains comparatively acid-free; an island amid the country's sea of coal-burning, sulfur-belching power plants. The reason is the region's regular dust storms. The calcium-rich dust acts as a buffer, neutralizing sulfuric and nitric acid particles before they fall to Earth.

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"Beijing city is surrounded by some desert areas," Zhifan Xu and Guilin Han of the Chinese Academy of Sciences wrote in their study, which appears in the April issue of the journal Atmospheric Environment. "The soil dust from these areas can contribute a large amount of alkaline material to precipitation and to neutralize the acidic ions."

Xu and Han found the effect was greatest in late spring and summer, when westerly winds periodically howl out of arid regions, and entrain dust on their way to Beijing. During such episodes, rainwater reached a peak pH of 7.62 and was routinely measured above the preindustrial global background level of 5.2.

But the researchers note that average pH has dropped 1.5 units since the 1980s. Government efforts to stem deforestation, desertification, and topsoil erosion in agricultural areas are at least partly to blame.

"In order to improve air quality in Beijing, the Chinese government has taken some powerful measures to control sand-dust storms in northwest China since the 1990s," they wrote. Since then, calcium measured in rainwater has fallen by nearly 40 percent.