Megacities' smog mapped from space

• A satellite survey shows U.S., Europe cities less smoggy
• China, India, the Middle East and Africa have more smog
• Reasons for smog include: traffic, industry and heating

U.S. and European "megacities" look less smoggy over the past decade, while ones in China, India, the Middle East and Africa look worse, reports a satellite survey.

Cities now hold roughly half of the world's population, with megacities of more than 2 million people growing rapidly. Air pollution dogs these cities, mainly from traffic, industry and heating.

In the American Journal of Climate Change report led by Pinhas Alpert of Israel's Tel Aviv University, researchers looked at aerosol pollution data collected by three NASA satellite instruments from 2002 to 2010. The instruments observed the 189 largest cities worldwide.

Among the worsening cities, Bangalore in India, saw a 34% average increase during the study period. Among the improving cities, Houston saw a 31% decrease in aerosols over the time period and Stockholm saw a 23% drop. Seattle and Portland looked more smoggy in the study, but Alpert said in a statement that he suspects that wildfires account for the increased smogginess, rather than industrial pollution.

As well as providing a snapshot of pollution, the measurements provide a check on ground monitors of pollution, the study says.