VOA标准英语2013--New Ways of Tracking Greenhouse Gas Emissions Developed（在线收听）

New Ways of Tracking Greenhouse Gas Emissions Developed

With more than 18 million people living, working and driving in Los Angeles, the city often is covered in a hazy layer of smog. Stan Sander, senior research scientist at the U.S. space agency's Jet Propulsion Laboratory near LA, describes this layer of visible air.

"That haze is caused by the fact that the air is trapped inside a layer that’s a few hundred meters to a thousand meters in altitude above the LA basin. So it collects those emissions from the cars and other sources and forms that layer," said Sander.

Jet Propulsion Lab scientist Riley Duren says these pollutants include greenhouse gases such as carbon dioxide and methane, and they do not just hang over the city.

"So these gases have a small local effect but the bigger impact is on the climate. We’re all in this together. It takes several weeks or months for these gases to mix, but they end up in the atmosphere and they affect everywhere - not just in the local city," said Duren.

Los Angeles is part of a complicated experiment called the Megacities Carbon Project. The goal is to monitor greenhouse gases and look for long-term trends to see if environmental policies to lower these pollutants actually work.

Sixteen highly sensitive monitoring devices are being installed throughout southern California on rooftops and media towers. These instruments work continuously to analyze what is in the air.

Scientists also use what is called "remote sensing" to monitor the air. Instruments placed on airplanes and a satellite look at sunlight bouncing off the surface of the earth. By looking at how the air changes the quality of that light, scientists can "see" the fingerprints of carbon dioxide and methane in the air.

There is one more remote sensing instrument on top of historic Mt. Wilson northeast of Los Angeles. NASA’S Stan Sander says this remote sensing equipment looks down throughout the LA Basin and analyzes the air.

"What we’re hoping to do here on Mt. Wilson is create a sort of pattern or model for the way other cities might be able to measure their greenhouse gas emissions in a very similar way," he said.

Riley Duren says that while developed countries are trying to reduce emissions...

"In the developing world, particularly in South America, Africa and Asia, we’re seeing explosive growth in cities because of the combined effects of urbanization and economic growth," he said.

He says many of these growing cities are at higher risk for the impact of climate change.