Reuters

Researchers measuring pollution during last year's 405 closure find air quality improved by as much as 83 percent.

Nature doesn’t normally present the kind of opportunity that "Carmageddon" did in Los Angeles last year, an opportunity to catch a glimpse of atmosphere that’s typically saturated with pollutants on a suddenly pristine day. As you'll recall, Los Angeles shut down a 10-mile stretch of one of its busiest highways, the 405, for a July weekend in 2011 (the city reprised the closure this past weekend to finish the project). Locals predicted apocalyptic gridlock. Instead, out of fear of just such a scene, drivers stayed home in dramatic numbers – from the 405, but also throughout the entire region.

"In environmental science, generally, if you make measurements in the real world, you can’t really manipulate the real world," says Suzanne Paulson, a professor of atmospheric and oceanic sciences at UCLA. "So when there are natural experiments like this – if you can call it natural – we like to take advantage of them."

Paulson and colleague Yifang Zhu measured pollutants in the air during Carmageddon last year and have recently released their pretty astounding findings. Air quality near the normally busy highway improved by 83 percent that day last July, relative to comparable weekends. Elsewhere in West Los Angeles, the improvement was equally dramatic. Air quality improved by 75 percent on that side of the city and in Santa Monica, and by 25 percent throughout the entire region, as a measure of the drop in ultrafine particulate matter associated with tailpipe emissions.

"We saw what we expected: you take motor vehicles away, the air gets really, really clean," Paulson says, "which tells us that most of the pollution is from motor vehicles from one type or another in this area."

There's little heavy industry around this stretch of the 405 Freeway; to the extent that this part of the city feels soaked in smog when you step outside for a breath of fresh air – that’s cars. Almost all cars.

The researchers found that particulate matter dropped significantly within minutes of the road closure (accordingly, it ramped back up the moment traffic resumed). And this is significant for policy reasons. When it comes to the environment, we’re often talking about making difficult or expensive investments in the short term that may not pay off for years to come. But this research underscores that changes in transportation policy or vehicle technology could yield practically instantaneous improvements in the quality of air (and quality of life) for people today.

"In the broadest picture," Paulson says, "what these measurements gave me was a view into what a future would be like where either people were using much more mass transit, and/or they’re driving vehicles that are really very clean and that remain very clean throughout their lives."

Typical internal combustion engine cars, of course, tend to get dirtier as they age.

Heading into "Carmageddon" last year, drivers and scientists alike had no idea what to expect. But because drivers stayed off roads throughout the area en masse, the event turned into a researcher’s dream, as well as, now, a boon for environmental advocates who until now have had to paint a more abstract picture of what our air would look and feel like in a world with fewer highways.

"This tells us that whatever we do to either clean up the tailpipe emissions of the motor vehicles, or to reduce people being in cars by themselves will improve air quality," Paulson says. "As vehicle fleets turn over as we get more and more electric cars and more and more hybrids – which are more common on the west side of L.A. than in most places – we’re going to see cleaner air. We think we can kind of see that already."

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