Science of L.A.'s 'Carmageddon' proves (shock!) that cars cause much of LA's air pollution

Suzanne Paulson, UCLA professor of atmospheric and oceanic sciences, saw "Carmageddon" as an opportunity to make use of a "natural experiment." She and a colleague "measured pollutants in the air during the LA freeway shutdown last year, and have now released their 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."

More: L.A.'s 'Carmageddon' Produced Dramatic, Instantaneous Air Quality Improvements (The Atlantic).

Another "Carmaggedon" just took place in LA. Wonder if there will be more science to come from this edition.

(Image: Dallas Traffic 10/19/11 1227pm, a Creative Commons Attribution Non-Commercial Share-Alike (2.0) image from nffcnnr's photostream)


  1. I read somewhere long ago, that it was industry as the real culprit, and blaming it on the cars, was a way to keep industry off the hook, and make us feel bad.  Did not find this old chestnut in a quick google.

    1. I have heard anecdotally that the airline industry (who used to have a lot of manufacturing in LA) caused most of it. And when they moved out the air quality improved dramatically.

  2. Remember the air quality changes during the New England blackout a few years back?  I guess it’s rather hopeful: if we could do something about our point-emitters that it would make a real and tangible improvement in our lives.

  3. I wish for a future where most of us can telecommute. I work from home as often as possible. Hate driving for no good reason except to make “face time”.

  4. In the fifties the city of LA kicked the big rubber plant out of downtown because of all the smoke that they released. The downtown area had become nearly unlivable from the cloud of smoke that hung over the city. The plant went away, the cloud stayed. Some smart guys from Cal Tech studied the problem and concluded it was not industry that was the problem, it was all the cars! No way, said drivers. It took ten years for the world to catch up to those scientists and acknowledge that cars were, in fact, the source of what was by then known as smog. Another ten years and a lot of legal arm twisting before the car makers and gas sellers began to clean up their act.

    1. In the fifties the city of LA kicked the big rubber plant out of downtown because of all the smoke that they released.

      Oops, there goes another….

    2. That said, air quality in LA *is* apparently much, much better than it used to be. I’ve been told that, a few decades ago, the San Gabriel Mountains were often not very visible from Caltech’s campus only a few miles away.

      1. Not the only place to “clean up its’ act”, the Ruhr Valley in Germany was a heavily populated toxic wasteland back in the 60s and 70s, and I seem to remember reading that the Pittsburgh area was not too pleasant either, back in those days.

      1. Do any electric cars have purely regenerative brakes, or doesn’t magnetism cut it?

        And surely a bit of rubber and brake dust is pretty negligible?

  5. Tel Aviv has a smog problem very similar to Los Angeles.

    Until Yom Kippur. 

    You can experience it for yourself. Book a few days in a hotel in Tel Aviv that will overlap with Yom Kippur (and go food shopping the day before if you don’t want to fast. The city WILL shut down.)

    By 2 o’clock on Yom Kippur, it’s like you’re living in a totally different city. The quiet and the clean air are too obvious to ignore. 

  6. I don’t get it: why do we need a Carmageddon to get this info? Why not measure air quality on Christmas Day and the following day, when traffic also drops dramatically? Or why not just measure it continuously for a month, and correlate it with varying traffic?

    Also, what the hell does “83% improvement in air quality” mean? What’s the measure of “quality”? Obviously it means a lower amount of pollutants, but what number is then increasing by 83%? Does it mean pollution is reduced by 83% of the normal amount, which is a huge 6x reduction in pollution? Or is quality somehow an inverse measure of pollution, which means an 83% increase is only a 1.83x reduction in pollution? Don’t just copy numbers people, think numbers.

    1. I wonder about this too, and just spent 10 minutes unsuccessfully searching for the actual paper or press release. I don’t know of many measures that INCREASE with reduced pollution, except maybe some measures of atmospheric transmission. Though, the quoted scientists have published papers on optical properties of aerosols, so maybe that’s exactly what they are saying… I shouldn’t have to guess!

    1. But it’s not only 2 significant data points, really. The article refers to comparing to comparable weekends, plural, and I presume the historical data goes back further than that. Also, there were multiple collections points around the highways and the city. Finally, there’s existing data about what types of pollutants internal combustion engines put out.

      Even for events that happen only once or rarely (astronomy and particle physics is filled with them) you can still do good science. People have been measuring pollution in this city for a while, and can come up with an number that says, “the probability of measuring an 83% difference completely by chance or random variation is very small… , therefore it’s almost certain the change is real.” Then you look at potential causes. If the difference in traffic is the only known difference between that day and others AND the type of pollution is known to be expelled by cars (i.e. the mechanism is credible), it is pretty good science to think the traffic did it.

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