Ronald Cohen, a UC Berkeley atmospheric chemist, and his graduate student, Joshua Laughner, surveyed the skies above dozens of U.S. cities. The results revealed a surprising discovery about the smog that’s suspended over Los Angeles: a main ingredient in the pollution is fading away slower than it was before.
Cohen said:
That’s certainly part of why we’re in a moment in Los Angeles where it’s harder to get the air cleaner.
Their findings lead scientists one step closer to explaining why the once-steady improvements in air quality have nearly stalled, even though nitrogen oxide emissions continue to decline. Their research also suggests that the reason may have something to do with the particular chemistry of L.A.’s air. If so, the situation may complicate clean-up efforts in the future. The study has been published in the journal Science.
The pollutants Cohen and Laughner zoned in on were nitrogen oxides (collectively known as NOx). They identified cities where the levels of NOx have fallen out of tandem with emissions in the past few years. Since the pollutant is so abundant in L.A., the discrepancy is particularly significant.
John Seinfeld, a Caltech atmospheric chemist who was not involved in the work, said:
The results of this study will be important in pointing the way toward future reductions in smog.
How does NOx end up in the atmosphere? Nitric oxide (NO) and nitrogen dioxide (NO2) combined make up NOx. They are generated by motor vehicles and industrial machinery such as boilers, turbines, power plants, and cement kilns. NOx molecules become a pollutant (like ozone) when they mix with volatile organic compounds (VOC) then are exposed to sunlight. VOC’s come from vehicles, as well as numerous household and commercial products.
Laughner and Cohen wanted to know if the falling NOx emissions (decreasing at a rate of 7% a year between 2006 and 2013) had any correlation with the NOx levels in the air. So, they used satellite data from 28 U.S. cities (including Indianapolis, Memphis, and New York) to measure nitrogen dioxide – because it’s a good proxy for NOx as a whole. They zoned in on city centers and measured how rapidly the levels dropped with distance.
They discovered that the length of time the molecules lingered in the air before being broken down by chemical reactions appeared to follow different patterns over time in different cities.
Cohen said:
It changed over the last decade, and it changed in really interesting ways. In some places, the lifetime got longer, in some places it got shorter. In some places, it did one and then the other. In cities like Los Angeles, Memphis and Washington D.C., those NOx lifetimes fell from 2006 until around 2010. Then they started to creep up again.
Could this be why the amount of NOx in L.A. was steadily dropping for years, then suddenly it began to even out? They concluded that the differences in NOx lifetimes in L.A. and other cities could be a result of the particular chemical makeup of the atmospheres above each location.
Cohen said:
What we can’t do is explain why some cities behave in one way and why in another. We think it’s because we don’t understand the organic molecules in those cities.
Overall, L.A. emission cuts have made an impact. For instance, back in 2006, there were over 41 metric tons of NOx in the air, but by 2010, there were about 27 metric tons. Since then, however, it has flattened out at 25 metric tons. Second to L.A. in NOx levels is Chicago with almost 17 metric tons, and third is Detroit with just below 12 metric tons.
The reason L.A. has the most NOx is because so many people drive cars, and the city lies in a basin – which makes it hard for airborne chemicals to escape.
Seinfeld said:
Los Angeles is always in the lead. This is where smog first appeared.
Although, when analyzing the concentrations of NOx, L.A. came in third with four quadrillion molecules per cubic centimeter. New York City had the most in this category with 4.7 quadrillions, and Chicago was second with 4.1 quadrillions.
On the bright side, L.A. has had the best improvement in concentrations, dropping by 45%, New York and Washington D.C. came in second with 40%, and Chicago places third with about 36%.
The study only checked the levels up through 2013, so the situation above L.A. may have improved, says Laughner. He said:
Some surface measurements of NOx in Pasadena do show a decrease in the last few years.
For the next phase of their study, they plan to figure out what factors are driving these differences in NOx behavior. Cohen is confident it has something to do with VOCs. A more thorough understanding of atmospheric chemistry will help scientists come up with strategies to improve air quality over cities. That is the ultimate goal here.
