Volume 6, Issue 40
Sniffing Out Smog
Atmospheric chemist Ron Cohen studies air quality in places ranging from UC's Sagehen Creek Reserve to the Canadian tundra. Image credit: Michael Barnes.
If smog were a kitchen creation, the recipe would go something like this: Start with a miasma of organic hydrocarbons from spilled gasoline, incomplete combustion and trees. Add nitrogen oxides from combustion in factory furnaces and vehicle engines. Zap with a dose of sunlight, and wait. The result: a heaping serving of photochemical smog.
Made up of components such as nitric acid and ozone gas, smog is nasty stuff. Nitric acid is a component of acid rain, while ozone kills human lung cells and contributes to global warming.
Atmospheric chemist Ron Cohen studies how these pollutants form, tracks where and how far they travel, and how they get removed from the atmosphere. He then uses this knowledge to understand air quality and the interactions of pollutants with climate. A Berkeley professor of chemistry and earth and planetary sciences, his work provides the factual underpinnings for climate and air pollution models that, ultimately, help keep us all breathing more easily.
Cohen's studies in the Sierra Nevada proved that smog forms twice as fast as previously thought. The effect makes daytime air cleaner within Sacramento than around foothill forests. Image credit: courtesy Ron Cohen
"Most of my work is figuring out how to make observations that test parts of these models; I ground truth them," Cohen says. "The goal is to provide the underlying science for the people who have to make social and policy judgments."
Cohen tracks air pollution from its molecular origins through its metamorphosis into ugly yellow smog. To do this, he designs and builds instruments capable of measuring minute amounts of the chemicals that contribute to air pollution. Armed with these super sniffers, he can track exactly how fast smog forms and how much of it is in the environment.
"We try to understand how these molecules get into the atmosphere, what their chemistry is, and what they're doing to climate," Cohen says.
Cohen's smog-sniffing instruments are mounted high above UC's Blodgett Forest Research Station.
For the past decade, Cohen has deployed these instruments to study smog formation in the Sacramento Valley. Air patterns there are as regular as the tides. During the day, winds sweep nitrogen oxides belched out by trucks and businesses up into the Sierra Nevada. At night, the cooling air tumbles back into the valley again.
"We can see how factors such as temperature or decreases in emissions on weekends affect the chemistry because everything else holds more or less constant. It lets us tease apart how the chemistry works much better than we've been able to do anywhere else," he says.
Cohen placed his instruments around Sacramento, in the Sierra foothills and just west of Lake Tahoe. Each step of the way, he measured the relative concentrations of smog components. By combining this data with wind speed and temperature information, Cohen could measure precisely how fast these reactions were occurring.
Weekday versus weekend air quality in the Bay Area. Weekend air is much cleaner because fewer big rigs are on the road. Image credits: SCIAMACHY instrument, courtesy Tim Bertram and Andreas Richter
In the foothills, Cohen found double the nitric acid he expected, indicating the photochemical reactions that produce smog run twice as fast as previously thought. And by the time the air masses near Lake Tahoe, most of the nitric acid has already disappeared. That means ozone levels in hamlets like Auburn and Kyburz can be far worse than at the state capitol. And air pollution from Sacramento isn't fouling the lake; the nitric acid entering its limpid waters comes from local traffic instead. From a broader perspective, his results help link regional air quality to larger climate trends.
One big question mark that remains in the models is how much ozone comes from natural sources like lightning. Cohen seeks to quantify this natural background by testing the air over some of the world's most remote places. He sets up the sampling instruments inside a specially outfitted NASA DC-8, straps himself into the extra comfy seats, and settles in for up to 12 hours of cruising across places such as the Canadian tundra and the unbroken waves between Hawaii and Alaska.
"Even there we see quite a strong signature of emissions from Asia. There's no place, at least in the Northern Hemisphere, where you can escape," Cohen says.
Cohen loads his custom-built smog detecting instruments into a NASA DC-8 to study air quality over pristine environments. Here, the plane is at Diamond Head, Hawaii, for a sampling mission over the Pacific Ocean. Image credit: Scott Sandholm
Despite these trace plumes of pollution, nitrogen oxide levels past Midway Island are so low that they don't inhibit natural ozone removal processes. Cohen is trying to identify the threshold concentrations where that shift back to ozone production starts to occur. If nitrogen oxide levels over the Central Pacific are already near that threshold, then lowering pollution a smidgen more might make a tremendous difference in air quality and maybe even global warming.
"As a society, we're in an amazing spot," Cohen says. "We have a pending catastrophe-climate change- that we can do something about if we use science wisely. And as a scientist, there's an incredible wealth of fun, interesting questions to work on while trying to help the social system address the question."