Smelling the air

We simply call it the “air.” But simple it is not.

Most of the air we breathe is oxygen and nitrogen, with a smattering of argon, water vapor, and carbon dioxide. The remaining fraction of a percent is composed of tiny traces of other gases and airborne particles. But despite their minuscule quantities, those other chemical compounds play a big role in the atmosphere, affecting climate and air pollution, which in turn can influence the health and livelihood of every organism on Earth.

Those trace compounds comprise hundreds of thousands of different chemicals—many of which remain unidentified. And many of the chemicals don’t come directly from any single source, but are instead produced in the atmosphere through a series of complex reactions. Unraveling this chemistry isn’t an easy task, but it’s a crucial one.

The particles suspended in the air, called aerosols, are an especially important component of the atmosphere. For one reason, they pose a major health threat. According to the Global Burden of Disease , a UN study published in December 2012 to quantify the global health impacts from a variety of risk factors, household air pollution (such as the particles produced while burning cooking fuel) contributed to 3.5 million premature deaths worldwide in 2010. Pollution from particulates in the ambient air contributed to 3.1 million deaths.

Moreover, aerosols can have a warming and cooling effect on the climate, depending upon their size and chemical makeup. For example, black-carbon aerosols absorb sunlight and help warm the planet while sulfate particles reflect heat. Aerosols are also important in forming clouds, which reflect sunlight and cool the planet. But scientists don’t yet know the full impact that aerosol particles can have on the climate. Indeed, such particles remain one of the biggest sources of uncertainty in climate models.

So to better understand both climate change and air pollution—and to inform effective policies to stem their impacts—scientists are using new tools to measure aerosol composition and to tease apart the complex chemistry of the air.

Building a better sniffer

To that end, researchers at the University of California, Berkeley, a small Berkeley-based company called Aerosol Dynamics and a Massachusetts-based company named Aerodyne Research have built a new type of instrument that they say captures the most complete characterization of aerosols yet in an automated fashion. The device, which is called TAG-AMS (for thermal desorption aerosol gas chromatograph-aerosol mass spectrometer), is a marriage of two instruments: the TAG and the AMS. Researchers at the University of California, Berkeley, and Aerosol Dynamics developed the TAG, while Aerodyne built the AMS, and has commercialized the TAG-AMS. Aerodyne sold the first TAG-AMS unit in summer 2013.

The AMS, which Aerodyne has been developing, refining, and selling for more than 15 years, measures the size and the chemical composition of submicron particles in real-time. The machine sucks in an air sample and, within seconds, vaporizes the particles, which are then analyzed with a mass spectrometer. The conventional way that scientists make these measurements is to pump air through filters that catch the particles. The samples have to be taken back to the lab where they’re analyzed, meaning you can’t measure changes in the air on timescales much shorter than a day. But not only does the AMS take data immediately, it can make measurements every few seconds. Almost 200 research groups worldwide now use the AMS, says John Jayne, one of the developers of the AMS at Aerodyne.

In 2004, during a campaign to measure the air over Nova Scotia, Jayne and others from Aerodyne worked alongside Allen Goldstein, an atmospheric chemist at UC Berkeley, and Susanne Hering, the president and founder of Aerosol Dyanmics. Goldstein and Hering had been developing their own instrument that detects individual organic chemical species—a device dubbed TAG (since it “tags” individual chemicals in aerosol particles.)

Goldstein is an expert on gas chromatography, a technique that separates chemical species by passing them through a narrow tube about 20 meters long. Lining the interior of the tube is a chemical that reacts with chemical species in different ways, causing each species to pass through at different rates. The species, now separated, can then be measured with a mass spectrometer.

Whiffs of caffeine

He and Hering began developing TAG in 2001. Hering had invented a so-called impactor technique to collect particles by blasting the air sample against a collection surface, on which the particles stick. At the time, Goldstein was using his gas-chromatography methods to measure gases called volatile organic compounds, and he realized that by combining his techniques with Hering’s impactor approach, they could create an instrument to measure the individual chemical species of airborne particles. The resulting TAG instrument was automated and could take data every hour—a big advantage over traditional filter methods.

“It gives you information that you don’t otherwise have,” Hering says. “It gives you this time resolution of hundreds of thousands of compounds. It just gives you a lot more information for understanding what’s happening in the atmosphere.” The instrument is sensitive enough to pick up traces of chemicals on the order of a nanogram per cubic meter. In test runs from the Berkeley campus, the researchers were able to detect whiffs of caffeine, engine emissions, burning wood, and even cannabis and cocaine.

No other instrument can make these measurements in an automated fashion, says Allen Robinson of Carnegie Mellon University in Pittsburgh, an expert on measuring airborne particles and one of the TAG’s first users. Its automated capability allows researchers to take this type of data on a more routine basis, says Robinson, who wasn’t directly involved in developing the instrument. “It was a really big advance from that perspective.”

But while TAG is good at picking out individual organic chemical species, it can’t measure the aerosol composition as a whole, and thus enters the AMS. The AMS produces an overall snapshot of the aerosols: a mass spectrum of the total aerosol content, including organic and inorganic compounds. Combining TAG with AMS would result in a two-in-one instrument that could characterize aerosols more completely than ever before, providing the detailed data of TAG with the contextual information of AMS.

Recognizing the promise of such an instrument, Goldstein, Hering, and Aerodyne wrote a proposal in 2007 to the Department of Energy’s Small Business Innovation Research program, and soon, TAG-AMS was born. To save costs, the TAG and AMS components share a mass spectrometer. Aerodyne sells the instrument as an add-on to the AMS, tapping into the market of current AMS users.

Between gas and particles

Meanwhile, Hering and Goldstein have continued to collaborate, developing two other versions of TAG. The first, called 2D TAG, differentiates chemical species according to two properties: volatility and polarity. The original TAG separates chemicals according to volatility, but when faced with a particularly complex sample, chock-full of chemicals, a second property that allows you to distinguish one from another becomes extremely helpful.

Goldstein’s group at Berkeley realized that in addition to aerosols, their TAG instruments were collecting trace amounts of semi-volatile compounds, molecules that sometimes form in particles and sometimes in a gas phase, depending on the atmospheric conditions. “They’re not really particles and not really gases, and they sort of go back and forth,” explains Gabriel Isaacman, one of Goldstein’s graduate students. Isaacman helped develop the second TAG version, SV-TAG, which targets these semi-volatile compounds. To collect them, the researchers swapped the impactor with a new stainless-steel filter developed by Aerosol Dynamics, the large surface area of which allows it to capture and measure these elusive chemicals.

Previous instruments were limited to analyzing only volatile compounds, but SV-TAG aims to change that. “One of the really exciting things about this instrument is that it’s striving to measure a range of compounds that’s never been well-measured,” Isaacman says. This past summer, he deployed the SV-TAG on a field campaign in Alabama as part of the Southeast Atmospheric Study, an effort to determine the origin of aerosols in the southeast US. The SV-TAG has yet to be commercialized, but Goldstein says he hopes it will be among the next generation of Aerodyne’s TAG instruments.

Still, because of their simplicity and low cost, filters remain the dominant means that scientists use to measure aerosols. “But, I believe that will change,” Goldstein says. More researchers are using TAG instruments, and, he adds, while TAG instruments are now used primarily for scientific research, they may eventually be developed to the point such that regulatory agencies can use them to routinely monitor the air.

Of course, building instruments is just the first step. There’s still getting the data, analysis and developing the expertise to understand what it all means. “Dealing with thousands of compounds at once is super daunting,” Isaacman admits. “But it’s really kind of cool.”

Marcus Woo is a freelance science writer based in the San Francisco Bay Area who has written for National Geographic News, New Scientist, and other outlets.