Tiny troubles in the air: Measuring scant compounds and their big impact on air quality
One of the main goals of NCAR’s atmospheric chemists and their colleagues elsewhere is to deconstruct the recipe of airborne compounds that cooks up the unique pollution stew found in a particular urban area. These molecular ingredients may exist for as little as a few seconds before being converted into another form, and some compounds make up less than one part per billion of the atmosphere. Two new instruments developed at NCAR—both exquisitely sensitive yet tough enough to be used aboard research aircraft—are helping scientists measure many of these chemicals with unprecedented precision.
On the trail of trace gases
Some types of pollution emerge from tailpipes and smokestacks in particle form, ready to shroud urban skies and clog sensitive lungs. Other pollutants take shape more indirectly. Ozone and so-called secondary organic aerosols—some of the most hazardous pollutants for human health and air quality—form only after sunshine has interacted with other atmospheric constituents. “These secondary pollutants are actually byproducts of the cleansing mechanism built into Earth’s atmosphere,” says NCAR chemist Eric Apel.
The starting points for these pollutants are nitrogen oxides and many types of volatile organic compounds (VOCs). The latter emerge mainly when fossil fuels, forests, and grasslands are burned, although intact forests can also contribute. Sunshine oxidizes the VOCs to produce ozone and secondary organic aerosols, which eventually rain out or settle out—the end point of a natural cleansing process. But a typical urban setting provides too many VOCs for nature’s scrubbers to keep up with, and pollution can build to unhealthy levels.
NCAR’s new Trace Organic Gas Analyzer (TOGA) is producing some of the most vivid pictures to date of how VOCs evolve. Designed for the rigors of airborne use, the instrument gathers and concentrates air samples at a rapid clip, and it features a gas chromatograph that can be heated and cooled very quickly, which enhances its ability to analyze fast-changing VOC levels.
About every two minutes, TOGA profiles as many as 50 VOCs that come and go in a 45-second window. All of these species are measured at a precision of less than 10 parts per trillion, and some can be gauged in the parts-per-quadrillion range. “We’re very pleased with its performance,” says Apel of TOGA, which he developed with Daniel Riemer (University of Miami) and NCAR’s Alan Hills. “It’s a big step forward for our community.”
VOCs are important in their own right, but they can also serve as useful clues to other goings-on. For example, methyl tertiary butyl ether (MTBE) is a VOC traditionally added to gasoline to increase its oxygen content. MTBE has been phased out of gasoline in the United States and Canada due to environmental concerns, but it’s still commonly used in Asia. Because MTBE survives up to four days in the atmosphere, scientists can use its presence or absence to determine whether or not a given air mass recently passed over Asia. During the 2006 phase of NASA’s Intercontinental Chemical Transport Experiment, TOGA’s ability to detect minuscule amounts of MTBE allowed for just this type of attribution.
The critical difference
If you’ve just installed carpet in your home, you may encounter plenty of airborne formaldehyde for a few days. In the open atmosphere, it’s a scarce quantity. Because it’s so quickly oxidized, formaldehyde makes up only 1 to 50 parts per billion in polluted urban areas and about 10 to 100 parts per trillion elsewhere. Yet formaldehyde is a major player in ozone formation, and often other VOCs are briefly oxidized into formaldehyde before further reactions unfold.
All this makes formaldehyde a vexing but important substance to keep track of. Most airborne VOC sensing devices, including TOGA, aren’t designed to measure it. However, a new NCAR instrument does the job with flying colors. It relies on a technology called difference frequency generation (DFG) that produces the mid-infrared light commonly used to detect VOCs without the bulky setup normally needed. “DFG is used at only a handful of labs around the world, mostly for fundamental research rather than field work,” says Dirk Richter, who brought the technique to NCAR after graduate study under Rice University’s Frank Tittel.
Like a painter who mixes two common dyes to synthesize a rare third color, the DFG process takes two near-infrared laser beams and mixes them in a crystal to produce the difference frequency. The result is a narrow beam of light in the desired mid-infrared wavelength. In three 2006 field campaigns, the instrument became the world’s first airborne DFG system.