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Environment

Studying Pigsties

Air Pollution: A new technique to measure stinky compounds will help fight pig farm odors

by Janet Pelley
July 7, 2010

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Credit: Kristoffer Jonassen
Pig manure releases more than 400 nose-wrinkling compounds.
Credit: Kristoffer Jonassen
Pig manure releases more than 400 nose-wrinkling compounds.

In the last two decades, people living near pig farms have raised a stink over their porcine neighbors and the miles-long plumes of oppressive smells that they generate. Now, a new technique described in Environmental Science & Technology (DOI: 10.1021/es100483s) provides a way to track compounds wafting off pig manure, making it easier to regulate farm aromas.

The volatile organic compounds behind pig manure's stench only reach parts-per-billion levels, but they are so potent that people gag miles away. And neighbor complaints can shut down farm expansion plans, says Larry Jacobson, an agricultural engineer at the University of Minnesota, St. Paul. With factory farming on the upswing in most developed countries and China, controlling this odor has become a serious economic issue.

But these stinky compounds are hard to measure, says Steve Trabue, a research chemist at the U.S. Department of Agriculture in Ames, Iowa. "They occur at low concentrations, they are polar and therefore stick to the insides of air-sampling bags, and the sulfurous compounds break down in the bags when the air is humid," he says.

Even the data collected from these frustrating air-sampling bags has limited use, says Anders Feilberg, an environmental chemist at Aarhus University in Denmark. Because scientists have to drag these bags back to the lab for gas chromatography/mass spectrometry analysis, existing methods only provide snapshots of emissions and prevent continuous sampling.

So Feilberg and his colleagues found an alternative method that did not require sample collection: proton-transfer-reaction mass spectrometry (PTR-MS). Used by other researchers to analyze volatile compounds in indoor air and smokestack exhaust, PTR-MS can simultaneously identify and quantify compounds in real-time. To test the technique's ability to monitor odors, all the scientists had to do was pipe exhaust air from a pig barn used for research directly into a PTR-MS machine on site.

The researchers then measured the levels of 17 key compounds in barn exhaust, including some of the sulfurous and polar molecules that were previously hard to analyze, such as hydrogen sulfide, methanethiol, and 3-methyl-1H-indole. And because PTR-MS allowed the scientists to sample the barn exhaust continuously over several days, the researchers could construct a daily emission pattern for the first time. They found that, in their barn, the stinky compounds peaked in the afternoon. 

USDA's Trabue says that PTR-MS will improve attempts to regulate pig farm odors, by helping assess new stench-mitigation technologies and provide more continuous data for modelers. Minnesota's Jacobsen agrees, calling the method "a breakthrough way to assess odors."

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