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Where there’s smoke, there’s toxic acid

U. COLORADO (US) — Researchers have discovered a chemical in smoke that may be harmful to humans.

The molecule, isocyanic acid, is similar to methyl isocyanate, the gas that leaked from a pesticide plant in Bhopal, India, in 1984 killing more than 3,000 people within weeks.

“The molecule has hardly been measured before—certainly not in the atmosphere,” says Joost de Gouw, coauthor of the new paper published this week in the Proceedings of the National Academy of Sciences. “So it was a complete surprise to find it in such large quantities.”

De Gouw and colleagues at the Cooperative Institute for Research in Environmental Sciences, or CIRES—a joint institute of the National Oceanic and Atmospheric Administration and the University of Colorado-Boulder—were first able to detect isocyanic acid when they developed and tested a new instrument, a mass spectrometer designed to measure gaseous acids in the air.

In the laboratory, they found biomass burning—the burning of trees or plant material— produced levels of the molecule approaching 600 parts per billion by volume, or ppbv.

“There is this molecule in smoke that we can now measure and it is there in significant quantities,” de Gouw says. “There are good reasons to believe that it can have significant health impacts.”

In the human body, isocyanic acid dissolves to form charged cyanate molecules, and the researchers found that the acid was very soluble at the pH level of human blood. This means it could potentially enter the bloodstream, says de Gouw. When the exposure levels of isocyanic acid are greater than 1 ppbv, the charged cyanate molecules are expected to be present at levels that can contribute to a variety of human health problems like cardiovascular disease, cataracts, and rheumatoid arthritis.

Once the researchers discovered that fires produced the gas at the U.S. Forest Service Fire Sciences Laboratory in Missoula, Mont., they then took their instruments out of the lab to see whether smoke in a “real” environment also gave off this chemical.

“We had a new tool to look around us and we just explored,” de Gouw says. “It was basically our chemical curiosity at work.”

Previous studies have shown that burning coal produces isocyanic acid, and the CIRES researchers have discovered the chemical also is present in tobacco smoke and smoke from the combustion of other plant materials. In rural areas of developing countries where biofuels are used for cooking and heating, exposure levels of the acid could be harmful, according to the research team.

But does a real fire, as opposed to a lab fire, give off the acid? The team didn’t have to wait long to find out. Starting on Labor Day 2010, the Fourmile Canyon wildfire raged in the foothills above Boulder, Colo., burning more than 6,000 acres and destroying 169 homes. Scientists at the NOAA Earth System Research Laboratory in Boulder wasted no time in learning what they could about the event.

The team’s spectrometer detected levels of the acid up to 200 pptv in the air at the site, which was downwind from the fire. “Boulder has a world-class atmospheric chemistry building and only once in its lifetime is it going to have a full-on hit from a wildfire,” de Gouw says. “So just about everyone in that building turned on their instruments.”

One possibility was that the acid would only be prevalent in the immediate vicinity of a fire, de Gouw notes. “But that didn’t happen,” he adds. “We were miles away and it was still there.”

The researchers didn’t constrain their measurements to wildfires. They also used their equipment to find the levels of isocyanic acid in the urban environment of Los Angeles. “In LA we find even when there are no fires there is a little of this acid,” de Gouw says. “So smoke may not be the only source of it in the atmosphere.”

Since more isocyanic acid was measured in the atmosphere during the day, sunlight could be sparking the chemical reactions that make it, de Gouw suspects. Another potential source in urban air could be emissions from diesel engines outfitted with the latest generation of pollution control equipment that is now being introduced in California and Europe, he says.

“We know so little about isocyanic acid’s behavior in the atmosphere that we want to do a number of follow-up studies, “ de Gouw says. “We have some data in our paper but that is just the beginning and we need to do a lot more work.”

Researchers from from NOAA’s Earth System Research Laboratory contributed to the work.

More news from the University of Colorado: www.colorado.edu/news/

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