Atmosphere ‘pretty good’ at cleaning itself

The earth’s atmosphere seems to be better at ridding itself of pollutants than had previously been realized.

An NOAA-led research team has found that the atmosphere’s self-cleaning capacity is actually pretty stable. Its analysis shows that global levels of the hydroxyl radical, a critical player in atmospheric chemistry, don’t in fact vary much from year to year.

Levels of hydroxyl, which help clear the atmosphere of many hazardous air pollutants and some important greenhouse gases — but not carbon dioxide — dip and rise by only a few percent every year, and not by up to 25 percent, as was once estimated.

“Now we know that the atmosphere’s ability to rid itself of many pollutants is generally well buffered or stable,” says NOAA research chemist Stephen Montzka. “This fundamental property of the atmosphere was one we hadn’t been able to confirm before.”

The finding adds confidence to projections of future air pollutant loads. The hydroxyl radical, comprised of one oxygen atom and one hydrogen atom, is formed and broken down so quickly in the atmosphere that it has been extremely difficult to measure on global scales.

“In the daytime, hydroxyl’s lifetime is about one second and is present at exceedingly low concentrations,” said Montzka. “Once created, it doesn’t take long to find something to react with.”

To estimate variability in global hydroxyl levels — and thus the cleansing capacity of the atmosphere — researchers studied  longer-lived chemicals that react with hydroxyl.

But this technique produced estimates of hydroxyl that swung wildly in the 1980s and 1990s. Nobody knew whether this was due to errors in emissions estimates for methyl chloroform, for example, or to real swings in hydroxyl levels.

The solution came thanks to the Montreal Protocol, which meant that production of methyl chloroform all but stopped in the mid 1990s. This allowed the team to gather a more precise picture of hydroxyl variability, based on the observed decay of remaining methyl chloroform.

The group’s findings improve confidence in projecting the future of Earth’s atmosphere.

“Say we wanted to know how much we’d need to reduce human-derived emissions of methane to cut its climate influence by half,” Montzka said.

“That would require an understanding of hydroxyl and its variability. Since the new results suggest that large hydroxyl radical changes are unlikely, such projections become more reliable.”