Conservation and Environment

From NOAA News

Scientists will be better able to predict future pollution levels in the Earth's atmosphere

Aidan Colton at NOAA's Mauna Loa Observatory (MLO) demonstrates how early flask samples were filled at the site. Air collected year-round at MLO and eight other remote sites around the world has been analyzed for the industrial solvent methyl chloroform. Variability in the decay of this chemical has helped scientists understand the oxidizing or cleansing power of the global atmosphere and its sensitivity to natural and human-induced perturbations.   Photo Credit: NOAA

An international, NOAA-led research team took a significant step forward in understanding the atmosphere’s ability to cleanse itself of air pollutants and some other gases, except carbon dioxide. The issue has been controversial for many years, with some studies suggesting the self-cleaning power of the atmosphere is fragile and sensitive to environmental changes, while others suggest greater stability. And what researchers are finding is that the atmosphere’s self-cleaning capacity is rather stable.

New analysis published online today in the journal Science shows that global levels of the hydroxyl radical, a critical player in atmospheric chemistry, do not 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; not by up to 25 percent, as was once estimated.

“The new hydroxyl measurements give researchers a broad view of the ‘oxidizing’ or self-cleaning capacity of the atmosphere,” said Stephen Montzka, the study’s lead author and a research chemist at the Global Monitoring Division of NOAA’s Boulder, Colo., laboratory.

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

The new 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.”

NOAA's Patricia Lang prepares to measure methane levels inside a flask that is part of NOAA's global air sampling network. Network measurements, made from remote sites around the world, were critical in helping an international team of scientists understand the oxidizing or cleansing power of the global atmosphere and its sensitivity to natural and human-induced perturbations. Methane levels were a key point of comparison in the new study, published in Science.   Photo Credit: NOAA

The radical is central to the chemistry of the atmosphere. It is involved in the formation and breakdown of surface-level ozone, a lung- and crop-damaging pollutant. It also reacts with and destroys the powerful greenhouse gas methane and air pollutants including hydrocarbons, carbon monoxide and sulfur dioxide. However, hydroxyl radicals do not remove carbon dioxide, nitrous oxide or chlorofluorocarbons.

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

The industrial chemical methyl chloroform, for example, is destroyed in the atmosphere primarily by hydroxyl radicals. By comparing levels of methyl chloroform emitted into the atmosphere with levels measured in the atmosphere, researchers can estimate the concentration of hydroxyl and how it varies from year to year.

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