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Science

Research Looks at Self Cleaning Ability of 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.
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.

An international team of researchers led by the National Oceanic and Atmospheric Administration (NOAA) has delved into understanding the self-cleaning properties of our atmosphere and found that, relatively speaking, the atmosphere has a seemingly stable capacity to rid itself of most pollutants.

The study, published online in the journal Science, shows that global levels of a critical particle in the atmospheric chemistry, hydroxyl radical, does not vary much from year to year, and certainly not by up to 25 percent as had once been 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. This fundamental property of the atmosphere was one we hadn’t been able to confirm before.”

Hydroxyl radical is of the utmost import to the atmosphere for its ability to breakdown surface-level ozone, methane, and air pollutants such as hydrocarbons, carbon monoxide and sulfur dioxide. It is unable, however, to remove carbon dioxide, nitrous oxide or any of the chlorofluorocarbons.

Instead of trying to summarize a very interesting yet technical aspect of this study, I will quote from the NOAA press release;

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.

This technique produced estimates of hydroxyl that swung wildly in the 1980s and 1990s. Researchers struggled to understand whether the ups and downs were due to errors in emissions estimates for methyl chloroform, for example, or to real swings in hydroxyl levels. The swings would be of concern: Large fluctuations in hydroxyl radicals would mean the atmosphere’s self-cleaning ability was very sensitive to human-caused or natural changes in the atmosphere.

To complicate matters, when scientists tried to measure the concentration of hydroxyl radical levels compared to other gases, such as methane, they were seeing only small variations from year to year. The same small fluctuation was occurring when scientists ran the standard global chemistry models.

An international agreement helped resolve the issue. In response to the Montreal Protocol – the international agreement to phase out chemicals that are destroying the Earth’s protective stratospheric ozone layer – production of methyl chloroform all but stopped in the mid 1990s. As a result, emissions of this potent ozone-depleting gas dropped precipitously.

Without the confounding effect of any appreciable methyl chloroform emissions, a more precise picture of hydroxyl variability emerged based on the observed decay of remaining methyl chloroform. The scientists studied hydroxyl radicals both by making measurements of methyl chloroform from NOAA’s international cooperative air sampling program and also by modeling results with state-of-the-art models.

As with many of the stories I end up covering here at Planetsave, this most recent study has benefits for predicting the future of our planet’s atmosphere, and hopefully acting accordingly.

“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.”

Source: NOAA




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