The atmosphere of the Earth has increased its capacity to remove air pollutants, including **methane**, a potent gas that contributes to **global warming**.
Published in Nature Communications, a recent study on the **self-cleaning capacity of the atmosphere** focused on measuring the amount of its elusive cleanser, the hydroxyl radical (OH), nicknamed the “atmospheric detergent” by Nobel laureate Paul Crutzen.
Applying an advanced method to analyze two long-term measurements of **air samples from New Zealand and Antarctica** since the late 1980s, the research by the National Institute of Water and Atmospheric Research (NIWA) revealed a significant trend in the atmosphere’s capacity to remove pollutants.
The research highlights that without the increased **cleaning capacity of hydroxyl**, methane would have contributed even more to global warming.
## A long-term study on how the atmosphere can remove contaminants
The **long-term study**, conducted by scientists from NIWA along with researchers from Victoria University of Wellington, GNS Science, and a collaborator from Finland, shows that the self-cleaning capacity of the atmosphere has been strengthening in the **southern hemisphere** since around 1997.
The research, spanning 33 years, focused on the most potent oxidant in the atmosphere, OH, and used **radiocarbon monoxide** (14CO) as a reliable marker. This rare form of carbon monoxide is produced when cosmic rays impact the Earth’s atmosphere; its production and removal rate by OH are well-known.
OH is highly reactive and short-lived, says NIWA atmospheric scientist Sylvia Nichol. “OH is a tiny chemical scrubber. It is formed by a **hydrogen atom** and an oxygen atom, with a free unpaired electron. It is generated in the atmosphere when **sunlight interacts with ozone** in the presence of **water vapor**.
“It reacts with harmful trace gases, such as carbon monoxide and methane, in the lowest layer of the atmosphere, the troposphere, which extends up to an average height of 11 kilometers (36,000 feet) from the Earth’s surface.
“In the 1970s, a key discovery was that OH is produced in the troposphere through reactions that allow the oxidation of **gases like carbon monoxide, methane, and ethane**. Although the lifetime of OH can be less than a second, it plays a crucial role in the troposphere,” explains Nichol.
Since the highly reactive hydroxyl controls the atmospheric lifetimes of most gases, its presence is essential for regulating **concentrations of some greenhouse gases**, particularly methane, says Nichol.
“Although hydroxyl radicals appear in minimal amounts for a short period, they remove carbon monoxide and nearly 90% of methane from the air, making them vital for maintaining air quality.”
The **dynamics of OH**, along with its very low concentrations, make it notoriously difficult to observe and quantify directly, adds NIWA senior technician Gordon Brailsford, who has spent decades collecting air samples.
“Ultraviolet light influences the production of hydroxyl, so the levels of this **atmospheric cleanser** vary greatly on a daily and annual basis. OH is only formed during the day, meaning it drops to almost zero at night and is more prevalent in summer.”
Previous attempts to monitor OH trends used methyl chloroform, but this compound was phased out under the **Montreal Protocol** in 1987 to protect the ozone layer, making its use impractical, notes Brailsford.
Records from two remote monitoring stations in the southern hemisphere since the late 1980s have generated quality data for analysis. “Regular and consistent measurements over 33 years at two sites provide the first evidence of a long-term increase in OH,” according to Brailsford.
## Implications of the strengthening of OH
The **Baring Head Atmospheric Research Station**, located on the outskirts of the windy capital of New Zealand, Wellington, is internationally recognized for its long-term monitoring of clean air.
“About 4,000 kilometers further south, the joint New Zealand-U.S. laboratory Arrival Heights on **Ross Island in Antarctica** is far removed from human pollution, and air samples are collected even during the five months of darkness each year. Both sets of measurements are the world’s longest and most consistent records of 14CO as a marker for changes in **atmospheric chemistry**.”
Processing the samples involves many steps, says lead technician Rowena Moss, who has dedicated over 10,000 hours to the project. “Large air samples, up to 1,000 liters, were collected in gas cylinders, then dried, compressed, cooled to **remove ambient CO2**, and concentrated to a microscopic amount of carbon monoxide and its isotopes.
“The samples from the two **observation stations** have been revealing about the role of OH in removing pollutants,” says lead author of the paper, atmospheric and climate scientist Dr. Olaf Morgenstern, whose work has expanded a previously developed “chemistry-climate” model.
“Data from New Zealand since 1997 show an annual decrease of 12% (+/- 2%) in 14CO. Measurements from Antarctica show an even larger drop of 43% (+/- 24%) but only during December to January, the peak of summer in the southern hemisphere.
“The results of this research suggest that the oxidizing capacity of the atmosphere, **driven by hydroxyl**, has been strengthening in recent decades. The findings confirm and support our models and corroborate those worldwide suggesting that OH has been increasing globally.”
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