The hydroxyl (OH) radical—a molecule made up of one hydrogen atom and one oxygen atom with a free (unpaired) electron—is one of the most reactive gases in the atmosphere. It acts like a detergent in the air, breaking down other gases. In particular, OH is the main check on the concentration of methane, a potent greenhouse gas that is second only to carbon dioxide in contributing to global warming.
New research led by a postdoctoral fellow at NASA has shown that hydroxyl radicals are recycling themselves and maintaining a steady atmospheric concentration even in the face of rising methane emissions. Understanding the role of OH is critical for determining the lifetime of methane in the atmosphere.
The animated map above, based on a model by atmospheric chemist and lead author Julie Nicely of NASA’s Goddard Space Flight Center, shows the global primary production of OH on July 1, 2000. Note how the concentration tracks with the movement of sunlight across the globe. The higher levels of OH over populated lands are likely due to OH recycling.
Scientists once thought that rising methane emissions could cause the amount of hydroxyl radicals to be depleted on a global scale; this would extend the lifetime of methane gas, which currently is cleansed from the atmosphere in about nine years. But while looking at primary sources of OH and methane and how they react, Nicely and colleagues also took into account secondary sources of OH—recycling that happens after OH breaks down methane and then reforms in the presence of other gases.
“OH concentrations are pretty stable over time,” Nicely said. “When OH reacts with methane, it does not necessarily go away, especially in the presence of nitrogen oxides (NO and NO2). The breakdown products from the reaction with methane then react with NO or NO2 to reform OH again. So OH can recycle back into the atmosphere.”
Nicely and colleagues plugged satellite observations of various gases from 1980 to 2015 into a computer model in order to simulate the possible sources for OH in the atmosphere. These include reactions with nitrogen oxides, water vapor, and ozone. They also tested a potential source of new OH: the enlargement of the tropical regions on Earth.
OH also forms when ultraviolet sunlight reacts with water vapor (H2O) and ozone (O3) in the lower atmosphere. Over the tropics, water vapor and ultraviolet rays are particularly plentiful. Recent scientific evidence suggests that this climate region may be stretching farther north and south due to rising temperatures and changes in air circulation patterns. So if tropical regions expand, this natural factory for OH could also grow over time, leading to more OH globally. Nicely cautions that this tropical expansion process is slow—roughly 0.5 to 1 degree in latitude every 10 years—but the effect may still be important.
Nicely and colleagues concluded that while the tropical widening effect and the recycling through reactions are relatively small sources of OH, together they essentially replace the OH used up by the breakdown of methane.
“The absence of a trend in global OH is surprising,” said Tom Hanisco, an atmospheric chemist at NASA Goddard who was not involved in the research. “Most models predict a ‘feedback effect’ between OH and methane. In the reaction of OH with methane, OH is also removed. The increase in NO2 and other sources of OH, such as ozone, cancel out this expected effect.” He added that since this study looks at the past 35 years, there is no guarantee that OH levels will continue to recycle in the same way as the atmosphere continues to evolve with climate change.
Nicely views the results as a way to fine-tune and update the assumptions that researchers make when they describe and predict how OH and methane interact throughout the atmosphere. “This could add clarification on the question of will methane concentrations continue rising in the future? Or will they level off, or perhaps even decrease?” she said. “This is a major question regarding future climate.”