Free Radicals! Sounds like a sign one might have seen outside Berkeley in the 60s, doesn’t it? But we are not talking about unwashed hippies here. Today’s topic is OH, or hydroxyl as it is known the scientific community. It is a molecule left over when sunlight breaks down water vapor in the atmosphere. What makes it a free radical is an extra electrical charge, meaning it can easily bond with other molecules — like methane.
We know that methane can be a powerful greenhouse gas that contributes significantly to a warmer planet. Whether it comes from wells or cow stomachs, it is estimated to trap as much as 12 times more heat than carbon dioxide. Now scientists at NASA have discovered that hydroxyl acts as a detergent, breaking down methane in the atmosphere then recycling itself so it can repeat the process again.
The hydroxyl radical, a molecule made up of one hydrogen atom and one oxygen atom with a free electron, is one of the most reactive gases in the atmosphere and regularly breaks down other gases, effectively ending their lifetimes. In this way OH is the main check on the concentration of methane. The research has been published by the Journal of Geological Research — Atmospheres.
“OH concentrations are pretty stable over time,” says lead author Julie Nicely of the Goddard Space Flight Center in Greenbelt, Maryland. “When OH reacts with methane it doesn’t necessarily go away in the presence of other gases, especially nitrogen oxides (NO and NO2). The breakdown products of its reaction with methane react with NO or NO2 to reform OH. So OH can recycle back into the atmosphere.” The research examined data on atmospheric gases from 1980 through 2015.
One factor contributing to the amount of hydroxyl molecules in the atmosphere is a widening of the tropical zones over time at the rate of about 0.5 to 1 degree of latitude per decade. Scientists believe the wider tropical zones may be related to new wind patterns as the air over Equator gets warmer. Wider zones mean more moist air is available for the creation of hydroxyl molecules. Think of it as a buffering process in which the Earth attempts to offset the negative effects of methane by breaking it down into less harmful components.
While that might give comfort to those who assume the Earth will somehow shrug off the insults visited upon it by humanity, anyone who has ever dabbled in organic chemistry knows that at some point, buffering reaches a limit. Once that happens, things go to hell in a hand basket very quickly. In other words, this discovery is good news but it is no bulwark against humans continuing to use the Earth as a communal toilet.
“The absence of a (downward) trend in global OH is surprising,” says Tom Hanisco, an atmospheric chemist at 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 then issues the same cautions financial advisers do when advising clients — past performance is no guarantee of future performance. As the atmosphere continues to evolve with global climate change, OH levels may not continue to recycle in the same way in the future.
Nicely says the value of the research is its ability to help climate scientists fine tune and update the assumptions about how the climate is affected by various inputs, including human activity. “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? This is a major question regarding future climate that we really don’t know the answer to,” she says.