Hydroxyl (OH) radical is one of the most reactive components of the Earth’s atmosphere. It is made up of one hydrogen atom, and one oxygen atom with a free electron just waiting to spring into chemical (re)action. The hydroxyl radical’s reactivity has earned it the nickname “air detergent” – because of the way it effectively breaks down other gasses in the atmosphere, especially methane.
Methane (CH4) is a product of various processes on Earth – for example, decomposition and oxidation (burning) reactions. It is a common byproduct of agricultural and fossil fuel activity, and one of the most powerful greenhouse gasses, 84 times more potent than carbon dioxide. Fortunately, it doesn’t remain in the atmosphere as long as CO2– its estimated lifetime is about nine years, which limits its global warming potential (GWP) to 28-36 over 100 years.
What degrades methane so quickly is the hydroxyl (OH) radical. However, concentrations of methane have been rising over the last few decades. This has led to the assumption that as the methane concentration rises, the hydroxyl radicals will get depleted from the atmosphere, causing methane to hang in there longer.
However, a new study, led by a postdoctoral fellow at NASA Julie M. Nicely, suggests that hydroxyl radical manages to “recycle” itself back into the atmosphere by reacting with other gasses. Nicely and her team used satellite observations of atmospheric gases from 1980 to 2015 and included them into a computer model which simulates the possible sources of atmospheric OH. What they found that the breakdown products of OH-CH4reaction can further react with nitrogen oxides (NO or NO2) to reform OH again. These two gasses are a common product of human activity.
The story doesn’t end with nitrogen oxides since hydroxyl radicals are also produced when ultraviolet light reacts with water vapor and ozone in the lower parts of the atmosphere. In relation to that, the study has also tested the possible influence of the world’s tropical zones and their expansion. How are the two exactly connected?
Probably due to increasing temperatures and changing air circulation patterns, tropical regions of the world are slightly expanding. Conveniently, both water vapor and ultraviolet radiation needed to create the hydroxyl radical are abundant in the tropics. Plainly stated, more tropical areas means more OH sources. Although the expansion is slow and modest – only 0.5 to 1 degree in latitude every 10 years – Nicely stresses that even that small change in combination with other small sources can help replace the hydroxyl lost in the breakdown of methane.
The new findings came as a surprise to the scientific community. Many models already use the assumption of OH depletion in response to rising methane concentrations. Tom Handisco, a chemist at Goddard Institute, who was not a part of the research, also finds the results surprising: “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.”
However, he added that the study looks only at the past thirty-five years, and there are no guarantees that the OH levels will manage to recycle themselves back as the atmosphere continues to change along with the global climate. Nicely points out that the study is not intended to give any long-lasting predictions, but to help make them by updating knowledge and models of hydroxyl-methane interaction.
“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 concluded.
Nicely, J.M. et al. (2018). Changes in Global Tropospheric OH Expected as a Result of Climate Change Over the Last Several Decades. Journal of Geophysical Research: Atmospheres, Vol 123, p. 10,774–10,795. https://doi.org/10.1029/2018JD028388
“Detergent-like Molecule Recycles Itself in Atmosphere”. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/144358/detergent-like-molecule-recycles-itself-in-atmosphere
“Greenhouse Gas ‘Detergent’ Recycles Itself in Atmosphere: NASA Study”. November 30, 2018: https://www.nasa.gov/feature/goddard/2018/greenhouse-gas-detergent-recycles-itself-in-atmosphere-nasa-study