The atmosphere is getting better at cleaning itself – but that’s not all good news

Imagine for a moment the atmosphere is a kitchen sink. Wildfires, industry emissions, plants and microbes dump their grimy dishes into it in the form of noxious and planet-heating gases.

The only reason why these gases are not continuously accumulating in the atmosphere and we are not choking in a giant smog cloud is that the atmosphere makes its own detergent: hydroxyl.

The hydroxyl radical (OH) is generated in complex chemical cycles and removes organic gases by reacting with them. This includes the potent greenhouse gas methane – OH removes about 90% of it from the atmosphere.

An important question for climate scientists is whether our ongoing emissions could use up the OH detergent and leave the atmosphere less able to cleanse itself.

While that may seem likely, we also emit compounds like nitrogen oxides (from engines and power plants) that increase OH production. Which of the two processes dominates and whether OH levels are going up or down has been hotly debated.

But as we show in our new study, OH has been increasing and the atmosphere’s self-cleaning ability has been strengthening since 1997.

This finding gets us a step closer to understanding what happens to methane once it enters the atmosphere. While it is good news that the atmosphere’s scrubbing capacity has been increasing, it also suggests that methane emissions are rising faster than scientists and policy makers assumed.

Complex measurements

OH is very challenging to measure directly. It only exists for a second before it reacts again.

Instead, we used the radiocarbon content of carbon monoxide (14CO) as a footprint of OH activity. Only reaction with OH removes 14CO, which makes it a robust tracer and indicates how much OH is in the air.

The 14CO radioactive isotope (which is chemically the same as carbon monoxide but heavier) forms when cosmic rays start a chain of reactions in the atmosphere. We can calculate this production rate accurately and therefore know how much 14CO enters the atmosphere.

For each of the hundreds of data points used in our study, we used air samples collected at two remote stations in New Zealand and Antarctica, respectively, over the past 33 years.

From these samples, we isolated only the carbon monoxide, which we then turned into carbon dioxide and eventually into graphite (pure carbon) to measure how many of the graphite atoms represent the carbon isotope 14C.

Confirmation by modelling

We found a statistically significant decrease in 14CO over the past 25 years. This can only be caused by an increase in OH.

Our computer model that calculates climate and atmospheric chemistry confirms this. The combination of measurements and simulations shows that OH is increasing, but proves it only for the Southern Hemisphere where we have collected samples.

This is interesting because this part of the world is affected by the “grime” gases, including methane, that react with OH but is far from more industrialised regions that emit compounds that generate OH (especially nitrogen oxides).

If we can detect an OH rise in the more pristine southern hemisphere, chances are the increase is global. Indeed, our model shows that OH is likely rising faster in the northern hemisphere.

The simulations also suggest the main factors at play. Higher methane fluxes suppress OH, as expected, and by themselves would cause a downward trend. In contrast, nitrogen oxide emissions, ozone depletion in the stratosphere and global warming favour the formation of new OH, turning the balance to an overall increase.

These findings are a big step in the understanding of atmospheric chemistry. They show that rising OH levels have so far saved us from even faster rising atmospheric methane levels and the associated warming.

Currently, urban and industrial pollution of nitrogen oxides maintains this state. But the danger is that the very necessary efforts to clean up these pollutants could cut the OH supply to the atmospheric kitchen sink. With less detergent and the same input of grime, the dishwater will turn dirty.