 |
| MIT researchers developed a model to study how some natural, methane-cleansing molecules known as the “atmosphere’s detergent” will shift in a changing climate. Image Credit: MIT News; iStock (CC BY-NC-ND 3.0) |
Scientific Frontline: Extended "At a Glance" Summary: Hydroxyl Radicals and Methane Cleansing
The Core Concept: Hydroxyl radicals (\(\text{OH}^{\bullet}\)) function as the "atmosphere's detergent" by naturally breaking down methane and other pollutants, though their future atmospheric concentrations will fluctuate in complex ways as the planet warms.
Key Distinction/Mechanism: Because \(\text{OH}^{\bullet}\) is comprised of one oxygen atom, one hydrogen atom, and an unpaired electron, it is highly reactive and neutralizes greenhouse gases by pulling an electron or hydrogen atom away from them, reducing them into weaker, water-soluble forms. However, \(\text{OH}^{\bullet}\) levels face a climate-driven tug-of-war: rising global temperatures increase water vapor (which boosts \(\text{OH}^{\bullet}\) production), while simultaneously increasing biogenic volatile organic compound (VOC) emissions from plants (which deplete \(\text{OH}^{\bullet}\)).
Major Frameworks/Components:
- AquaChem Model: An advanced atmospheric chemistry model built upon the Community Earth System Model (CESM). It simulates Earth as an entirely ocean-covered "aquaplanet" to cleanly isolate atmospheric chemical reactions from complex land and ice dynamics.
- Water Vapor Boosting: The modeled thermodynamic response where a 2-degree Celsius increase in global temperatures raises atmospheric water vapor, theoretically boosting \(\text{OH}^{\bullet}\) production by roughly 9 percent.
- Biogenic VOC Emissions: The counteracting biological variable where natural plant emissions, such as isoprene, increase with warming temperatures. These emissions react with and break down \(\text{OH}^{\bullet}\), reducing its atmospheric levels by an estimated 6 percent.
Branch of Science: Atmospheric Chemistry, Earth and Atmospheric Sciences, and Climate Modeling.
Future Application: Upgrading the accuracy of global climate models to definitively predict future atmospheric methane accumulation. This will allow researchers to refine emission targets and develop better mitigation strategies for greenhouse gases and hazardous air pollutants.
Why It Matters: Methane is a highly potent greenhouse gas, and \(\text{OH}^{\bullet}\) is responsible for roughly 90 percent of its removal from the atmosphere. Accurately calculating the future availability of this molecular "detergent" is critical to understanding how long methane will linger in the atmosphere, which directly impacts the acceleration rate of global warming and human respiratory health.
Methane is a powerful greenhouse gas that is second only to carbon dioxide in driving up global temperatures. But it doesn’t linger in the atmosphere for long thanks to molecules called hydroxyl radicals, which are known as the “atmosphere’s detergent” for their ability to break down methane. As the planet warms, however, it’s unclear how the air-cleaning agents will respond.
MIT scientists are now shedding some light on this. The team has developed a new model to study different processes that control how levels of hydroxyl radical will shift with warming temperatures.
They find that the picture is complicated. As temperatures increase, so too will water vapor in the atmosphere, which will in turn boost the molecule’s concentrations. But rising temperatures will also increase “biogenic volatile organic compound emissions” — gases that are naturally released by some plants and trees. These natural emissions can reduce hydroxyl radical and dampen water vapor’s boosting effect.
Specifically, the team finds that if the planet’s average temperatures rise by 2 degrees Celsius, the accompanying rise in water vapor will increase hydroxyl radical levels by about 9 percent. But the corresponding increase in biogenic emissions would in turn bring down hydroxyl radical levels by 6 percent. The final accounting could mean a small boost, of about 3 percent, in the atmosphere’s ability to break down methane and other chemical compounds as the planet warms.
“Hydroxyl radicals are important in determining the lifetime of methane and other reactive greenhouse gases, as well as gases that affect public health, including ozone and certain other air pollutants,” says study author Qindan Zhu, who led the work as a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS).
“There’s a whole range of environmental reasons why we want to understand what’s going on with this molecule,” adds Arlene Fiore, the Peter H. Stone and Paola Malanotte Stone Professor in EAPS. “We want to make sure it’s around to chemically remove all these gases and pollutants.”
A natural neutralizer
The hydroxyl radical, known chemically as \(\text{OH}^{\bullet}\), is made up of one oxygen atom and one hydrogen atom, along with an unpaired electron. This configuration makes the molecule extremely reactive. Like a chemical vacuum cleaner, \(\text{OH}^{\bullet}\) easily pulls an electron or hydrogen atom away from other molecules, breaking them down into weaker, more water-soluble forms. In this way, OH reduces a vast range of chemicals, including some air pollutants, pathogens, and ozone. And changes in OH are a powerful lever on methane.
“For methane, the reaction with \(\text{OH}^{\bullet}\) is considered the most important loss pathway,” Zhu says. “About 90 percent of the methane that’s removed from the atmosphere is due to the reaction with OH.”
Indeed, it’s thanks to reactions with hydroxyl radical that methane can only stick around in the atmosphere for about a decade — far shorter than carbon dioxide, which can linger for 1,000 years or longer. But even as \(\text{OH}^{\bullet}\) breaks down methane already in the atmosphere, more methane continues to accumulate. Rising methane concentrations, in addition to human-derived emissions of carbon dioxide, are driving global warming, and it’s unclear how \(\text{OH}^{\bullet}\)’s methane-clearing power will keep up.
“The questions we’re exploring here are: What are the main processes that control \(\text{OH}^{\bullet}\) concentrations? And how will \(\text{OH}^{\bullet}\) respond to climate change?” Fiore says.
An aquaplanet’s air
For their study, the researchers developed a new model to simulate levels of \(\text{OH}^{\bullet}\) in the atmosphere under a current global climate scenario, compared to a future warmer climate. Their model, dubbed “AquaChem,” is an expansion of a simplified model that is part of a suite of tools developed by the Community Earth System Model (CESM) project. The model that the team chose to build off is one that represents the Earth as a simplified “aquaplanet,” with an entirely ocean-covered surface.
Aquaplanet models allow scientists to study detailed interactions in the atmosphere in response to changes in surface temperatures, without having to also spend computing time and energy on simulating complex dynamics between the land, water, and polar ice caps.
To the aquaplanet model, Zhu added an atmospheric chemistry component that simulates detailed chemical reactions in the atmosphere consistent with the applied surface temperatures. The chemical reactions that she modeled represent those that are known to affect \(\text{OH}^{\bullet}\) concentrations.
\(\text{OH}^{\bullet}\) is primarily produced when ozone interacts with sunlight in the presence of water vapor. For instance, scientists have found that \(\text{OH}^{\bullet}\) levels can vary depending certain anthropogenic and natural emissions, all of which Zhu incorporated separately and together into the AquaChem model in order to isolate the impact of each process on \(\text{OH}^{\bullet}\).
The emissions in particular include carbon monoxide, methane, nitrogen oxides, and volatile organic compounds (VOCs), some of which are emitted through human practices, and others that are given off by natural processes. One type of naturally-derived VOCs are “biogenic” emissions — gases, such as isoprene, that some plants and trees emit through tiny pores called stomata during transpiration.
Into the AquaChem model, Zhu plugged in data that were available for each type of emissions from the year 2000 — a year that is generally considered to represent the current climate in a simplified form. She set the aquaplanet’s sea surface temperatures to the zonal annual mean of that year, and found that the model accurately reproduced the major sensitivities of \(\text{OH}^{\bullet}\) chemistry to the underlying chemical processing as simulated in a more complex chemistry-climate model.
Then, Zhu ran the model under a second, globally warming scenario. She set the planet’s sea surface temperatures to warm by 2 degrees Celsius (a warming that is likely to occur unless global anthropogenic carbon emissions are mitigated). The team looked at how this warming would affect the various types of emissions and chemical processes, and how these changes would ultimately affect levels of \(\text{OH}^{\bullet}\) in the atmosphere.
In the end, they found the two biggest drivers of \(\text{OH}^{\bullet}\) levels were rising water vapor and biogenic emissions. They found that global warming would increase the amount of water vapor to the atmosphere, which in turn would boost production of \(\text{OH}^{\bullet}\) by 9 percent. However, this same degree of warming would also increase biogenic emissions such as isoprene, which reacts with and breaks down \(\text{OH}^{\bullet}\), bringing down its levels by 6 percent.
The team recognizes that there are many other factors that affect the response of isoprene emissions to surface warming. Rising \(CO_2\), not considered in this study, may dampen this temperature-driven response. Of all the factors that can shift \(\text{OH}^{\bullet}\) levels under global warming, the researchers caution that biogenic emissions are the most uncertain, even though they appear to have a large influence. Going forward, the scientists plan to update AquaChem to continue studying how biogenic emissions, as well as other processes and climate scenarios, could sway \(\text{OH}^{\bullet}\) concentrations.
“We know that changes in atmospheric \(\text{OH}^{\bullet}\), even of a few percent, can actually matter for interpreting how methane might accumulate in the atmosphere,” Zhu says. “Understanding future trends of \(\text{OH}^{\bullet}\) will allow us to determine future trends of methane.”
Funding: This work was supported, in part, by Spark Climate Solutions and the National Oceanic and Atmospheric Administration.
Published in journal: Journal of Advances in Modeling Earth Systems (To be released later today)
Authors: Qindan Zhu, Arlene Fiore, Jian Guan, Paolo Giani, Robert Pincus, Nicole Neumann, George Milly, Clare Singer, and Brian Medeiros
Source/Credit: Massachusetts Institute of Technology | Jennifer Chu
Reference Number: as032426_01
.jpg)
.jpg)