Dusty visible in the sunset, off the coast of Peru. Credit: Jan-Berend Stuut (NIOZ)
Key Points:
- Researchers believe methane oxidation in the atmosphere is masking the increase of biological methane emissions.
- Global atmospheric chlorine concentrations might be roughly 40% higher than previously estimated.
- A new study suggests Saharan dust clouds can help mitigate atmospheric methane.
A new study from an international team of researchers suggests Saharan dust clouds can help mitigate atmospheric methane. The findings have potential implications for understanding the global methane budget and reasons behind the accelerating increase in atmospheric methane.
The study incorporates a proposed new mechanism whereby blowing mineral dust mixes with sea-spray to form Mineral Dust-Sea Spray Aerosol (MDSA).
According to the paper, published in PNAS, MDSA is activated by sunlight to produce an abundance of chlorine atoms, which oxidize atmospheric methane and tropospheric ozone via photocatalysis. Largely composed of blowing dust from the Sahara Desert combined with sea salt aerosol from the ocean, MDSA is the dominant source of atmospheric chlorine over the North Atlantic.
The study relies on a combination of global modeling and laboratory and field observations, including air samples from Barbados showing seasonal depletion of the stable isotope 13CO, an anomaly that puzzled scientists for 20 years. They knew observed changes in 13CO and C18O were evidence of chlorine atoms reacting with methane, and that carbon monoxide is the first stable product in atmospheric methane oxidation. But the known sources of atmospheric chlorine could not account for the degree of depletion in 13CO, until now.
Using a global 3-D chemistry-climate model (CAM-Chem), the researchers found that when increased chlorine from the MDSA mechanism was incorporated into the model, the results agreed well with the Barbados data and explained the 13CO depletion.
If the MDSA effect observed in the North Atlantic is extrapolated globally, and if its efficiency is similar in other parts of the world, global atmospheric chlorine concentrations might be roughly 40% higher than previously estimated, according to the study results. Factoring this into global methane modeling could potentially shift understanding of the relative proportions of methane emissions sources.
“Methane emissions from biological sources such as wetlands and agriculture may be growing as global temperatures rise,” said Maarten van Herpen, lead author of the study. “But recent increases in dust from North Africa have probably increased methane oxidation in the atmosphere, partly masking the growth in biological methane emissions. Adjusting atmospheric modeling to take this into account may show that methane emissions from biological sources are rising even faster than we thought.”
In follow-up studies, the researchers are analyzing air samples from across the North Atlantic, provided by atmospheric observatories and commercial ships. They have collected 500 flasks so far, but they need a full year of data before they can draw conclusions.