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Dust from the Sahara Helps Remove Dangerous Atmospheric Methane


Findings published in PNAS improves understanding of global methane totals

Methane is a potent greenhouse gas, potentially accounting for up to one-third of global warming, some scientists estimate. A new study published in PNAS evaluates the effects of Saharan dust clouds on atmospheric methane. The international research team, which includes Professor John Mak from Stony Brook University’s School of Marine and Atmospheric Sciences (SoMAS), found that when mineral dust mixes with sea spray to form mineral dust sea-spray aerosol (MDSA), this MDSA is activated by sunlight to produce an abundance of chlorine atoms, ultimately mitigating methane totals. The research findings may have far-reaching implications for understanding global methane totals and the reasons behind its accelerating increase in the atmosphere.

Air sampling station, Ragged Point, Barbados, 1999. Air samples collected weekly from this site and published in Mak et al. (2003) were used in the most recent PNAS paper.
Credit: John Mak

The researchers used a combination of global modeling and field and laboratory observations. Central to the new research were observations of atmospheric carbon monoxide and its stable isotopes (13CO) that had been made 20 years prior by Mak’s research group and published in the Journal of Geophysical Research in 2003.

In that paper, the authors hypothesized that the observed seasonal changes in the abundance of 13C in atmospheric carbon monoxide (13CO) were evidence of chlorine atoms reacting with methane; however, the known sources of atmospheric chlorine could not account for the degree of depletion of observed 13CO, until this latest research.

“At the time we did not have the knowledge or skill to pursue that hypothesis, but this international research group has followed up with more recent ideas,” explains Mak, a co-author of the new paper. “This is an example of how quality observations and measurements can be useful in the future in unanticipated ways.”

In the new study, led by Maarten van Herpen, and titled “Photocatalytic Chlorine Atom Production on Mineral Dust–Sea Spray Aerosols Over the North Atlantic,” the researchers conclude that MDSA is the dominant source of atmospheric chlorine over the North Atlantic.

By using a global 3D chemistry-climate model, the research team found that when increased chlorine from the MDSA mechanism was incorporated into the model, the results matched the Barbados data and explained the carbon monoxide depletion.

Schematic overview of the MDSA mechanism. Credit: PNAS

The authors write, in summary and significance: “We demonstrate a mechanism in which a mix of Sahara dust and sea spray aerosol activated by sunlight produces large amounts of active chlorine. This mechanism resolves a number of unexplained observations and significantly revises our understanding of atmospheric chlorine, reducing uncertainties in the source budget. If the MDSA effect observed in the North Atlantic is extrapolated globally, and if its efficiency is similar in other parts of the world — two areas that aren’t yet well understood and require further research — global atmospheric chlorine concentrations might be roughly 40 percent higher than previously estimated, the study finds. Factoring this into global methane modeling could potentially shift our understanding of the relative proportions of methane emissions sources.”

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