A study published in Proceedings of the National Academy of Sciences explored the effects of Saharan dust clouds on atmospheric methane. Its findings have potentially far-reaching implications for understanding the global methane budget and factors behind the rapid increase in atmospheric methane.
The study, titled “Photocatalytic Chlorine Atom Production on Mineral Dust-Sea Spray Aerosols over North Atlantic,” involved a proposed new mechanism by which blowing mineral dust mixes with sea-spray to form mineral dust-sea spray aerosols (MDSA).
The results suggest that MDSA is activated by sunlight to produce more chlorine atoms, which oxidize atmospheric methane and tropospheric ozone through photocatalysis. Largely composed of blowing dust from the Sahara Desert combined with sea salt aerosols from the ocean, MDSA is the dominant source of atmospheric chlorine in the North Atlantic, the study found.
The study relied on a combination of global modeling and laboratory and field observations, including air samples from Barbados that showed seasonal depletion of stable isotopes. 13CO, an anomaly that has puzzled scientists for 20 years. They know the observed changes in the 13CO and C18O is evidence of chlorine atoms reacting with methane, and that carbon monoxide is the first stable product of atmospheric methane oxidation. But the known sources of atmospheric chlorine cannot account for the level of depletion 13CO, until now.
Using a global 3D chemistry-climate model (CAM-Chem), the research team found that when additional chlorine from the MDSA mechanism was incorporated into the model, the results agreed with the Barbados data and explained the 13Low CO.
If the effect of MDSA observed in the North Atlantic is extrapolated to the whole world, and if its effectiveness is the same in other parts of the world – two areas that are not well understood and need more research – the global concentrations of chlorine in the atmosphere will be almost 40% higher than previously estimated, the study found. Factoring this into global methane modeling could potentially shift our understanding of the relative proportions of sources of methane emissions.
Methane is a potent greenhouse gas, with a Global Warming Potential (GWP) that is 83 times higher than carbon dioxide over 20 years and 30 times higher over 100 years, accounting for almost a third of modern warming. Atmospheric methane concentrations, now nearly 2.6 times higher than in pre-industrial times, are rising at a rapid rate, with the largest annual increases on record occurring in 2020 and 2021.
Anthropogenic methane emissions are known to be responsible for most of the overall increase, with high natural emissions and changes in atmospheric chemistry resulting from anthropogenic emissions of various gases also playing a part.
While the cause of the recent acceleration is not well understood, this study may have found an important clue. Its conclusion that there is more active chlorine than previously thought, which affects 13C, shows the potential increase in methane from biological sources such as agriculture and wetlands. This suggests that biological methane may play a slightly larger role than previously estimated.
“Methane emissions from biological sources such as wetlands and agriculture may grow as global temperatures rise,” said Maarten van Herpen, lead author of the PNAS study. “But the recent increase in dust from North Africa is likely increasing methane oxidation in the atmosphere, which is 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 much faster than we thought.”
“If these findings are included in methane budgets it is likely to increase our assessment of how much methane comes from biological sources,” said University of Copenhagen professor Matthew Johnson, co-author of the study.
“While methane oxidation from MDSA is relatively small in terms of global methane, our data show that it causes large changes in the abundance of 13C of methane, which is used to determine the source contribution. The past few years have seen atmospheric methane increase at an increasing rate, more than ever before, and it is important to understand the cause. Models must take into account the altered chlorine isotope shift to get a clear picture of the increase in biological methane, which has been identified as a critical tipping point.”
How the MDSA mechanism might work in other parts of the world is not well understood and requires more research, the study argues. Follow-on research is ongoing.
“Our current research is focused on getting a better understanding of what exactly influences how much methane MDSA particles are removed from the atmosphere,” said van Herpen.
“To do that, we analyzed air samples from across the North Atlantic, provided by atmospheric observatories and commercial ships. Seafarers are helping to advance our research by filling flasks with air as they cross the African dust cloud. We have collected 500 flasks so far.
More information:
van Herpen, Maarten MJW et al, Photocatalytic chlorine atom production in mineral dust–sea spray aerosols in the North Atlantic, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2303974120
Provided by the Royal Netherlands Institute for Sea Research
Citation: Sahara dust boosts methane removal, study finds (2023, July 24) retrieved July 24, 2023 from https://phys.org/news/2023-07-sahara-methane.html
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