Recent research by Maarten van Herpen et al., reveals a new mechanism whereby blowing mineral dust mixes with sea-spray forms Mineral Dust-Sea Spray Aerosol (MDSA). The research was funded in part by the NGO Spark Climate Solutions.
The findings imply that sunlight activates MDSA to produce large amounts of chlorine atoms, which then use photocatalysis to oxidize atmospheric methane and tropospheric ozone. According to the study, MDSA, which is primarily made up of sea salt aerosol from the ocean and wind-borne dust from the Sahara Desert, is the main source of atmospheric chlorine over the North Atlantic.
The research is based on a combination of global modeling and laboratory and field observations, including air samples from Barbados that showed seasonal depletion of the stable isotope 13CO, an anomaly that scientists had been puzzled by for two decades.
They knew that changes in 13CO and C18O indicated chlorine atoms reacting with methane, and that carbon monoxide is the first stable product of atmospheric methane oxidation. However, until now, the recognized sources of atmospheric chlorine could not account for the degree of depletion in 13CO.
Van Herpen et al. discovered that when excessive chlorine from the MDSA mechanism was incorporated into a global 3-D chemistry-climate model (CAM-Chem), the results were in line with the Barbados data and described the 13CO depletion.
Global atmospheric chlorine concentrations may be about 40% higher than previously thought if the MDSA effect observed in the North Atlantic is extrapolated globally and if its efficacy is similar in other parts of the world, the study finds.
However, these two areas are poorly understood and require further research. This could potentially alter the perception of the relative proportions of the sources of methane emissions if it is taken into account in global methane modeling.
With a Global Warming Potential (GWP) 83 times greater than carbon dioxide over 20 years and 30 times greater over 100 years, methane is a powerful greenhouse gas that contributes to about one-third of current warming. Currently almost 2.6 times higher than in pre-industrial times, atmospheric methane concentrations are rising more quickly, with the largest annual increases ever recorded occurring in 2020 and 2021.
The majority of the overall increase is known to be caused by anthropogenic methane emissions, with increased natural emissions and changes in the atmosphere’s chemistry brought on by anthropogenic emissions of different gases also making a contribution.
While the cause of the recent acceleration is unknown, the van Herpen et al. study may have discovered an important clue. Its conclusion that there is more active chlorine than previously thought, affecting 13C, suggests that methane from biological sources like agriculture and wetlands could increase. This suggests that biological methane played a slightly greater role than previously thought.
“Methane emissions from biological sources such as wetlands and agriculture may be growing as global temperatures rise. 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,” states Maarten van Herpen, the study’s lead author.
When these findings are incorporated into methane budgets it is likely to increase our assessment of how much methane comes from biological sources. While methane oxidation from MDSA is relatively small in terms of global methane, our data shows it is causing large changes in the abundance of 13C in methane, which is used to determine source contributions.
Matthew Johnson, Study Co-Author and Professor, University of Copenhagen
Matthew Johnson adds, “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 need to take the revised chlorine isotope shift into account to get a clear picture of the increase in biological methane, which has been identified as a critical tipping point.”
The study claims that the MDSA mechanism is not well understood in other parts of the world and that more research is needed. Additional research is being conducted by the investigators.
“Our current research is focused on getting a better understanding of what exactly influences how much methane MDSA particles are removing from the atmosphere,” stated van Herpen,
van Herpen concludes, “To do that, we are analyzing air samples from across the North Atlantic, provided by atmospheric observatories and commercial ships. Seafarers are helping advance our research by filling flasks with air as they cross through the African dust cloud. We have collected 500 flasks so far. Early results are looking very encouraging, but we need a full year of data before we can draw conclusions.”
van Herpen, M. M. J. W., et al. (2023) Photocatalytic chlorine atom production on mineral dust–sea spray aerosols over the North Atlantic. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2303974120.