Active chlorine in the atmosphere is poorly constrained. Similarly, its role in the oxidation of the potent greenhouse gas methane, causing uncertainty in global methane budgets, remains obscure.
A new study proposed a photocatalytic mechanism for chlorine atom production that occurs when Sahara dust mixes with sea spray aerosol. They found a hidden link between Saharan dust clouds and atmospheric methane levels – a strong greenhouse gas.
Jan-Berend Stuut was part of a group of worldwide scientists investigating a mechanism that could fundamentally alter our comprehension of methane emissions and their connection to global warming. Potentially enormous consequences could be had for the future of our planet’s climate.
Jan-Berend Stuut said, “Sahara dust plays a series of important roles in our planet’s climate. In high regions of the atmosphere, it controls the amount of sunlight reaching the earth’s surface. And it can trap heat in the lower part of the atmosphere. The dust is also fertilizing phytoplankton in the oceans. Plankton captures CO2 from the atmosphere. The iron and silica-rich dust is their only food source far away from land.”
This new study introduces a novel mixing mechanism for mineral dust, and sea spray called the “Mineral Dust-Sea Spray Aerosol” (MDSA). The findings imply that sunlight activates MDSA to produce large amounts of chlorine atoms, using photocatalysis to oxidize atmospheric methane and tropospheric ozone. Over the North Atlantic, MDSA is the main source of atmospheric chlorine.
Stuut:Â “Under the influence of sunlight, the iron from desert dust forms so-called chlorine radicals from sea salt (NaCl). These ions bind to the greenhouse gas methane (CH4), removing it from the atmosphere.”
“Methane is a greenhouse gas that causes ~30x as much warming as CO2. Therefore, removing this greenhouse gas is also a natural way to combat warming.”
Thanks to this mechanism, scientists could better understand global CH4 budgets. The lighter carbon isotope 12C is eliminated more strongly than the heavier carbon isotope 13C due to the chemical interactions of these chlorine radicals. In the Caribbean, the same finding had already been made.
Stuut said, “Measurements from Barbados showed that atmospheric methane was enriched in 13C, especially in summer and autumn. But there was no explanation for this.”
The study emphasizes the need for a detailed understanding of how the MDSA mechanism may operate in other parts of the world.
Van Herpen said, “Follow-on research is underway. Our current research is focused on better understanding of what influences how much methane MDSA particles are removed from the atmosphere. To do that, we analyze 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 conclude.”
Journal Reference:
- Maarten M. J. W. vam Herpen, Qinyi Li et al. Photocatalytic chlorine atom production on mineral dust–sea spray aerosols over the North Atlantic. PNAS. DOI: 10.1073/pnas.2303974120