Decoding the place with the cleanest air on the planet

The Southern Ocean has clean air with very few aerosols - tiny particles of matter that form naturally or as a result of human activity.

Open (circled above) and closed (circled below) honeycomb clouds in the Southern Ocean. Photo: Tahereh Alinejadtabrizi/npj Climate and Atmospheric Science — Open (circled above) and closed (circled below) honeycomb clouds in the Southern Ocean. Photo: *Tahereh Alinejadtabrizi/npj Climate and Atmospheric Science*

The Southern Ocean is far from any human activity, but that’s not the only reason why the air here is the cleanest in the world, Science Alert reported on April 14. In a new study published in the journal Nature , a team of experts from Monash University and the University of Melbourne discovered new information about cloud structures that shows how the Southern Ocean and its unique honeycomb-like clouds play a major role in regulating the climate. They also discovered the key role that clouds and rain play in cleaning the air.

These honeycomb clouds can have either a closed cell structure—the bright, white clouds that reflect sunlight back into space, cooling the Earth—or an open cell structure, which allows more sunlight to reach the Earth. “These complexities are a source of error in Earth climate modeling because they are not properly incorporated. It is important to strike the right balance between open and closed cells, otherwise the results can be misleading,” the team wrote.

The team believes they have found the right balance and a better understanding of the role rainfall plays in cleaning the air. Using satellite imagery, they compared the honeycomb cloud with aerosol measurements from the Kennook/Cape Grim Observatory and rainfall from nearby gauges and found that the cleanest air samples were associated with the presence of open honeycomb clouds.

Open-cell honeycomb clouds hold more moisture, about six times more than closed-cell white clouds. They look thin from satellites, but actually provide the most effective showers for washing away aerosols. Closed-cell clouds, which look dense, are less effective.

Open honeycomb clouds are most common in winter, resulting in the cleanest air at that time. In addition to the Southern Ocean, these clouds also occur in the North Atlantic and North Pacific during winter.

While aerosol concentrations in the air above the Southern Ocean are influenced by a number of factors, rainfall plays a key role. “Rainfall is key, especially from these open-cell honeycomb clouds. Rain washes aerosols out of the sky in the same way a washing machine cleans clothes,” the team writes.

Thu Thao (According to Science Alert )

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