The Southern Ocean, encircling Antarctica, is renowned for having the cleanest air on the planet. This pristine condition is largely due to its remote location, which is far from industrial pollution and human activities. Additionally, the constant westerly winds, known as the "Roaring Forties," help disperse any pollutants that may reach the region, maintaining the air's purity. This unique environment provides scientists with a natural laboratory for studying atmospheric composition and climate change without the interference of pollution.
A renowned global atmospheric gas monitoring station is located at Cape Grim in northwestern Tasmania, Australia. Image source: Phys.org
According to the World Health Organization, air pollution, a global public health crisis, results in 7 million deaths annually. Studies have shown that air pollution increases the risk of heart disease, stroke, and lung cancer. So, is there still pristine, pure air on Earth?
Four years ago, scientists from Colorado State University published a paper in the Proceedings of the National Academy of Sciences stating that the air above the Southern Ocean around Antarctica is the cleanest in the world, describing the region as a "truly pristine place."
A report in The Conversation suggests that compared to other areas, there is less use of industrial chemicals and fossil fuels, resulting in fewer human emissions. However, the reason this area remains a bastion of clean air has been a mystery.
A study published in the journal Npj Climate and Atmospheric Science now reveals that clouds and rain play a crucial role in purifying the atmosphere.
Satellite Images Reveal Honeycomb Cloud Patterns
Aerosols, tiny solid particles or droplets suspended in the air, come from both industrial sources and natural elements like sea salt or dust stirred up by the wind. Aerosol levels above the Southern Ocean are influenced by various factors. The quantity of sea spray and the seasonal growth of phytoplankton (a source of sulfate particles in the air) both affect aerosol content. The air is cleanest in winter when sulfate production is lower.
The Southern Ocean is also the cloudiest place on Earth, leading to frequent, brief showers. Researchers at Monash University in Australia have explored the role of clouds and rain in air purification. One major challenge has been the lack of high-quality observations of clouds, rainfall, and aerosols in the region.
Thanks to new-generation satellites, researchers can now study cloud patterns in unprecedented detail. They developed a computer program to identify different types of clouds over vast areas of the Southern Ocean, focusing on unique honeycomb patterns.
Honeycomb patterns are significant for climate regulation. On one hand, when these honeycomb cells are "packed" with clouds, they become whiter and brighter, reflecting more sunlight back into space and helping keep the Earth cool. On the other hand, open or "clear" honeycomb cells allow more sunlight to penetrate.
Whether honeycomb cells are open or closed also relates to the amount of rainfall they can produce. These complex interactions cause some inaccuracies in current climate models, as they do not properly include these features. Accurately simulating the "open and closed" states of honeycomb cells is crucial, otherwise, the results can be vastly different.
Heavy Showers "Wash Away" Aerosols
In April this year, the Cape Grim observatory in Australia launched a cloud and precipitation experiment to gather higher resolution data on clouds, rain, and sunlight.
Researchers compared the honeycomb cloud patterns with aerosol measurements from the observatory and meteorological data on rainfall. They found that the cleanest air days were associated with open honeycomb clouds. These clouds cause intense, sporadic showers that "wash" aerosol particles out of the air.
Compared to fluffy, white, cloud-filled closed honeycomb cells, open cells contain more moisture and produce more rain. Data showed that rainfall from open honeycomb cells was six times greater than from closed ones.
Researchers found that less cloudy conditions, as seen from satellites, actually trigger the most effective rain showers that cleanse aerosols. While cloud-filled or closed honeycomb cells may appear cloud-heavy, they are less effective at cleaning the air.
During the cleanest air seasons in winter, empty honeycomb cells are more common. Analyses indicate that large weather systems control cloud patterns. When relentless storms sweep over the Southern Ocean, they generate these open and closed honeycomb cells.
This new research further unravels why the Southern Ocean has the cleanest air in the world. Rainfall, especially from these transparent, open honeycomb clouds, is key to purifying the air across the Southern Ocean. This explains why air samples contain microbial components from the ocean.
These honeycomb patterns also exist in the North Atlantic and North Pacific during winter. Thus, the study also helps explain how these clouds remove aerosols in these regions. This research will aid in improving climate models for more accurate predictions.
