The Southern Ocean has the cleanest air on Earth. We have just discovered why

By Tahereh Alinejadtabrizi, Monash University, Steven Siems, Monash University, Yi Huang, The University of Melbourne

The Southern Ocean is renowned for having the cleanest air on Earth. But the precise reasons why have remained a mystery, until now.

There’s more to it than just a lack of human activity. Yes, there are fewer people down there using industrial chemicals and burning fossil fuels. But there are natural sources of fine particles too, such as salt from sea spray or dust whipped up by the wind.

Regardless of origin, fine solid particles or liquid droplets suspended in air are known as “aerosols”. We consider clean air to have low levels of aerosols, without discriminating between natural or industrial sources.

Our recent research discovered clouds and rain play a crucial role in scrubbing the atmosphere clean.

Understanding the role of clouds and rain

Aerosol levels over the Southern Ocean are influenced by a range of factors. These include the amount of salt spray and seasonal variation in the growth of tiny plant-like organisms called phytoplankton, which are a source of airborne sulphate particles.

Fewer sulphates are produced during winter, which is when the air over the Southern Ocean is most pristine.

But that’s not the full story. The Southern Ocean is also the cloudiest place on Earth. It experiences short-lived, sporadic showers like nowhere else. We wanted to understand the role of clouds and rain in cleaning the air.

The biggest barrier to understanding these processes has always been the lack of high-quality observations of clouds, rainfall and aerosols in this poorly observed region of the world.

Thankfully, a new generation of satellites allows us to study images of clouds in unprecedented detail. We developed a computer program to recognise different cloud patterns over a vast area of the Southern Ocean.

In particular we were on the lookout for distinctive honeycomb-shaped patterns in the cloud field. These honeycomb-like clouds are of great interest because they have a major role in regulating the climate.

https://theconversation.com/the-southern-ocean-has-the-cleanest-air-on-earth-we-have-just-discovered-why-226811