Scientists identify the ‘cleanest’ air on Earth

The Southern Ocean is the place to breathe deep, it seems. Atmospheric scientists say they have identified a region south of 40 degrees latitude that is “truly pristine” because the boundary layer air that feeds the lower clouds is essentially free from aerosols produced by anthropogenic activities or transported from distant lands.

The summer airborne bacterial community in this region, directly south of Australia, is dominated by marine bacteria emitted in sea spray, they say.

The study – the first to measure the ocean’s bioaerosol composition – was led by Sonia Kreidenweis from Colorado State University in the US and used data collected on board the Australian research ship R/V Investigator.

The findings are published in the journal Proceedings of the National Academy of Sciences.

“We were able to use the bacteria in the air over the Southern Ocean [SO] as a diagnostic tool to infer key properties of the lower atmosphere; for example, that the aerosols controlling the properties of SO clouds are strongly linked to ocean biological processes, and that Antarctica appears to be isolated from southward dispersal of microorganisms and nutrient deposition from southern continents,” says co-author Thomas Hill.

“Overall, it suggests that the SO is one of very few places on Earth that has been minimally affected by anthropogenic activities.”

The team sampled the air in the marine boundary layer, the lower part of the atmosphere that has direct contact with the ocean, as the R/V Investigator, which is operated by Australia’s national science agency CSIRO, steamed south to the Antarctic ice edge.

They collected airborne microbes, and using DNA sequencing, source tracking and wind back trajectories, determined that their origins were marine.

Bacterial composition also was differentiated into broad latitudinal zones, suggesting aerosols from distant land masses and human activities, such as pollution or soil emissions driven by land use change, were not traveling south into Antarctic air.

Kreidenweis, Hill and colleagues say the results contrast with those from studies of oceans in the subtropics and Northern Hemisphere, which show that most microbes came from upwind continents.

Plants and soil are strong sources of particles that trigger freezing of supercooled cloud droplets, known as ice-nucleating particles. This process reduces cloud reflectivity and enhances precipitation, increasing the amount of sunlight reaching the surface and altering Earth’s radiative balance.

However, over the Southern Ocean, the researchers say, sea spray emissions dominate the material available for forming liquid cloud droplets. Ice-nucleating particle concentrations, rare in seawater, are the lowest recorded anywhere on the planet.

In fact, the air over the Southern Ocean was so clean that there was very little DNA to work with.