Unraveling Earth’s Cleanest Air Mystery

The Southern Ocean is celebrated for its pristine air quality, long an enigma until recent findings shed light on the mystery.

The cleanliness isn’t solely due to limited human activity. While the region sees minimal industrial chemical use and fossil fuel burning, natural sources like sea salt from spray or wind-driven dust also contribute.

Aerosols, which are fine solid particles or liquid droplets in the air, define the cleanliness of the atmosphere, irrespective of their origin—be it natural or industrial.

Our latest study reveals that clouds and rain are pivotal in purifying the atmosphere.

Exploring the Impact of Clouds and Rain

Aerosol concentrations in the Southern Ocean are shaped by various factors, including salt spray levels and seasonal fluctuations in phytoplankton growth, which produce airborne sulphate particles.

Sulphate production decreases during winter, coinciding with the region’s peak air quality.

However, this isn’t the entire narrative. The Southern Ocean stands out as the cloudiest area globally, experiencing unique, brief rain showers. Our goal was to elucidate how clouds and rain contribute to air purification.

Understanding these processes has been challenging due to limited high-quality data on clouds, rainfall, and aerosols in this sparsely monitored region.

Fortunately, advanced satellite technology now offers detailed cloud imagery. We developed a program to identify various cloud patterns across the expansive Southern Ocean, focusing on distinctive honeycomb formations. These clouds play a significant role in climate regulation.

When a honeycomb cell is cloud-filled or “closed,” it appears whiter, reflecting more sunlight and cooling the Earth. Conversely, “open” cells allow more sunlight penetration.

The complexity of these patterns poses challenges in accurately modeling Earth’s climate; hence, it’s crucial to account for open and closed cell balances to avoid significant discrepancies.

The honeycomb cell status also impacts rainfall potential. These cells, spanning 40-60km in diameter, are visible from space, enabling satellite-based studies.

Our study aligns well with this month’s launch of a cloud and precipitation experiment at Cape Grim in Tasmania, aiming to gather high-resolution data on clouds, rain, and sunlight.

Removing Aerosols from the Atmosphere

We analyzed honeycomb cloud patterns using aerosol data from the Kennaook/Cape Grim observatory and rainfall records from a nearby gauge. Our findings revealed that days with the cleanest air correlated with open honeycomb clouds, likely due to their ability to produce intense rain showers that cleanse aerosol particles.

Interestingly, open cells, despite appearing less cloudy, hold more moisture and produce six times more rain than the filled, closed cells. This counterintuitive result means less cloud cover by satellite often leads to more effective rain showers for aerosol removal, while cloudier appearances are less effective.

During the cleanest winter months, open honeycomb patterns were predominant. Additionally, our analysis indicates that large-scale weather systems dictate the cloud field’s appearance, with storm movements creating both open and closed honeycomb cells.

Cleaner Air and Improved Climate Predictions

Our study provides a significant insight into the Southern Ocean’s pristine air quality, highlighting rainfall, particularly from clear, open honeycomb clouds, as the key factor in air purification. We were the first to identify these clouds’ pivotal role in cleansing the air passing over the Southern Ocean.

Similar honeycomb cloud patterns appear in the North Atlantic and North Pacific during winter, suggesting our findings may also explain aerosol removal in these regions, aiding in refining climate models for more accurate predictions.

Rain effectively removes aerosols from the atmosphere, akin to how a washing machine cleans clothes. Following a cold front passage, the air becomes noticeably cleaner, benefiting those on Australia’s southern coast as fresh air sweeps in from the Southern Ocean.

We extend our gratitude to CSIRO, ANSTO, and the Bureau of Meteorology for their valuable contributions to this research.

Read the original article on: Science Alert

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