Source of dust key to ice forming in clouds

Dust particles from river deltas can more potently influence ice formation in clouds than those from deserts, researchers have found. At low temperatures, atmospheric dust particles are known to enable the aggregation of ice particles to form ice crystals in clouds. In the absence of dust, called ice-nucleating particles, water in clouds can remain in liquid form despite freezing temperatures.

The ice thus formed in clouds, which reflects back sunlight, can significantly impact the amount of sunlight reaching the Earth. Whether this ice formation in clouds adds to global warming or helps cool the planet, depends on the amount of ice in clouds, the amount of ice nucleating particles present and the nature of these particles, say the researchers from the University of Leeds and the National Centre for Atmospheric Science, UK.

Therefore, we need to be able to understand and quantify the various global sources of ice-nucleating particles, they said in their study published in the journal Science Advances. "Only a small fraction of the dust particles in the atmosphere has the capacity to nucleate ice and we are only just starting to understand their sources and global distribution.

"At present, climate models tend not to represent these high-latitude sources of dust, but our work indicates that we need to," said Benjamin Murray, an atmospheric scientist at Leeds who supervised the study. During summer and autumn, when the air is usually dry, the silt from river deltas - microscopic pieces of rock, minerals and vegetation - is picked up by winds and carried over hundreds of miles reaching altitudes where it facilitates cloud ice formation.

In this study, the researchers looked at dust at high-latitudes, coming from a delta on the south coast of Alaska from the Copper River Valley, which extends for more than 450 kilometres and is estimated to transport 70 million tonnes of glacial sediment every year.

Previous studies have looked at atmospheric dust at low and mid latitudes kicked up by storms in the deserts across Africa and Asia, including the Sahara. The dust particles from the river delta, rich with biological material, helped form cloud ice more effectively than those coming from the Sahara, the researchers found. Further, they found that the ice formation enabled by Alaskan delta dust were driven by microscopic fragments of substances essential for life whereas, that enabled by dust from Sahara were driven by particles of a mineral called potassium feldspar.

"We knew that deserts like the Sahara are very important at supplying ice-nucleating particles to the atmosphere, but this paper shows that river deltas like the Copper River Valley are also very important. "Huge quantities of dust are emitted from places like the Copper River, and we need to understand these emissions to improve our climate models," said Sarah Barr, the lead author of the paper.