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A team of researchers, including scientists from the National Centre for Atmospheric Science and the Facility for Airborne Atmospheric Measurements, have used the FAAM research aircraft to examine desert dust from the Sahara as it is transported over the Atlantic several days later. Scientists were using a new technique to measure the concentration of ice-nucleating particles in the desert dust. These are particles in the atmosphere that ice crystals can form on -  without them, liquid water droplets can cool to -40℃ without freezing. There have been very few measurements of these ice-nucleating particles within the air near to desert sources.

Price H IceD 14 1200pxThe FAAM aircraft collected samples of atmospheric aerosols (solid or liquid particles such as dust, soot and sea salt suspended in the air) in the region around Cape Verde as part of the ICE-D campaign. The concentrations of ice-nucleating particles in these samples were successfully measured over a range of altitudes from 30m to 3,500m. The concentrations of ice-nucleating particles active at temperatures  between -25 and -10℃ were quantified, and have been published in a paper led by Dr. Hannah Price, FAAM Instrument Scientist. The measurements showed a correlation between the concentration of ice-nucleating particles and the desert dust concentrations. This project was part funded by Prof. Ben Murray’s European Research Council fellowships which supported Hannah Price, Jim McQuaid and Ben Murray to make the INP measurements and write the paper.

The formation of ice crystals in the air is interesting to researchers because it can affect the lifetime of clouds, precipitation, and atmospheric electrification. Desert dust is one of the most important sources of ice-nucleating particles in the world, it can trigger the formation of ice crystals all over the world’s atmosphere, not only in the air close to arid regions. The Sahara is the largest single source of desert dust, exporting 100 teragrams (100 billion kg) of dust to the tropical Atlantic and beyond.

Despite the fact that the dusty air over the Atlantic was from a geographically diverse range of sources, the measurements collected by FAAM seemed in this particular case to be similar to some laboratory-based scenarios. This could suggest that laboratory measurements have some use in predicting the amount of ice crystals that may form in desert dust aerosols, and that the concentration of ice-nucleating particles within desert dust may be only weakly dependent on the source of the dust.

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The aircraft was based out of Praia on the island of Santiago and flights focused on the oceanic region around Cape Verde with flights as far away as the Canary Islands. This region is strongly impacted by dust from sources across North Africa with the dust layers typically below 5 km.

Authors on the paper who belong to the National Centre for Atmospheric Science or the Facility for Airborne Atmospheric Science include Hannah Price, Tom Choularton, Zhiqiang Cui, Martin Gallagher, Alan Blyth, Ryan Neely and Jamie Trembath.

Pictures are provided courtesy of Hannah Price.