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The tiny specks of dust that have a big impact on the climate

From desert soils to glacier silt, plumes of dust particles can freeze clouds and clear skies. Understanding their impact on the weather is critical to improving our climate models.

Climate Change and Environment

Ice nucleating particles (INPs), the small grains of airborne matter, are of crucial importance to climate modelling. Yet little is known about where these particles originate, or their concentration in the atmosphere. “It has become apparent over the last 5 years or so that global climate models put far too much ice into clouds. When you correct that, the models predict much stronger warming,” says Benjamin Murray, coordinator of the MarineIce project, which was supported by the European Research Council. “So quantifying the amount of ice in clouds is absolutely critical for reducing uncertainty in climate models.”

Seeds of change

INPs are generally tiny objects much smaller than the width of a human hair. They include soil dust, soot, bacteria, sea spray, pollen and more. Once airborne, they help moisture in the air to condense into droplets, forming clouds. Cloud droplets can be very cold – with temperatures as low as -38 °C, and the presence of INPs in the air column encourages these supercooled droplets to become ice crystals. “This conversion from water to ice is critical to the climate,” explains Murray. “When you trigger the transition from liquid cloud to ice cloud, you go from many small droplets to few large ice crystals, which are able to fall out of the atmosphere.” A small injection of INPs, as little as one nanoscale particle per litre of air, is enough to transform a cloudy sky into a clear one. In marine environments, this removes the reflective layer of cloud and exposes the darker ocean surface, allowing the sun’s heat to become absorbed by the planet, and dramatically changing the climatic implications. This process is particularly important in subpolar regions, around 45 to 75 degrees latitude. “The concentration of INPs is critical for understanding the phase of clouds and reflectivity in marine environments,” adds Murray. “You get massive cloud systems occurring in a supercool state, that are sensitive to INPs.”

Counting crystals

To provide better estimates of INPs in the atmosphere, Murray and his colleagues at the University of Leeds designed and built the Portable Ice Nucleation Experiment, or PINE chamber. A 10-litre vessel inside this fridge-sized device collects ambient air and condenses it into a cloud, counting the number of ice crystals formed in the process. The team also developed the Lab On A Chip, Nucleation by Immersed Particle Instrument (LOC-NIPI), which uses microfluidics to rapidly count ice crystals in water droplets. “One of the traditional ways of measuring INPs is to make droplets by hand on a cold stage; you can look at maybe 10 droplets an hour,” says Murray. “With this, we can easily generate 100 droplets a second.” Installed in ground-based and airborne laboratories, these devices are contributing to a rapid advancement in our knowledge of the distribution of INPs. One surprising source of these particles was discovered during aerial surveys carried out by the MarineIce project. “We got lucky and picked up plumes of dust coming out of glacial valleys in Iceland,” he notes. “We demonstrated this is blown all around the region, and is a major contribution to INPs in high latitudes.” Further research in the Arctic showed a complex and heterogenous distribution of INPs, highlighting how little is understood about this phenomenon. The team are now commercialising the PINE chamber through the EU-funded CountIce project, which should prove a boon to climate scientists.

Keywords

MarineIce, cloud, ice, INP, nucleating particle, dust, soot, desert, plume, clear, weather, climate

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