Mineral dust in the atmosphere has a significant influence on the global climate, by scattering and absorbing radiation both from the Sun and Earth's surface. Dust particles can also act as nuclei for the formation of clouds and ice, influencing the properties and lifespan of clouds. Dust deposited on snow and ice changes its colour and hence affects the rate of snow melt. And dust can also deposit nutrients and pollutants to terrestrial and marine ecosystems, altering their biogeochemical cycles. Although dust emission is usually associated with deserts from arid and semi-arid regions, dust events also occur in cold regions at high latitudes. Iceland is a very active dust source in the northern hemisphere, with dark sandy areas often dominated by amorphous basaltic glass. A lack of information on the chemical and mineralogical compositions of Icelandic dust has prevented realistic estimations of its local and regional impacts. In work recently published in Atmospheric Chemistry and Physics, an international team of researchers investigated the chemical composition and mineralogy of dust samples from five major Icelandic dust hotspots. Their results will allow more accurate estimates of the radiative impact of Icelandic dust and its contribution to Arctic warming.
"Blood rain" is a relatively common sight in the UK, when red dust carried on strong winds from the Sahara falls and leaves a dusty layer on cars and other surfaces, showing just how far dust can travel in the atmosphere. But while arid and semi-arid hot regions are familiar sources of dust, there are also sources in cooler areas at higher latitudes. Dust regions in the Northern Hemisphere (including Alaska, Canada, Greenland and Iceland contribute about 3% to global dust emissions.
Wind lifts dust from soil surfaces, and with its windy climate and lack of tree cover, Iceland is one of the most active dust sources in the world. Experiencing between 34 and 135 dust events per year, Iceland is comparable to dust-active areas in arid regions. However, Iceland is also a volcanic hotspot, and 10% of its land surface is covered in glaciers. Volcanic activity within these glaciers is common, and glacial flood plains supply Icelandic dust hotspots with fine volcanic sediments.
Often dominated by amorphous basaltic glass, Iceland's sandy areas have a dark surface. These cold deserts are markedly different from those in arid regions, which is important because the chemical and mineralogical composition of dust affects how it interacts with the climate and the ecosystem.
One of the key characteristics of Icelandic dust is that it is rich in iron. As iron oxide minerals strongly absorb solar radiation, the speciation of the iron regulates the light absorption properties of the dust. Icelandic dust, therefore, can potentially warm up the lower atmosphere in the region. Once deposited to the ice sheet in the Arctic, the light-absorbing minerals might also accelerate their melt.
A lack of iron also limits seasonal growth in the sub-polar North Atlantic Ocean. Hence Icelandic dust may influence the biogeochemical processes there, by depositing soluble iron, stimulating primary production and enhancing carbon uptake.
After collecting samples from five Icelandic dust hotspots, the team used atmospheric chambers to generate and suspend dust aerosols. They then collected the PM10 fractions (particles with aerodynamic diameters under 10 µm).
At Diamond, PhD candidate Clarissa Baldo was part of the team using XANES spectra at the Fe K-edge (collected on beamline I18) to examine the iron speciation in the samples qualitatively.
This was my first visit to a synchrotron, and I thoroughly enjoyed my two days at Diamond. There was a spacious lab in which to prepare the samples, and the beamline staff were extremely helpful in setting up the XANES experiments. They guided us through the data analysis, and were always available to answer questions, even after we had left Diamond.
The team's results reveal significant differences between Icelandic dust and that from low latitudes. The team attribute these differences to the basaltic composition of the parent sediments and the low levels of chemical weathering. Glacial processes, producing glacial flour (fine sediments) that accumulates in the floodplains where the dust hotspots are located, are also a factor.
The total iron content in Icelandic dust is higher than in northern African dust. In northern African and Asian dust, iron is mainly found as clay minerals and iron oxides. However, the contribution of iron oxide species in northern African and Asian dust is different from Icelandic dust. Icelandic dust has more magnetite and less goethite and hematite.
The research team has put together a comprehensive dataset of Icelandic dust, including chemical composition, mineralogy, iron speciation and iron solubility.
The distinct chemical and mineralogical composition of Icelandic dust indicate that it may have a potentially significant impact on the radiation balance in the subpolar and polar regions. The results also show that Icelandic dust deposits soluble iron and can impact primary productivity in the North Atlantic Ocean.
Baldo C et al. Distinct chemical and mineralogical composition of Icelandic dust compared to northern African and Asian dust. Atmospheric Chemistry and Physics 20.21 (2020): 13521-13539. DOI: 10.5194/acp-20-13521-2020.
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