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      Rainfall drives atmospheric ice-nucleating particles in the coastal climate of southern Norway

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      Atmospheric Chemistry and Physics
      Copernicus GmbH

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          Abstract

          <p><strong>Abstract.</strong> Ice-nucleating particles (INPs) active at modest supercooling (e.g. −8<span class="thinspace"></span>°C; INP<sub>−8</sub>) can transform clouds from liquid to mixed phase, even at very small number concentrations (&amp;lt;<span class="thinspace"></span>10<span class="thinspace"></span>m<sup>−3</sup>). Over the course of 15 months, we found very similar patterns in weekly concentrations of INP<sub>−8</sub> in PM<sub>10</sub> (median<span class="thinspace"></span> = <span class="thinspace"></span>1.7<span class="thinspace"></span>m<sup>−3</sup>, maximum<span class="thinspace"></span> = <span class="thinspace"></span>10.1<span class="thinspace"></span>m<sup>−3</sup>) and weekly amounts of rainfall (median<span class="thinspace"></span> = <span class="thinspace"></span>28<span class="thinspace"></span>mm, maximum<span class="thinspace"></span> = <span class="thinspace"></span>153<span class="thinspace"></span>mm) at Birkenes, southern Norway. Most INP<sub>−8</sub> were probably aerosolised locally by the impact of raindrops on plant, litter and soil surfaces. Major snowfall and heavy rain onto snow-covered ground were not mirrored by enhanced numbers of INP<sub>−8</sub>. Further, transport model calculations for large (&amp;gt; 4<span class="thinspace"></span>m<sup>−3</sup>) and small (&amp;lt;<span class="thinspace"></span>4<span class="thinspace"></span>m<sup>−3</sup>) numbers of INP<sub>−8</sub> revealed that potential source regions likely to provide precipitation to southern Norway were associated with large numbers of INP<sub>−8</sub>. The proportion of land cover and land use type in potential source regions was similar for large and small numbers of INP<sub>−8</sub>. In PM<sub>2. 5</sub> we found consistently about half as many INP<sub>−8</sub> as in PM<sub>10</sub>. From mid-May to mid-September, INP<sub>−8</sub> correlated positively with the fungal spore markers arabitol and mannitol, suggesting that some fraction of INP<sub>−8</sub> during that period may consist of fungal spores. In the future, warmer winters with more rain instead of snow may enhance airborne concentrations of INP<sub>−8</sub> during the cold season in southern Norway and in other regions with a similar climate.</p>

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          Ice nucleation by particles immersed in supercooled cloud droplets.

          The formation of ice particles in the Earth's atmosphere strongly affects the properties of clouds and their impact on climate. Despite the importance of ice formation in determining the properties of clouds, the Intergovernmental Panel on Climate Change (IPCC, 2007) was unable to assess the impact of atmospheric ice formation in their most recent report because our basic knowledge is insufficient. Part of the problem is the paucity of quantitative information on the ability of various atmospheric aerosol species to initiate ice formation. Here we review and assess the existing quantitative knowledge of ice nucleation by particles immersed within supercooled water droplets. We introduce aerosol species which have been identified in the past as potentially important ice nuclei and address their ice-nucleating ability when immersed in a supercooled droplet. We focus on mineral dusts, biological species (pollen, bacteria, fungal spores and plankton), carbonaceous combustion products and volcanic ash. In order to make a quantitative comparison we first introduce several ways of describing ice nucleation and then summarise the existing information according to the time-independent (singular) approximation. Using this approximation in combination with typical atmospheric loadings, we estimate the importance of ice nucleation by different aerosol types. According to these estimates we find that ice nucleation below about -15 °C is dominated by soot and mineral dusts. Above this temperature the only materials known to nucleate ice are biological, with quantitative data for other materials absent from the literature. We conclude with a summary of the challenges our community faces.
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            THE TERMINAL VELOCITY OF FALL FOR WATER DROPLETS IN STAGNANT AIR

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              Primary biological aerosol particles in the atmosphere: a review

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                Author and article information

                Journal
                Atmospheric Chemistry and Physics
                Atmos. Chem. Phys.
                Copernicus GmbH
                1680-7324
                2017
                September 19 2017
                : 17
                : 18
                : 11065-11073
                Article
                10.5194/acp-17-11065-2017
                90182c8e-db7b-4bd9-8584-f235ca8d4d6b
                © 2017

                https://creativecommons.org/licenses/by/3.0/

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