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Discussion papers
https://doi.org/10.5194/acp-2019-729
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-729
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Sep 2019

Submitted as: research article | 18 Sep 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Characterization of aerosol particles at Cape Verde close to sea and cloud level heights – Part 2: ice nucleating particles in air, cloud and seawater

Xianda Gong1, Heike Wex1, Manuela van Pinxteren1, Nadja Triesch1, Khanneh Wadinga Fomba1, Jasmin Lubitz1, Christian Stolle2,3, Tiera-Brandy Robinson3, Thomas Müller1, Hartmut Herrmann1, and Frank Stratmann1 Xianda Gong et al.
  • 1Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 2Leibniz-Institute for Baltic Sea Research Warnemünde (IOW), Rostock, Germany
  • 3Institute for Chemistry and Biology of the Marine Environment, University of Oldenburg, Wilhelmshaven, Germany

Abstract. Ice nucleating particles (INPs) in the troposphere can form ice in clouds via heterogeneous ice nucleation. Yet, atmospheric number concentrations of INPs (NINP) are not well characterized and although there is some understanding of their sources, it is still unclear to what extend different sources contribute, nor if all sources are known. In this work, we examined properties of INPs at Cape Verde from different sources, the oceanic sea surface microlayer (SML) and underlying water (ULW), the atmosphere close to both sea and cloud level as well as cloud water.

Both enrichment and depletion of NINP in SML compared to ULW were observed. The enrichment factor (EF) varied from roughly 0.4 to 11, and there was no clear trend in EF with temperature.

NINP in PM10 sampled at Cape Verde Atmospheric Observatory (CVAO) at any particular temperature spanned around 1 order of magnitude below −15 °C, and about 2 orders of magnitude at warmer temperatures (>−12 °C). NINP in PM1 were generally lower than those in PM10 at CVAO. About 83 ± 22 %, 67 ± 18 % and 77 ± 14 % (median ± standard deviation) of INPs had a diameter > 1 µm at ice activation temperatures of −12, −15, and −18 °C, respectively. Among the 17 PM10 samples at CVAO, three PM10 filters showed elevated NINP at warm temperatures, e.g., above 0.01 std L−1 at −10 °C. However, for NINP in PM1 at CVAO, this is not the case. At these higher temperatures, often biological particles have been found to be ice active. Consequently, the difference in NINP between PM1 and PM10 at CVAO, suggests that biological ice active particles were present in the super-micron size range.

NINP in PM10 at CVAO was found to be similar to that on Monte Verde (MV, at 744 m a.s.l.) during non-cloud events. During cloud events, most INPs on MV were activated to cloud droplets. When highly ice active particles were present in PM10 filters at CVAO, they were not observed in PM10 filters on MV, but in cloud water samples, instead. This is direct evidence that these INPs which are likely biological are activated to cloud droplets during cloud events.

In general, Cape Verde was often affected by dust from the Saharan desert during our measurement. For the observed air masses, atmospheric NINP in air fit well to the concentrations observed in cloud water. When comparing concentrations of both sea salt and INPs in both seawater and PM10 filters, it can be concluded that sea spray aerosol (SSA) only contributed a minor fraction to the atmospheric NINP. Therefore it can be said that, unless there would be a significant enrichment of NINP during the formation of SSA particles, NINP was mainly dominated by mineral dust at cold temperatures with few contributions from possible biological particles at warmer temperatures.

Xianda Gong et al.
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Short summary
In this study, we examined number concentrations of ice nucleating particles (INPs) at Cape Verde in the oceanic sea surface microlayer and underlying seawater, airborne close to both sea and cloud level as well as in cloud water. The results show that most INPs are super-micron in size, that INP number concentrations in air fit well to those in cloud water and that sea spray aerosols at maximum contributed a small fraction of all INPs in the air at Cape Verde.
In this study, we examined number concentrations of ice nucleating particles (INPs) at Cape...
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