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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
30 Aug 2017
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.
Concentration and variability of ice nuclei in the subtropic, maritime boundary layer
André Welti1, Konrad Müller1, Zoë L. Fleming2, and Frank Stratmann1 1Leibniz-Institute for Tropospheric Research (TROPOS), Leipzig, Germany
2National Centre for Atmospheric Science (NCAS), Department of Chemistry, University of Leicester
Abstract. Measurements of the concentration and variability of ice nucleating particles in the subtropical, maritime boundary layer are reported. Filter samples collected in Cape Verde over the period 2009–2013 are analyzed with a drop freezing experiment sensitive to detect the few, rare ice nuclei active at low supercooling. The data-set is augmented with continuous flow diffusion chamber (SPIN) measurements at temperatures below −24 °C from a two month field campaign at Cape Verde in 2016. The data set is used to address the questions: What are typical concentrations of ice nucleating particles active at a certain temperature, what affects their concentration, what is their composition and where are their sources?

To investigate what the most common ice nuclei are and to identify the sources, bulk chemical aerosol composition obtained from the utilized filter samples is tested for correlations with ice nuclei concentration. It is shown that no significant correlation between the rare ice nuclei and the bulk aerosol chemical composition, which could serve as tracer for a specific aerosol class e.g. of maritime origin, can be made.

Concentration of ice nuclei is found to increase exponentially with decreasing temperature. It indicates that several groups of particles with different ice nucleation properties (size, composition) are contributing to the ice nuclei concentration at different temperatures. The concentration of ice nuclei active at a specific temperature varies over a wide range. The frequency with which a certain ice nuclei concentration is measured within this range is found to follow a log-normal distribution. The log-normal frequency distribution can be explained by random dilution associated with turbulent, long-distance transport. To investigate the geographic origin of ice nuclei, source attribution of air masses from dispersion modeling is used to classify the data into 7 typical situations. While no source could be attributed to the ice nuclei active at temperatures higher than −12 °C, concentrations at lower temperatures tend to be elevated in air masses originating from the Saharan desert.

Citation: Welti, A., Müller, K., Fleming, Z. L., and Stratmann, F.: Concentration and variability of ice nuclei in the subtropic, maritime boundary layer, Atmos. Chem. Phys. Discuss.,, in review, 2017.
André Welti et al.
André Welti et al.
André Welti et al.


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Short summary
We report on ambient concentrations of ice nuclei, measured on Cape Verde Islands. Concentrations are found to exponentially increase by seven order of magnitude from −5 °C to −38 °C. At each temperature, the frequency distribution of observed concentrations can be described by a log-normal distribution, typical for random dilution of substances during transport. Correlating concentrations to aerosol sources shows, that there is no easy answer to what ice nuclei are made of or where they are from.
We report on ambient concentrations of ice nuclei, measured on Cape Verde Islands....