Concentration and variability of ice nuclei in the subtropic, maritime boundary layer

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.