Ground-based measurements of immersion freezing in the eastern Mediterranean

Abstract

Ice nuclei were measured in immersion-freezing mode in the eastern Mediterranean region using the FRIDGE-TAU (FRankfurt Ice-nuclei Deposition freezinG Experiment, the Tel Aviv University version) chamber. Aerosol particles were sampled during dust storms and on clean and polluted days (e.g., Lag BaOmer). The aerosol immersion-freezing potential was analyzed in the laboratory using a drop-freezing method. Droplets from all the samples were found to freeze between −11.8 °C and −28.9 °C. Immersion-freezing nuclei (FN) concentrations range between 0.16 L[superscript −1] and 234 L[superscript −1], while the activated fraction (AF) ranges between 8.7 × 10[superscript −8] and 4.9 × 10[superscript −4]. The median temperature at which the drops from each filter froze was found to be correlated with the corresponding daily average of PM[subscript 10], PM[subscript 2.5] and PM[subscript 10]–PM[subscript 2.5]. A higher correlation value between FN concentrations and PM[subscript 10]–PM[subscript 2.5] suggests that the larger particles are generally more effective as FN. The measurements were divided into dust storms and "clean" conditions (this is a relative term, because dust particles are always present in the atmosphere is this region) based on the air mass back trajectories and the aerosol mass concentrations (PM[subscript 10]). Droplets containing ambient particles from dust storm days froze at higher temperatures than droplets containing particles from clean days. Statistically significant differences were found between dust storms and clean conditions primarily in terms of the initial temperature at which the first drops froze, the median freezing temperature and the aerosol loading (PM values). FN concentrations and AF values in dust storms were larger by more than a factor of 2 than in the clean conditions. This observation agrees with previous studies showing that some dust particles are almost always present in the atmosphere in this region. Measurements of aerosol particles emitted from wood burning bonfires during a Lag BaOmer holiday showed that although a high concentration of particles was emitted, those particles' effectiveness as FN was relatively poor. The most likely reason for the low FN efficiency is the combination of relatively low fire temperatures and high organic carbon fraction in the aerosols.