Immersion Freezing Experiments Using PM10 Filter Samples from Cape Verde

Ice formation largely influences the properties of clouds and hence it has an important impact on weather and climate. In particular, immersion freezing, where a so-called ice nucleating particle (INP) immersed in a droplet catalyzes the nucleation of ice, is found to be an important ice forming mechanism in atmospheric mixed-phase clouds (Ansmann et al, 2009). In this study, selected PM10 filter samples from the Cape Verde atmospheric observatory (CVAO; 16,848°N, 24.871°W) are re-used to investigate the atmospheric INP number concentration at CVAO in the immersion mode. The samples are collected on a tower 30m above the sea level, representing the subtropical marine boundary layer. Besides marine sources, aerosol transported from the Saharan desert, the Sahel zone and SW Europe contribute to the PM10 particle composition at CVAO (Fomba et al., 2014). Using the drop freezing technique described in Conen et al. (2012) the concentration of INPs above 253K is determined. Multiple samples are cut out of each PM10 filter, are immersed in water in 0.5ml freezing tubes and subject to temperatures down to 253K, allowing ice formation by immersion freezing. As there are multiple particles immersed in each freezing tube, freezing is dominated by efficient, typical rare (one in a million), INPs. Number concentrations of INPs per volume of atmospheric sample air are calculated according to Vali (1971). Time series of the INP concentration (1-3 day time resolution) are analysed at different temperatures for seasonal trends and the impact of Saharan dust outbreaks. We find that rare INPs (1-100 m-3) active between -8ºC and -18ºC are always present in the background aerosol independent of airmass origin or total aerosol concentration. Comparing with data collected in DeMott et al. (2010), the CVAO data support the trend of the Fletcher (1962) parameterisation. In addition, the INP number concentration is investigated with regard to the influence of wind direction, temporal pattern, aerosol source region and chemical composition. The focus is on dust events, polluted versus pristine aerosol and aerosol from biological origin.

References:

Ansmann, A., and Coauthors: Evolution of the ice phase in tropical altocumulus: SAMUM lidar observations over Cape Verde. J. Geophys. Res.-Atmos., 114, D17208, doi: 10.1029/2008jd011659, 2009.

Conen, F., Henne, S., Morris, C. E., and Alewell, C.: Atmospheric ice nucleators active ≥ -12 °C can be quantified on PM10 filters, Atmos. Meas. Tech., 5, 321-327, doi:10.5194/amt-5-321-2012, 2012.

DeMott P. J. and Coauthors: Predicting global atmospheric ice nuclei distributions and their impacts on climate. Proc. Natl. Acad. Sci. USA, 107(25), 11217–11222, doi: 10.1073/pnas.0910818107, 2010.

Fomba, K. W., Müller, K., van Pinxteren, D., Poulain, L., van Pinxteren, M., and Herrmann, H.: Long-term chemical characterization of tropical and marine aerosols at the Cape Verde Atmospheric Observatory (CVAO) from 2007 to 2011, Atmos. Chem. Phys., 14, 8883-8904, doi:10.5194/acp-14-8883-2014, 2014.

Vali, G.: Quantitative Evaluation Of Experimental Results On Heterogeneous Freezing Nucleation Of Supercooled Liquids. J. Atmos. Sci., 28, 402-409, doi:10.1175/1520-0469(1971)028<0402:QEOERA>2.0.CO;2, 1971.