Can Aerosols from mid-latitudes inhibit the liquid to ice transition in Arctic Clouds?

Despite being remote from industrialized areas, Arctic regions are regularly under the influence of elevated concentrations of pollutants from lower latitude. During winter, meteorological conditions are favourable to increase of aerosol loadings, mainly due to surface temperature inversion and reduction of wet scavenging (dryness of the atmosphere). In spring the aerosol concentration decreases as the weather gets warmer (i.e. through increase of wet scavenging and/or reduction of transport efficiency).

Aerosols can interact with the atmospheric water by acting as Cloud Condensation Nuclei (CCN) or Ice Nuclei (IN). Both nuclei facilitate vapour-liquid or liquid-ice phase transition, potentially resulting in higher number of liquid droplets or ice crystals respectively. Changes in cloud thermal emissivity and in cloud reflectivity induced by aerosol-cloud interactions can lead to warming or cooling at the surface depending on cloud liquid water path, microphysical properties (particle size and phase) and altitude.

Previous studies have shown that aerosols can act more or less efficiently as ice nuclei depending on their chemical composition and size. Also it has been proposed that acidification of aerosols could impact their ability to act as ice nuclei and would therefore play an important role in determining microphysical properties of Arctic ice clouds. In this study we are focusing on the potential impact of aerosols on cloud thermodynamic phase and especially on liquid-ice phase transition.

We combined POLDER-3/PARASOL and MODIS/AQUA satellite measurements to retrieve cloud properties that are further analysed in view of biomass burning and anthropogenic aerosols concentration determined from the FLEXPART numerical transport model. Thanks to the combination of POLDER and MODIS observations, a cloud phase index is derived to analyse statistics of ice and liquid cloud fractions as function of atmosphere thermodynamic conditions, other cloud properties and aerosol loadings (related through modelled CO concentrations).

We report here preliminary results of this study illustrating a potential correlation between average temperature of liquid-ice phase transition and aerosol concentrations. Various hypothesis for this observation will be discussed. In particular it remains unclear from these early results whether the ice-water transition dependence on CO concentration is solely linked to the decrease of cloud effective radius by the first indirect effect or caused by other factors.