Seasonal variability of Saharan desert dust and ice nucleating particles over Europe

Abstract

Dust aerosols are thought to be the main contributor to atmospheric ice nucleation. While there are case studies supporting this, a climatological sense of the importance of dust to atmospheric ice nucleating particle (INP) concentrations and its seasonal variability over Europe is lacking. Here, we use a mesoscale model to estimate Saharan dust concentrations over Europe in 2008. There are large differences in median dust concentrations between seasons, with the highest concentrations and highest variability in the lower to mid-troposphere. Laboratory-based ice nucleation parameterisations are applied to these simulated dust number concentrations to calculate the potential INP resulting from immersion freezing and deposition nucleation on these dust particles. The potential INP concentrations increase exponentially with height due to decreasing temperatures in the lower and mid-troposphere. When the ice-activated fraction increases sufficiently, INP concentrations follow the dust particle concentrations. The potential INP profiles exhibit similarly large differences between seasons, with the highest concentrations in spring (median potential immersion INP concentrations nearly 10 5 m -3, median potential deposition INP concentrations at 120% relative humidity with respect to ice over 10 5 m -3), about an order of magnitude larger than those in summer. Using these results, a best-fit function is provided to estimate the potential INPs for use in limited-area models, which is representative of the normal background INP concentrations over Europe. A statistical evaluation of the results against field and laboratory measurements indicates that the INP concentrations are in close agreement with observations.