How the Emitted Size Distribution and Mixing State of Feldspar Affect the Ice Nucleating Particle Number Concentration in a Global Model

The effect of aerosol particles on ice nucleation and, in turn, the formation of ice and mixed phase clouds is recognized as one of the largest sources of uncertainty in climate prediction. We have developed an improved dust module in NASA GISS Earth System ModelE, which distinguishes eight different mineral species and accretions between iron oxides and the other minerals. By taking into consideration soil aggregates and their partial fragmentation at emission, based on brittle fragmentation theory, combined with the emission of large particles, we achieve a robust improvement of the simulated mineral fractions in dust aerosols versus measurements, in comparison to simpler assumptions on the emitted size distributions of the mineral fractions. We apply the improved model to calculate ice nucleating particle concentrations from K-feldspar abundance for an active site parameterization for a range of activation temperatures and external and internal mixing assumption. We find that the globally averaged INP concentration is reduced by a factor of two to three, compared to a simple assumption on the size distribution of emitted dust minerals. The decrease can amount to a factor of five in some geographical regions. The results vary little between external and internal mixing and different activation temperatures, except for the coldest temperatures. In the sectional size distribution, the size range 2-4 μm contributes the largest INP number. The results suggest that extending measurements of INP to a size range up to at least 10-20 μm will improve simulated INP concentrations.