Using Brittle Fragmentation Theory to represent Aerosol Mineral Composition

Abstract:

Improved estimates of dust aerosol effects upon climate require the characterization of dust mineral and chemical composition. Regional variations in soil mineral composition lead to variations in dust aerosol composition. Yet, deriving aerosol mineral content also requires knowledge of the parent soil size distribution along with the fragmentation of soil particles and aggregates during the emission process. These processes modify the size distribution and mineral abundance of the emitted aerosols compared to the parent soil. An additional challenge for modeling is that global atlases of soil texture and composition are based on wet sieving, a technique that breaks the aggregates, particularly phyllosilicates, that are encountered in natural soils, drastically altering the original size distribution of the soil that is subject to wind erosion.

We propose both a semi-empirical and theoretical method to constrain the size-resolved mineral composition of emitted dust aerosols based on global atlases of soil texture and composition. Our semi-empirical method re-aggregates clay phyllosilicate minerals into larger soil particle sizes and constrains the size distribution of each emitted mineral based on observed mineral distributions at the source. Our theoretical method extends Kok’s brittle fragmentation theory to individual minerals. To this end we reconstruct the undisturbed size distribution for each mineral as a function of soil texture and soil type and calculate the emitted size distribution applying brittle fragmentation and assuming homogeneous fragmentation properties among the mineral aggregates. These approaches were tested within the NASA GISS Earth System ModelE. We discuss the improvements achieved and suggest future developments.