The effects of the size-resolved mineralogical composition of dust particles on the tropospheric photochemistry

Mineral dust particles can perturb the troposheric photochemical system by modifying the actinic flux and by providing the reactive surfaces for heterogeneous reactions. Both processes strongly depend on size and composition of mineral dust particles. Given that the current chemical transport models include gas-phase and heterogeneous chemistry among other processes, adequate representation of mineral dust properties in all relevant processes is required. This study considers size-resolved mineralogical composition to provide consistent treatments of dust in both radiative and chemical modeling. The goal is to investigate the effect of the size-resolved mineral dust on tropospheric chemistry through heterogeneous reactions and photolysis and to evaluate the relative importance of assumptions involved in such modeling.

We consider several representative cases of Asian dust loadings in the clean and polluted marine environments. The radiative fluxes and the photolysis rate constants are calculated with the NCAR TUV code which was modified to incorporate a new block of spectral optical characteristics of aerosols. The overall heterogeneous loss rates are calculated by integrating the gas-to-particle diffusion rate constant using the Fuch-Sutugin approximation in the transition regime for a given dust particle size distribution. The range of the uptake coefficient is estimated by taking into account the mineralogical composition of dust particles based on recent measurements.

Several types of the dust size distribution reported from measurements as well as those commonly considered in the models are used in order to investigate how the size distribution affects spectral actinic fluxes and the rates of heterogeneous reactions. The data on the size-resolved mineralogical composition, especially the iron oxide content and the alkalinity, are used to constrain both the spectral optical properties and the uptake coefficients. The results will be presented and discussed in the context of findings reported by previous studies.