The Effect of Size-resolved Mineralogical Composition on the Heterogeneous Chemistry on Dust Particle Surfaces

Mineral dust aerosols play an important role in tropospheric chemistry by providing a reactive site for acidic gases and by altering photolysis rates. These processes strongly depend on size and composition of dust particles. Due to the complex nature of mineral dust, a number of simplified assumptions on the properties of dust particles were made in past studies. The goal of this study is to investigate how size and composition of dust particles affect heterogeneous loss rates (khet) of gaseous species, focusing on the relative importance of the assumptions involved in such modeling. We also address the implications to the tropospheric photochemistry. We examine the importance of several factors controlling heterogeneous loss rate on dust particles. The values of khet were calculated by integrating a gas-to-particle diffusion rate constant following the Fuchs-Sutugin approximation in the transition regime. Several dust size distributions from field measurements or commonly used in modeling studies were considered. The uptake coefficients for individual minerals as well as those of authentic dust samples reported from laboratory measurements were included in our model. We consider dust as a mixture of three species: iron oxide-clay aggregates, quartz, and calcite. The range of mass fractions of these species was constrained from measurements. A number of modeling experiments was performed to investigate the effect of the size-resolved mineralogical composition on khet compared to the bulk composition-resolved case or bulk case with a prescribed uptake coefficient. The results of this work will be discussed in the context of findings of previous studies as well as our companion study, addressing the effects on photolysis rates.