Understanding the Transport, Chemical Transformation, and Biogeochemical Impact of Mineral Dust

Mineral dust can influence the Earth’s climate by directly absorbing and scattering incoming solar radiation, indirectly influencing the Earth’s radiative budget by altering cloud microphysics and lifetime, and its deposition has been suggested to play a controlling role in marine primary productivity which can cause fluxuations in atmospheric carbon dioxide (CO2) concentrations. During this dissertation research, synergistic methods were developed in order to study the complex processes controlling mineral dust transport, chemical transformation, and the impact of its deposition on marine ecosystems. To evaluate these processes, we combine calculations made by the three dimensional (3-D) global chemical transport model (CTMs) GEOS-Chem and a dust/biota assessment tool with remotely-sensed data from the Moderate-resolution Imaging Spectroradiometer (MODIS), Multi-Angle Imaging SpectroRadiometer (MISR), and Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and active satellite retrievals from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO).;Horizontal and vertical dust transport pathways simulated in GEOS-Chem were compared to remotely-sensed aerosol optical depth (AOD) and aerosol extinction data over the major global dust emission and transport regions (“dusty” regions). The model-satellite intercomparison demonstrates that while GEOS-Chem, MODIS, MISR, and CALIPSO agree in the horizontal and vertical distributions of mineral dust, GEOS-Chem-predicted AOD values display a positive bias in comparison to remotely-sensed data over the majority of “dusty” regions (particularly in the lowest 4 km of the troposphere). This result reveals that GEOS-Chem may be over predicting mineral dust emissions or contains uncertainties in mineral dust optical properties. The implementation of a new dust mass size distribution scheme systematically reduced mineral dust AOD values and improved model-satellite comparisons. The improved agreement between GEOS-Chem and remotely-sensed AODs was achieved without reducing mineral dust emission rates, which in the past has been common practice in order to improve model-satellite/measurement comparisons.;The influence of Patagonian dust and soluble/bioavailable iron (sol-Fe) deposition on marine primary productivity was evaluated using GEOS-Chem (implemented with a prognostic dust-iron (Fe) dissolution scheme), a dust/biota assessment tool, and remotelysensed SeaWiFS chlorophyll concentration ([Chl- a]) data. General conclusions based on the model simulations of two large Patagonian dust outbreaks reveal that the synoptic meteorological patterns of strong high and low pressure systems control the horizontal and vertical transport pathways over the South Atlantic and Southern Ocean (SAO/SO). The dust outbreaks examined during this study, which are representative of large summertime outflows of mineral dust from South American continental sources, reveal that atmospheric fluxes of mineral dust from Patagonia are not likely to be the major source of bioavailable Fe to oceanic regions characterized by high primary productivity, however, even if these dust events are not causing large visual algal blooms, they may still play a controlling factor in background [Chl-a] in the SAO.;Past modeling studies calculating bioavailable nutrient deposition to global remote oceanic regions tend to use simplified parameterizations and numerous a priori assumptions. However, during this research, the most comprehensive dust-nutrient dissolution mechanism to date was developed and implemented into GEOS-Chem in order to produce bioavailable nutrient deposition datasets for usage in future marine biogeochemical and climate model predictions. Expanding on our past dust-Fe dissolution mechanism (acid-based dissolution), we added: organic ligand-promoted dissolution processes, Fe(II)/Fe(III) photochemical redox cycling, acid-based phosphorus/phosphate (PO₄3-) dissolution, and nitrate (NO₃-) formation during atmospheric transport. Overall, the model predicted ∼0.17, 0.20, and 75 Tg of sol-Fe, PO₄3-, and NO₃- were deposited to the global oceans during a yearlong simulation. Our model results agree with in situ measurements/laboratory data in the fact that the bioavailable fractions of these micronutrient, associated with mineral dust, were highly variable both spatially and temporally.