Mineral dust and its impact on global process including climate and ocean biogeochemical cycles – the role of chemistry
As one of the most mass abundant types of aerosol emitted into the atmosphere, wind-blown mineral dust aerosol accounts for one third to one half of the mass of the total aerosol budget. It is estimated that between 1000 and 3000 Tg of mineral dust is uplifted into the atmosphere annually with Saharan desert being the largest global contributor. In addition, human activities, such as improper agricultural and grazing practices, contribute to 20-50% of the atmospheric loadings. The improper agricultural and grazing practices have resulted in the desertification of land throughout the globe. The estimated budget of mineral dust is therefore likely to increase due to the predicted increase of human activities as well as the expansion of arid regions. By the year 2100, mineral dust aerosol production is anticipated to increase by 10% from its current level. Mineral dust aerosol plays an important role in the coupled global processes of chemistry, climate, biogeochemical cycles, and health. Because mineral dust may undergo processing as it is transported in the atmosphere, its impact on global processes may change over the course of its “life history”.
In our laboratory, we are trying to better understand how mineral dust can impact not only the chemical balance of the atmosphere but also its impact on climate, biogeochemical cycles and human health as described below.
Climate
Mineral dust has substantially different optical properties than atmospheric gases and can absorb and scatter solar radiation. The feature results in a direct modification to the radiative budget of the atmosphere the surface below, known as a direct radiative effect. The magnitude and sign of the direct radiation for mineral dust are strongly dependent on its potential properties, which dictate the fractions of light dust absorbs or reflects as a function of wavelength. Mineral dust can also affect the global radiation balance indirectly through their interaction with clouds by acting as cloud nucleation nuclei (CCN). Mineral dust can change the size distribution, residence time, and optical properties of clouds, influencing radiative transfer and causing an unexpected global climate change. In addition, mineral dust can provide a surface site for heterogeneous reactions of atmospheric gases, causing a change of atmospheric balance. The modification of optical constants and thermodynamic properties of dust surfaces as well as the change of atmospheric gas phase compositions has an effect on the radiative forcing. Very small concentration of mineral dust deposited onto snow and sea ice can reduce albedo of snow. Small change of snow albedo can exert a large influence on climate by altering the timing of snow melt and triggering snow/ice-albedo feedback. Currently, researchers have a low level of scientific understanding about the effect of aerosol on radiative budget of the atmosphere, according to the latest report by the Intergovernmental Panel on Climate Change (ICPP). The scientific understanding is designed as “medium – low” and “low” for direct and indirect climate forcing, respectively. The radiative impact of mineral dust in the atmosphere is quite unclear due to the incomplete understanding concerning the diverse nature, the transport and removal processes, and the chemical and physical properties of the particles. We are doing laboratory experiments and modeling analyses of mineral dust optical properties that will significantly improve our understanding of the impact of mineral dust aerosol on global climate through direct radiative forcing(collaboration with Professors Paul Kleiber (Physics) and Mark Young (Chemistry)) We are also interested in the impact that chemistry has on cloud formation including the cloud condensation and ice nuclie activity of mineral dust aerosol.
Biogeochemistry
Iron is an essential element for all biological organisms including those in marine environments. It has been suggested that 30 % of the oceans are comprised with high nutrient low chlorophyll regions where phytoplankton primary productivity is limited by the amount of bioavailable iron. Mineral dust aerosol, mainly desert dust and dust from volcanic eruptions, is a source of iron and has been previously thought to account for ~ 95% of the globally averaged atmospheric iron budget. Approximately 450 Tg of mineral dust is annually deposited into ocean waters. Among many different iron-containing solid phases found in mineral dust, hematite (α-Fe2O3) has been thought to be the most important form for soluble iron and biological utilization and it is currently the only form of iron minerals incorporated into atmospheric chemistry models. However, a number of very recent studies indicate that this approach is far too simplistic and does not accurately reflect both the complexities of atmospheric sources of iron and/or the most important iron-mineral phases involved in dissolution. For an example, Fe-containing clays accounted for more than 90% of soluble iron whereas anthropogenic sources of soluble iron, mainly from combustion sources, from industrialized and biomass burning regions, contributes up to 5% - 30% of the total iron deposited into ocean regions. Chemical and photochemical processing of the mineral aerosols may reduce Fe(III) to a more soluble Fe(II) species.
While iron dissolution in Fe-containing dust aerosol can be linked to source material (mineral or anthropogenic), mineralogy and iron speciation, particle size, type of acid anions and other oxyanions (e.g., carbonate and phosphate) present in the medium and presence of light can be key factors regulating iron dissolution from mineral dust aerosol. In our laboratory studies, we combine dissolution measurements along with spectroscopy and microscopy to better understand aerosol iron bioavailability and how atmospheric processing of Fe-containing mineral dust aerosol through heterogeneous chemistry plays a role in the amount of bioavailable iron. Laboratory studies could be further important in better understanding and quantifying these reactions.
Health
Recently health studies were initiated with Professor Alejandro Comellas’ in the Department of Internal medicine. Since iron concentrations are extremely low in body fluids, there is the potential that iron-containing particles will influence the ability of bacteria to scavenge iron for growth, virulence and inhibit antimicrobial peptide (AMP) activity. Understanding the mechanisms of iron-containing particle induced bacterial growth and virulence provides valuable knowledge in the physicochemical characteristics of particles that potentially can cause detrimental effects on human health. This is especially true for susceptible populations. Furthermore, the physicochemical characteristics and properties of these particles can play an important role in this activity. Currently, we are investigating Pseudomonas aeruginosa (PA01) growth in the presence of iron-containing particles and AMPs. In addition, bacteria virulence is being tested in an invertebrate model, Drosophila melanogaster.