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Nuts and bolts of radiative forcing by mineral dust
Contributed by:
Irina N. Sokolik, University of Colorado at Boulder, USA
Interactions of dust with Earth's radiation field are more complex than those of most other atmospheric aerosols because mineral particles are able to scatter and absorb UV, visible, and infrared radiation, leading to either heating or cooling of the climate system. The direct radiative forcing is used to quantify a change in the planetary radiation balance. Negative forcing corresponds to cooling of the underlying surface-atmosphere system, while positive forcing corresponds to warming. At present, due to our limited knowledge of the properties of mineral aerosol, the magnitude and even the sign of its net (solar plus infrared) direct radiative forcing remains unclear. It is almost certainly positive in some locations and negative in others. The presence of dust alters the surface radiation budget, which affects surface temperature and thus various surface-air exchange processes. Furthermore, the atmospheric temperature profile and hence atmospheric dynamics may be affected by additional radiative heating or cooling occurring in the dust layer itself. Overall, the radiative impact of mineral aerosol is important relative to that of other types of aerosols'such as sulfates and smoke particles–due to the widespread distribution and large optical depth of mineral dust.
Part of the complexity in estimating dust radiative impact comes from the fact that dust sources and sinks are not uniformly distributed, and that the lifetime of mineral aerosol in the atmosphere is relatively short, at most a few weeks. Consequently the dust burden has a complex spatial and temporal pattern. Moreover, the physical and chemical characteristics of dust may evolve during the transport. Some of the major processes governing the evolution of the particle size spectrum have been identified and included in climate models (such as dust emission, sedimentation, and dry and wet removal), while others have been only hypothetically formulated (such as heterogeneous chemistry on dust particle surfaces, cloud processing, and interactions with other atmospheric aerosols).
A further complication arises because mineral aerosol is a collective term referring to widely varying mixtures that may include many constituents, including quartz, various clays (mainly kaolinite, illite, and montmorillonite), calcite, gypsum, hematite and others. These minerals each have very different physical and chemical properties (e.g., size spectrum, particle shape, density, solubility, chemical reactivity). The abundance of the various constituents depends on the place of dust origin, how it was mobilized, and physical and chemical transformation processes occurring during transport. Furthermore, because spectral optical constants (or refractive indices) vary widely from mineral to mineral, the optical properties of dust are determined by the relative abundance of each mineral and the details of how the minerals are mixed together. The differences in the refractive indices of dust samples collected at diverse locations cause large variations in the major dust optical characteristics and, consequently, in dust radiative impact [Sokolik et al., 1998]. No climate model has yet included this aspect of the radiative forcing problem.
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Figure 1. Single scattering albedo calculated at wavelength of 500 nm as a function of the ratio of dust volume to total particle volume. Solid curve is for dust particles aggregated with water (refractive index, n = 1.335); the dotted curve is for dust with sulfates (n = 1.55), and the dashed curve is for dust aggregated with soot (n = 1.96 -0.66i). |
In addition, the interaction of dust particles with other atmospheric aerosols and clouds can affect their properties. For instance, studies by Parungo et al. [1996] demonstrate that dust particles originating from China's desert are coated by sulfates or soot after passing through polluted industrial regions downwind of the desert. In contrast, Saharan dust transported over the Atlantic Ocean is often coated by sea-salt. Dust particles internally mixed with soot, sulfates, nitrates or aqueous solutions can have drastically different properties from those which are evident at the dust source. The ability of dust particles to scatter and absorb light can be altered in different ways depending on which species aggregate with dust particles. To illustrate, Figure 1 shows how the single scattering albedo of internally-mixed multicomponent particles changes as a function of the ratio of dust volume to total particle volume. The single scattering albedo is a key optical characteristic for calculating the heating or cooling effects of aerosols. As the fraction of dust increases from 0 to 1, the single scattering albedo of aggregates of dust-water (solid curve) and dust-sulfates (dotted curve) decreases, resulting in more heating. In turn, aggregation of dust with soot particles causes a decrease in the single scattering albedo (dashed curve), enhancing heating.
Coagulation of dust with other aerosol or cloud particles as well as uptake of atmospheric gases on dust particle surfaces followed by heterogeneous chemical reactions may both result in the formation of multicomponent aerosols (MCA). However, the relative importance of these processes remains unknown. Unfortunately, there is no reliable theory for modeling optical constants of the MCA aggregates [Sokolik and Toon, 1999].
While heterogeneous chemical reactions may be important in the formation of an internal mixture of dust with other aerosol compounds, in turn, the heterogeneous reactions on dust surfaces can be crucial in understanding tropospheric chemistry. There is growing evidence that heterogeneous chemical reactions on airborne mineral aerosol surfaces can play an important role in the tropospheric chemistry of SOX, NOY and O3 [Dentener et al., 1996]. To model the related chemical reactions, the size-resolved mineralogical composition of dust is required. For instance, the removal of SO2 and HNO3 from the gas phase depends on the alkalinity of dust aerosols, so the calcium content needs to be known. Hydrogen radicals like HO2 and OH react on atmospheric particles through pathways involving redox reactions with iron, so assessments of this process depend on measurements of Fe3+ abundance in dust samples. Furthermore, sulfates and nitrates can be formed via heterogeneous chemistry on dust particle surfaces, resulting in drastically modified radiative properties discussed above.
Therefore, incorporation of regionally and temporally varying size-resolved dust mineralogical composition into global and regional climate models is a desirable and promising approach to decrease the currently large uncertainties in the assessment of radiative forcing by the natural and anthropogenic components of the airborne mineral aerosol. Accomplishing this will require a great deal of effort to 1) make the relevant observations (surveys) of dust layers at various altitudes and locations, 2) achieve a better understanding of related physical and chemical process rates.
Although a large body of data on ground-level dust properties already exists, it is of limited usefulness because in most cases only a few chemical or physical properties were measured, in isolation from the others. In particular, a large gap exists between measurements of dust particle size distribution, optics, radiation, and chemistry. These need to be measured simultaneously (as in closure experiments) to test the hypothetical relationships between them. Another limitation is the deficiency found in different measurement techniques. Improvements are sorely needed in the methods employed to determine the composition of individual particles, their shapes, spectral refractive indices, aerosol absorption of light, and aerosol scattering phase function.
To gain a better understanding of diverse dust impact, a promising strategy is to perform comprehensive coordinated measurements of the physical and chemical properties of dust in targeted regions, on various space and time scales. Saharan, Central Asian, and Arabian Peninsula regions, which are the most important dust sources, are of primary importance. In this context, the ACE-Asia international experiment, focusing on Central Asia, will provide a unique opportunity to obtain coordinated measurements of dust chemical and physical properties at the dust source, in industrial and urban areas, and in the marine environment. Such measurements are urgently needed to obtain a quantitative understanding of the spatial and temporal variations of the Asian mineral aerosol and to determine the extent to which these variations are important for radiative and climate model simulations.
The above discussion reveals two major issues which must be resolved to improve modeling and prediction of dust impact: 1) quantification of dust production from both natural and anthropogenic sources, and 2) identification of the main processes controlling the evolution of the physical and chemical properties of dust during its life cycle.
Dust production
Information is needed on production rates and size, space and time scales from both natural and anthropogenic sources. There is a large body of data on the mineralogical and chemical nature of the Earth's soils, which demonstrates the complex spatial variability of soil composition. Existing global data sets of soil properties currently include soil texture and types but they do not provide information on size-resolved mineralogical composition of the parent soil. There is a clear need for a new data set to provide this missing information.
Along with the composition of bed surfaces, dust mobilization processes partly determine the initial particle size distribution of airborne minerals as well as their composition and the degree of particle aggregation. Therefore the airborne dust may exist as an external mixture of individual minerals and/or as a mixture of aggregated particles. As was demonstrated by Sokolik and Toon [1999] for a given composition and under similar atmospheric conditions, a mixture of aggregates can cause positive radiative forcing while a mixture of individual minerals gives negative forcing.
Significant progress has been made recently in incorporating surface mineralogy and surface roughness into dust emission schemes to simulate the size-resolved dust vertical flux from Saharan sources [e.g., Marticorena and Bergametti, 1995]. Such schemes can be extended to simulate size- and composition-resolved fluxes of dust from other major production regions (Africa, Asia, Middle East, India, Australia and USA). The overall goal is to quantify dust emission and relate the physical and chemical properties of dust to the source.
Evolution of dust properties during the dust life cycle
During dust transport, its initial characteristics, determined by a given dust source and production mechanism, may be altered by the following processes: deposition, sedimentation, wet and dry removal, heterogeneous nucleation, coagulation with other aerosols, heterogeneous chemistry, and cloud processing. It is important to establish the relative roles and characteristic scales of all these processes. This is a difficult problem due to the nonlinearity of the processes, possible feedbacks, and the overlap in time scales between aerosol processes and atmospheric dynamics.
Recent global and regional dust models consider only a few processes (dust production, dry and wet removal, and deposition) responsible for the evolution of particle size distribution, ignoring composition of mineral aerosol, coagulation, and heterogeneous processing [e.g., Tegen et al., 1996]. Dust composition is so fundamental that it must be included for models to be realistic. In turn, observations of dust composition will help to develop better physically-based treatments of the processes needed for climate models and for remote sensing applications. For instance, wet removal of dust is currently parameterized in models in an oversimplified fashion. If dust composition (and hence its hygroscopicity) were known, however, the treatment of this process could be greatly improved.
It would be an impossible task to incorporate every naturally occurring mineral into climate models. A practical alternative is to take into account a handful of major mineral components to correctly represent dust spectral optical properties and its heterogeneous chemical process rates. A multidisciplinary coordinated program of laboratory measurements, in situ and satellite observations together with modeling studies will be required to advance our knowledge in this area.
References
- Dentener, F., G. Carmichael, Y. Zhang, P. Crutsen, and J. Lelifeld, The role of mineral aerosols as a reactive surface in the global troposphere, J. Geophys. Res., 101, 22869-22890, 1996.
- Duce, R.A., Sources, distributions, and fluxes of mineral aerosols and their relationship to climate, in Aerosol Forcing of Climate, edited by R.J. Charlson and J. Heint-zenberg, pp.43-72, John Wiley, New York, 1995.
- Marticorena, B., and G. Bergametti, Modeling the atmospheric dust cycle: 1. Design of a soil-derived dust emission scheme, J. Geophys. Res., 100, 16415-16430, 1995.
- Parungo, F., et al., Asian dust storms and their effects on radiation and climate, Part 1, TR 2906, Science and Technology Corporation, Hampton, VA, 1995.
- Sokolik, I. N., and O. B. Toon, Direct radiative forcing by anthropogenic mineral aerosols, Nature, 381, 681-683, 1996.
- Sokolik, I. N., O. B. Toon, and R.W.Bergstrom, Modeling the radiative characteristics of airborne mineral aerosols at infrared wavelengths. J. Geophys. Res., 103, 8813-8826, 1998.
- Sokolik, I. N., and O. B. Toon, Incorporation of mineralogical
composition into models of the radiative properties of mineral
aerosol from UV to IR wavelengths.
J. Geophys. Res., in press. - Tegen, I., A. A. Lacis, and I. Fung, The influence on climate forcing of mineral aerosols from disturbed soils, Nature, 380, 419-422,1996.
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