Major Dust Source Research Articles

Introduction to special section on Aeolian Processes: Field Observations and Modeling

[1] Aeolian processes, involving erosion, transportation, and deposition of sediment by the wind, occur in a variety of environments including coastal areas, semiarid, and arid regions (e.g., cold and hot deserts), agricultural fields, and some planetary bodies, notably Mars and possibly Venus and Titan. Aeolian processes are responsible for the emission, transport, and deposition of dust and formation of areas of sand dunes. [2] This special section contains 14 papers that recognize the distinguished career of Dr. Dale Gillette. The papers are based upon oral and poster presentations given at the Sixth International Conference on Aeolian Research (ICAR VI), which was held at the University of Guelph, Ontario, Canada, 23–28 July 2006 (for a summary of the conference see Lancaster et al. [2006]). A special aspect of ICAR VI was the recognition of the long, multifaceted, and distinguished career of Dale Gillette, known to many for his seminal work on mineral aerosols and wind erosion processes. ICAR VI was the latest in a series of conferences on aeolian processes, sediments, and landforms that started in 1986 in Aarhus, Denmark, followed by conferences in Sandberg, Denmark (1990); Zzyzx, California (1994); Oxford, UK (1998); and Lubbock, Texas (2002). The next ICAR will be held in Argentina in 2010. [3] The papers in this volume were chosen for their close relationships to aspects of Dale Gillette's career and his major scientific contributions, as well as being a reflection of some of the most innovative contemporary approaches to studies of aeolian processes, sediments, and landforms. Many of the papers cite Dale's work and are clearly inspired by the fundamental concepts and approaches that he articulated. Thus, there are papers on dust emissions from disturbed and undisturbed surfaces [e.g., Macpherson et al., 2008] that build on the work of Gillette et al. [1980] as well as field studies of dust events and processes [McGowan and Clark, 2008] that continue and build upon the methodology developed during collaborative work at Owens Lake, California, the major source of mineral dust in the western United States prior to mitigation measures designed to reduce dust emissions [Cahill et al., 1996; Gillette et al., 1997a, 1997b, 1997c]. Further papers document experiments on dust deposition processes [Goossens, 2008] and field studies of dust deposition to soils and ecosystems [Bowker et al., 2008; Goldstein et al., 2008]. [4] Dale Gillette was a pioneer in conceptualizing and quantifying the effects of roughness elements (rocks, vegetation) on wind erosion and sediment transport processes [Gillette and Stockton, 1989; Musick and Gillette, 1990; Raupach et al., 1993; Stockton and Gillette, 1990], and this volume contains a number of papers [e.g., Brown et al., 2008; King et al., 2008; Sutton and McKenna Neuman, 2008; Okin, 2008; Shao and Yang, 2008] that build on the fundamental concepts that Gillette and his colleagues developed. Dale Gillette also maintained a long-term interest in the fundamentals of sediment mobilization and transport by wind [e.g., Gillette and Stockton, 1986; Gillette et al., 1996]; papers in this special section by Rasmussen and Sorensen [Rasmussen and Sorensen, 2008] as well as McKenna Neuman and Sanderson [McKenna Neuman and Sanderson, 2008] provide further understanding of the controls of sediment transport and creative ways in which to model these complex nonlinear processes. Throughout his career, Dale Gillette maintained a strong interest in modeling of aeolian processes. Many of the papers in this volume develop models that can be used to assess aeolian processes and their effects on the environment as well as to understand fundamental controls of the processes [e.g., Okin, 2008]. Although Dale never worked on dunes, the approach of the paper on barchan dune corridors [Elbelrhiti et al., 2008] adopts the approach to modeling advocated by him, being firmly based upon fundamental physical principles. [5] The papers in this volume clearly indicate the diversity and healthy state of aeolian research today. Challenges for the immediate future include the issue of scaling of detailed process studies, such as those reported here, on small spatial and temporal scales to regional and global issues such as the global dynamics of aeolian systems, including for example the contribution of dust deposition to soil development, and the human dimensions of change in aeolian systems. Despite considerable progress in recent years, there still remains a significant gulf between the modeling community and those engaged in field and experimental studies. Dale Gillette provides a stellar example of a person who has been able to bridge this gap and thereby contribute in a unique way to scientific understanding of aeolian processes as well as its application to real-world problems of wind erosion and air quality.

Micro-Pulse Lidar Measurements of Aerosol Vertical Structure over the Loess Plateau

Knowledge of the vertical distribution of aerosols in the free troposphere is important for estimating their impact on climate. In this study, direct observations of the vertical distribution of aerosols in the free tropo- sphere are made using surface Micro-Pulse Lidar (MPL) measurements. The MPL measurements were made at the Loess Plateau (35.95°N, 104.1°E), which is near the major dust source regions of the Taklimakan and Gobi deserts. The vertical distribution of the MPL backscattering sug- gested that non-dust aerosols floated from ground level to an altitude of approximately 9 km around the source re- gions. Early morning hours are characterized by a shallow aerosol layer of a few hundred meters thick. As the day progresses, strong convective eddies transport the aero- sols vertically to more than 1500 m.

Size-differentiated source profiles for fugitive dust in the Chinese Loess Plateau

Size-differentiated fugitive dust chemical source profiles are determined for the Chinese Loess Plateau, a major source of Asian dust. Fifteen loess samples at five sites (Yulin, Yanchi, Huanxian, Luochuan, and Xi’an) were collected, dried, sieved, resuspended, and sampled through TSP, PM 10, PM 2.5, and PM 1 inlets onto filters for analysis of 40 major and trace elements (Na to U), six ions (Cl −, NO 3 −, SO 4 2−, Na +, K +, and NH 4 +), organic carbon (OC) and elemental carbon (EC), and carbonate carbon (CO 3-C). The abundances of major species (>1%) include Al, Si, K, Ca, Fe, OC and CO 3-C, in four size-differentiated source profiles. OC accounted for ∼90% of total carbon in four size fractions for most of the profiles. Enrichment factors indicated that the 18 elements (Na, Mg, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Sr, Y, Zr, Ba) found in the samples were dominated by crustal sources. Enrichment factors for Co, As, Se, Mo, Cd, In, Sb, and Tl are one to two orders of magnitude larger than crustal-derived elements, suggesting the influence of anthropogenic pollution sources. Compared with loess samples, V, Cr, Ni, Cu, Zn, Sb, SO 4 2−, and NO 3 − are enriched in ambient PM 2.5 aerosol samples, implying that Asian dust contains pollution components in downwind regions of the desert. The elemental ratios of Si, K, Ca, Ti, Mn, and Fe to Al in Asian dust source regions compared well with those found at downwind regions such as Korea, Japan, and the North Pacific. These ratios can be used as fingerprints to trace the transport path of Asian dust.

Intercontinental transport of pollution and dust aerosols: implications for regional air quality

Abstract. We use the global model GOCART to examine the impact of pollution and dust aerosols emitted from their major sources on surface fine particulate matter concentrations at regional and hemispheric scales. Focusing on the North America region in 2001, we use measurements from the IMPROVE network in the United States to evaluate the model-simulated surface concentrations of the "reconstructed fine mass" (RCFM) and its components of ammonium sulfate, black carbon (BC), organic matter (OM), and fine mode dust. We then quantify the RCFM budget in terms of the RCFM chemical composition, source type, and region of origin to find that in the eastern U.S., ammonium sulfate is the dominant RCFM component (~60%) whereas in the western U.S., dust and OM are just as important as sulfate but have considerable seasonal variations, especially in the NW. On an annual average, pollution aerosol (defined as aerosols from fuel combustion for industrial and transportation uses) from North America accounts for 65–70% of the surface RCFM in the eastern U.S. and for a lower proportion of 30–40% in the western U.S.; by contrast, pollution from outside of North America contributes to just 2–6% (~0.2 μg m−3) of the total RCFM over the U.S. In comparison, long-range transport of dust brings 3 to 4 times more fine particles than the transport of pollution to the U.S. (0.5–0.8 μg m−3 on an annual average) with a maximum influence in spring and over the NW. Of the major pollution regions, Europe has the largest potential to affect the surface aerosol concentrations in other continents due to its shorter distance from receptor continents and its larger fraction of sulfate-producing precursor gas in the outflow. With the IPCC emission scenario for the year 2000, we find that European emissions increase levels of ammonium sulfate by 1–5 μg m−3 over the surface of northern Africa and western Asia, and its contribution to eastern Asia (≥0.2 μg m−3) is twice as much as the Asian contribution to North America. Asia and North America pollution emissions exert strong impacts on their neighboring oceans, but their influences over other continents are relatively small (≤10%) due to long traveling distances across the oceans and efficient removal during transport. Among the major dust source regions, Asia displays a significant influence over large areas in the northern hemisphere except over the North Atlantic and the tropics, where African dust dominates. We also notice that the African dust and European pollution can travel eastward through a pathway spanning across Asia and North Pacific to western North America; such a pathway is difficult to detect because these aerosols usually merge and travel together with Asian dust and pollution labeled as "Asian outflow".

The Evolution of Dust in the Early Universe with Applications to the Galaxy SDSS J1148+5251

Dusty hyperluminous galaxies in the early universe provide unique environments for studying the role of massive stars in the formation and destruction of dust. At redshifts above ~6, when the universe was less than ~1 Gyr old, dust could have only condensed in the explosive ejecta of Type II supernovae (SNe), since most of the progenitors of the asymptotic giant branch stars, the major alternative source of interstellar dust, did not have time to evolve off the main sequence since the onset of star formation. In this paper we present analytical models for the evolution of the gas, dust, and metals in high-redshift galaxies, with a special application to SDSS J1148+5251 (hereafter J1148+5251), a hyperluminous quasar at z = 6.4. We find that an average SN must condense at least 1 M☉ of dust to account for the observed dust mass in J1148+5251. Observationally, it is in excess of the largest dust yield of 0.02 M☉ found thus far in the ejecta of any SN. If future observations find this to be a typical SN dust yield, then additional processes, such as accretion onto preexisting grains or condensation around the active galactic nucleus, will need to be invoked to account for the large amount of dust in this and similar objects. The galaxy's star formation history is still uncertain, and current observations of the gas, metal, and dust contents of J1148+5251 can be reproduced by either an intensive and short burst of star formation (ψ 103 M☉ yr-1) with a duration of 108 yr or a much lower star formation rate (ψ ≈ 100 M☉ yr-1) occurring over the lifetime of the galaxy.

Numerical simulation of the October 2002 dust event in Australia

In comparison to the major dust sources in the Northern Hemisphere, Australia is a relatively minor contributor to the global dust budget. However, severe dust storms do occur in Australia, especially in drought years. In this study, we simulate the 22–23 October 2002 dust storm using an integrated dust model, which is probably the most severe dust storm in Australia in at least the past 40 years. The model results are compared with synoptic visibility data and satellite images and for several stations, with high‐volume sampler measurements. The model simulations are then used to estimate dust load, emission, and deposition, both for over the continent and for over the ocean. The main dust sources and sinks are identified. Dust sources include the desert areas in northern South Australia, the grazing lands in western New South Wales (NSW), and the farm lands in NSW, Victoria, and Western Australia, as well as areas in Queensland and Northern Territory. The desert areas appear to be the strongest source. The maximum dust emission is around 2000 μg m−2 s−1, and the maximum net dust emission is around 500 μg m−2 s−1. The total amount of dust eroded from the Australian continent during this dust event is around 95.8 Mt, of which 93.67 Mt is deposited on the continent and 2.13 Mt in the ocean. The maximum total dust load over the simulation domain is around 5 Mt. The magnitude of this Australian dust storm corresponds to a northeast Asian dust storm of moderate size.

Soluble and insoluble lithium dust in the EPICA DomeC ice core—Implications for changes of the East Antarctic dust provenance during the recent glacial–interglacial transition

Continental dust impurities in Antarctic ice provide information on climate changes in the dust source areas and on past atmospheric circulation. We investigated records of dust species from the last 45 ka in the East Antarctic EPICA DomeC (EDC) ice core with special emphasis on the lithium (Li) content of dust. We obtained two complementary Li-records using a new Ion Chromatography (IC) technique in line with Inductively Coupled Plasma-Sector Field Mass Spectroscopy (ICP-SFMS). Concentrations of soluble Li (Li +) were obtained using IC, while total concentrations of Li (Li T ) were obtained using ICP-SFMS, providing an ideal opportunity to investigate the soluble and insoluble chemistry of Li in East Antarctic dust over the last glacial–interglacial transition. The records show that changes in the solubility of Li are associated with climatic changes. For the late glacial period and the Antarctic Cold Reversal (ACR) a large fraction, up to 75%, of the Li T content is present as insoluble minerals whereas for the Holocene period it seems that Li is present mainly as soluble salts (Li +). We compared the concentrations of Li + with the concentrations of Ca 2+ and the mass and size characteristics of the dust, which were obtained using Coulter Counting (CC). Furthermore we compared the concentrations of Li T with the concentrations of Ba T . Our analysis suggests that the changes in solubility of Li along the EDC ice core are related to changes in compositions of the dust minerals. During the late glacial period, changes in the dust composition is characteristic of variations in the strength of the atmospheric circulation, while changes over the last glacial–interglacial transition are indicative of a change in the major dust source areas. The dust characteristics for the glacial and the Holocene periods indicate two different dust types. The glacial dust type partly disappeared after the ACR, while the Holocene dust type appeared significantly after around 16 ka BP and became dominant after the ACR. The relative increase in the Holocene dust type at the glacial–interglacial transition could be due to changed conditions in the potential source area or to changed patterns of atmospheric circulation, resulting in enhanced transport from a source area that was different from the glacial source areas.

Natural Minerogenic Dust and Human Health

Natural Minerogenic Dust and Human Health

Dusty weather forecasts using the MesoNH mesoscale atmospheric model

Several studies have shown the importance of aerosols in the Earth's radiative balance. The radiative effect of dust has earlier been quantified in global and regional climate models. Fewer studies have included prediction of aerosols online in numerical weather prediction (NWP) models. Predicting climate effect of aerosols is different from including aerosols actively in weather prediction because climate models give average responses over long timescales, whereas the purpose of weather prediction models is to calculate the right weather on short timescales. In this paper, we run a mesoscale NWP with four different assumptions on dust aerosols: (1) no dust, (2) climatological data set for dust aerosols, (3) forecasted dust using the Dust Entrainment and Deposition model (DEAD) and (4) same as assumption 3 but with decreased single scattering albedo. We show that the assumption on dust is important for predicting ground temperature and convective precipitation in the model. Dust changes the model physics through changing the radiative fluxes. We interpret the changes through analyzing the energy budgets for four zones close to the major dust sources. The zones include both convective and nonconvective areas. In convective areas over ocean, dust can decrease convective activity. The vertical gradient of the aerosols and their single scattering albedo determine how efficient they are. Over land, dust also influences the surface energy budget so that decreased latent heat fluxes result if dust plumes pass over areas where water evaporates from the surface.

A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

We present a Spitzer Space Telescopespectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The stars represent the superwind phase on the Asymptotic Giant Branch, which forms a major source of dust for the interstellar medium in galaxies. The spectra cover the wavelength range 5‐38� m. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The r elations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-� m C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-� m absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitr ite dust. We investigate the strength and central wavelength of the SiC and MgS dust bands as function of colour and metallicity. The line-to-continuum ratio of these bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J K vs. K A colours, and show that these colours are relatively insensitive to chemical type. Metal -poor carbon stars form amorphous carbon dust from self-produced carbon. This type of dust forms more readily in the presence of a higher C/O ratio. Low metallicity carbon dust may contain a smaller fraction of SiC and MgS constituents, which do depend on metallicity. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compared to the Galaxy.

Control of dust emissions by geomorphic conditions, wind environments and land use in northern China: An examination based on dust storm frequency from 1960 to 2003

The degree to which dust emissions are controlled by geomorphic conditions, wind environments and land use was investigated using the dust storm frequency (DSF) and data from more than 300 meteorological stations throughout northern China. Our analysis showed that most dust emissions originated in gobi deserts that developed in piedmont alluvial fans of the Kunlun, Qilian and Helan mountains. Dust emissions are low from other gobi desert regions, such as the northern Gurbantunggut and eastern Taklimakan, where high vegetation coverage restrained dust emissions or where dust-size particles are not abundant after a long period of strong wind erosion. Sandy deserts with relatively high vegetation coverage or an extensive cover by mobile sands are not a major dust source. Although the highest dust emissions did not appear in regions with the highest wind energy, DSF trends in each region from 1960 to 2003 were closely related to local wind activity. DSF was low in regions with high levels of human activity, where the mean DSF from 1960 to 2003 did not exceed 4 days/year; even from the 1960s to the early 1970s, the period with the greatest DSF, frequency did not exceed 8 days/year, which indicates that extensive land use did not contribute to DSF. The low DSF in these areas might result from the fact that although land use could produce abundant fine soil fractions, vegetation coverage and soil moisture remained higher than in the gobi deserts of arid China, thereby decreasing dust-storm occurrence.

Validating a dust production model by field experiment in Mu Us Desert, China

Field experiments of dust production have been seldom performed in Chinese deserts that are identified as one of main dust sources in the world, and such an experiment to validate a dust production model (DPM) has not been made in China until now. Saltation flux, dust emission flux, surface features and meteorological parameters (U*, Z0, Ri, etc.) were investigated in Mu Us Desert of China to verify the DPM model and accumulate dust emission data during the spring of 2002. It indicates that observed saltation fluxes from 0.07 to 8.00 g·m−1·s−1 are in good agreement with the ones predicted by the DPM model when the constant of the saltation flux equation is tuned to about 2.61, which corresponds to wind friction velocities from 0.26 to 0.35 m·s−1. Unfortunately, during three local dust emission events, the observed dust fluxes are from 1 to 3 µg·m−2·s−1, lower than the modeled ones, implying that the model needs to be improved further under lower wind velocity. In comparison with data from a sandy soil with physical crust and a loam soil, saltation fluxes of the loose sandy soil in Mu Us Desert are obviously higher, which suggests that deserts and sandy land with desertification are major dust sources in northern China.

Global Model Simulations of the Transport of Asian and Sahara Dust: Total Deposition of Dust Mass in Japan

We simulated the global transport of dust emitted from all major dust source regions on earth by using a global three-dimensional chemical transport model for the year 2000. A modified dust emission flux scheme and an advanced nonlocal diffusion scheme for accurately determining the atmospheric eddy diffusivity in the atmospheric boundary layer were implemented to improve the chemical transport model. We specifically examined and analyzed dust storms that occurred in China and northern Africa. Four scenarios representing the global transport of dust from dust storms originating in the Loess region and the Gobi, Takla Makan, and Sahara deserts, respectively, were simulated. The results of each scenario were then compared with those resulting from all major dust source regions on earth, to identify which deserts have a major impact on dust transport and its deposition in Japan. The dust from the Sahara desert, most from the western part that was found in this study, reached Japan in 9–10 days, while Gobi and Loess region dust took 2 to 3 days, and 4 days for Takla Makan dust. The model-calculated monthly total deposition of dust mass from all major dust source regions was compared with comprehensive measurements of total deposition collected at sampling stations distributed over Japan, and good agreement was generally found. From these analyses, it was also indicated that dust emitted from other deserts, including the Turkestan, Iranian, and Arabian, could have a significant impact on dust deposition on Japan.

Dust formation in Electric Arc Furnace: Birth of the particles

The characterization of Electric Arc Furnace (EAF) dust shows that bubble burst at the liquid steel surface is the principal source of dust emission. We have therefore developed an experimental device for studying this phenomenon. As in the case of the air–water system, the bubble-burst gives birth to two types of droplets: film drops and jet drops. The jet drop formation is observed with high-speed video. The film drop aerosol is collected on filters and then characterized by means of SEM, granulometric and gravimetric analyses. Results are presented and discussed. The quantification of both types of projections leads to the conclusion that the film drop projections represent the major source of dust. The amount of film drops greatly decreases with the parent bubble size. Under 4.5 mm in bubble diameter, no film drops are formed. Decreasing enough the bubble size would therefore represent an effective solution for reducing drastically the EAF dust emission.

Regional contrasts in dust emission responses to climate

Time‐series analysis of Earth's major dust source regions reveals common traits in responses of wind erosion to climate anomalies. Lag cross‐correlations of monthly mean aerosol optical depth, precipitation, vegetation, and wind speed are examined from 1979–1993. The response to monthly climate anomalies can differ greatly from the response to seasonal mean climate. The signs, magnitudes, and lags of highly significant (p < 0.01) correlations show that 14 important mineral dust source areas characterized by Prospero et al. (2002) fall into four response categories. Each category represents distinct mechanisms by which climate anomalies influence subsequent atmospheric dust loading on seasonal to interannual timescales. In most regions, precipitation and vegetation together strongly constrain dust anomalies on multiple timescales. In these regions, dry anomalies increase, and wet anomalies reduce, dust emission. Interestingly, in many other regions the contrary is true: Dust and precipitation anomalies correlate positively, consistent with sediment‐supply factors. The response timescales are consistent with loss of surface crusts (less than 1 month) and with alluvial transport and dessication (interannual lags). Supply‐limited dust emission appears more prevalent than previously thought and is not accounted for in models. Reproducing these wind erodibility responses in models may help remediate underprediction of observed seasonal to interannual dust variability.

Modelling the large particle environment of comet 67P/Churyumov–Gerasimenko

A method is presented to simulate images of a cometary dust trail in consistency with observed gas and dust production rates. It is also applicable to dust tails including neckline and antitail phenomena. Our main objective is to analyse the amount of mm-sized and larger particles within a given image. These are interesting to us because their emission by comets may constitute a major source of interplanetary dust. The model is applied to 67P/Churyumov–Gerasimenko, the target of ESA’s Rosetta mission. The employed parameters are consistent with a hydrodynamic coma model. Dust emitted during all apparitions of 67P since the Jupiter encounter in 1959 is taken into account. We compare simulated images with two images taken in May 2003 and April 2004. Both of these show a bright feature close to the projected orbit of the comet. We analyse the simulated images with respect to the age of the dust in this feature. We find that, within our model, the feature in 2003 is dominated by particles emitted during the same apparition. For 2004 we estimate that 25% of the intensity in the bright feature can be attributed to dust emitted during previous apparitions and 29% to particles emitted at true anomalies of (180 ± 5)° before observation. Concerning the sizes of dust grains in the bright feature, we find that in 2003, particles with radii between about 10 μm and 1 cm contribute nearly equally to the total intensity. In 2004 however, the structure is dominated by particles having sizes of 100 μm to 1 mm.

Provenance of long‐travelled dust determined with ultra‐trace‐element composition: a pilot study with samples from New Zealand glaciers

AbstractWe report high‐precision inductively coupled plasma mass spectrometric (ICP‐MS) compositional data for 39 trace elements in a variety of dust deposits, trapped sediments and surface samples from New Zealand and Australia. Dusts collected from the surface of alpine glaciers in the Southern Alps, New Zealand, believed to have undergone long‐distance atmospheric transport from Australia, are recognizable on account of their overabundances of Pb and Cu with respect to typical upper crustal values. Long‐travelled dust from Australia therefore scavenges these and other metals (e.g. Zn, Sb and Cd) from the atmosphere during transport and deposition. Hence, due to anthropogenic pollution, long‐travelled Australian dusts can be recognized by elevated metal contents.The relative abundance of 25 other elements that are not affected by atmospheric pollution, mineral sorting (Zr and Hf) and weathering[sol ]solubility (alkali and earth alkali elements) reflects the geochemistry of the dust source sediment. As a result, we are able to establish the provenance of dust using ultra‐trace‐element chemistry at regional scale. Comparison of long‐travelled dust chemistry with potential Australian sources shows that fits of variable quality are obtained. We propose that the best fitting potential source chemistry most likely represents the major dust source area. A binary mixing model is used to demonstrate that admixture of small quantities of local dust provides an even better fitting dust chemistry for the long‐travelled dusts. Copyright © 2005 John Wiley & Sons, Ltd.

Columnar aerosol optical properties at AERONET sites in central eastern Asia and aerosol transport to the tropical mid‐Pacific

The column‐integrated optical properties of aerosol in the central eastern region of Asia and midtropical Pacific were investigated based on Sun/sky radiometer measurements made at Aerosol Robotic Network (AERONET) sites in these regions. Characterization of aerosol properties in the Asian region is important due to the rapid growth of both population and economic activity, with associated increases in fossil fuel combustion, and the possible regional and global climatic impacts of related aerosol emissions. Multiyear monitoring over the complete annual cycle at sites in China, Mongolia, South Korea, and Japan suggest spring and/or summer maximum in aerosol optical depth (τa) and a winter minimum; however, more monitoring is needed to establish accurate climatologies. The annual cycle of Angstrom wavelength exponent (α) showed a springtime minimum associated with dust storm activity; however, the monthly mean α440–870 was >0.8 even for the peak dust season at eastern Asian sites suggesting that fine mode pollution aerosol emitted from population centers in eastern Asia dominates the monthly aerosol optical influence even in spring as pollution aerosol mixes with coarse mode dust originating in western source regions. Aerosol optical depth peaks in spring in the tropical mid‐Pacific Ocean associated with seasonal shifts in atmospheric transport from Asia, and ∼35% of the springtime τa500 enhancement occurs at altitudes above 3.4 km. For predominately fine mode aerosol pollution cases, the average midvisible (∼550 nm) single scattering albedo (ω0) at two continental urban sites in China averaged ∼0.89, while it was significantly higher, ∼0.93, at two relatively rural coastal sites in South Korea and Japan. Differences in fine mode absorption between these regions may result from a combination of factors including aerosol aging during transport, relative humidity differences, sea salt at coastal sites, and fuel type and combustion differences in the two regions. For cases where τa was predominately coarse mode dust aerosol in the spring of 2001, the absorption was greater in eastern Asia compared to the source regions, with ω0 at Dunhuang, China (near to the major Taklamakan dust source), ∼0.04 higher than at Beijing at all wavelengths, and Anmyon, South Korea, showing an intermediate level of absorption. Possible reasons for differences in dust absorption magnitude include interactions between dust and fine mode pollution aerosol and also variability of dust optical properties from different source regions in China and Mongolia.

Temporal variability of dust mobilization and concentration in source regions

The time‐varying properties of dust mobilization and concentration variability in source areas are studied in model simulations to understand the processes that control entrainment of dust into the atmosphere. Two different meteorological reanalysis data sets (National Centers for Environmental Prediction/National Center for Atmospheric Research and NASA Data Assimilation Office) and two different source parameterizations are used for the analysis, which is done for six different main dust source regions. The results show that ∼35–70% variance of dust mobilization is associated with diurnal variability of dust mobilization in the major dust sources regions, and this is similar using different meteorological analyses and source parameterizations. Synoptic‐scale variability is responsible for 6–50% of the dust mobilization, with more variation between different meteorology and parameterizations. The variability of dust concentrations in source regions is very sensitive to source meteorology and parameterization, with the synoptic variability responsible for 30–50% of the variability in some configurations or 6–40% in other configurations. Diurnal variability in concentrations varies from 20–50 to 25–80% depending on the model configuration. Analyses suggest that dust mobilization and dust concentration are more influenced by synoptic variability in Australia and east Asia than in North Africa and Arabian dust source regions, consistent with observations of cold fronts and cyclone movements systems driving dust mobilization and transport in Australia and east Asia source regions. Differences in variability between different meteorological data sets and source parameterizations suggest that possible dust feedbacks onto the dust cycle may be sensitive to source parameterization and model.

Characterization of MASDs of surface soils in north China and its influence on estimating dust emission

The micro-aggregated size distribution (MASD) of surface soil is an important parameter for modelling dust emission. However, there is no dataset of MASDs of all surface soil types in north China. The MASDs are here presented, measured by dry sieving, for typical surface soil samples, including sandy soil, gravelly sand soil, gravelly loam soil, loam soil and silt loam soil, collected from sandy deserts, Gobi deserts, oases, farmlands in steppe regions and steppe areas in north China. The MASDs of various surface soil types exhibit a combination of several log-normal distributions of five separated sizes with mean mass median diameters (MMDs) of 90, 210, 390, 600 and 980 μm, respectively, and mean standard deviations (SDs) of 1.25, 1.40, 1.25, 1.35 and 1.25 respectively. The log-normal distributions correspond to very fine sand, fine sand, medium sand, coarse sand and very coarse sand population. On the basis of characterization of the retrieved MASDs of various surface soil types in north China, dust emission fluxes are modelled by a dust production model (DPM model). It is shown that dust emission has been significantly influenced by MASDs. Fine sand and very fine sand are always associated with the highest dust emission fluxes. Emission fluxes of the medium sand, gravelly sand soil, gravelly loam soil and loam soil are lower than those of very fine sand and fine sand, but larger than those of the coarse sand. The differences in dust emission fluxes vary among the different soil types from 101 to 103 (μg · m−2 · s−1. Dust emission fluxes from sandy deserts and farmlands covered with sand sheets in north China rang from 101 to 104 μg · m−2 · s−1 while those from Gobi deserts, farmlands and steppes with gravelly desertification range from 101 to 102 μm · m−2 · s−1. The modelled results indicate that deserts and farmlands with sand are the major dust sources in north China.