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Mineral dust is the most abundant aerosol species by mass in the atmosphere, and it impacts global climate, biogeochemistry, and human health. Understanding these varied impacts on the Earth system requires accurate knowledge of dust abundance, size, and optical properties, and how they vary in space and time. However, current global models show substantial biases against measurements of these dust properties. For instance, recent studies suggest that atmospheric dust is substantially coarser and more aspherical than accounted for in models, leading to persistent biases in modelled impacts of dust on the Earth system. Here, we facilitate more accurate constraints on dust impacts by developing a new dataset: Dust Constraints from joint Observational-Modelling-experiMental analysis (DustCOMM). This dataset combines an ensemble of global model simulations with observational and experimental constraints on dust size distribution and shape to obtain more accurate constraints on three-dimensional (3-D) atmospheric dust properties than is possible from global model simulations alone. Specifically, we present annual and seasonal climatologies of the 3-D dust size distribution, 3-D dust mass extinction efficiency at 550 nm, and two-dimensional (2-D) atmospheric dust loading. Comparisons with independent measurements taken over several locations, heights, and seasons show that DustCOMM estimates consistently outperform conventional global model simulations. In particular, DustCOMM achieves a substantial reduction in the bias relative to measured dust size distributions in the 0.5–20 µm diameter range. Furthermore, DustCOMM reproduces measurements of dust mass extinction efficiency to almost within the experimental uncertainties, whereas global models generally overestimate the mass extinction efficiency. DustCOMM thus provides more accurate constraints on 3-D dust properties, and as such can be used to improve global models or serve as an alternative to global model simulations in constraining dust impacts on the Earth system.

The presence of coarse mineral dust in the atmosphere has been substantiated in several recent measurement campaigns, which include observations of particles up to and above 100μm in diameter. Yet, atmospheric dust models either do not include particles larger than 20μm or severely underestimate their concentrations. One possibility for the underestimated concentrations is that models do not represent enhancements of particle transport due to subgrid-scale topography. Here, large-eddy simulations are used in combination with Lagrangian particle tracking to assess the impact of gentle two-dimensional topography with 50 and 100m elevation on the vertical transport of coarse dust in neutrally stratified conditions. The presence of topography significantly increases the likelihood that 5 and 20μm particles reach several hundred meters in altitude. Further, topography increases this likelihood by orders of magnitude for larger 60μm particles. Three mechanisms are observed to contribute to the increased vertical transport: a strong upward mean flow region on the uphill side of the topography, ejection of particles downwind of the topography crest, and enhanced vertical dispersion in the wake of the crest. The compounding effects of these mechanisms provide a pathway for coarse dust emitted from the surface to reach elevations where they can be further transported into the free atmosphere by large-scale motions such as convective plumes. While these findings are motivated by mineral dust observations, they are generally applicable to other heavy aerosols such as pollen.

Quantifying the dust optical depth (DOD) and its uncertainty across spatiotemporal scales is key to understanding and constraining the dust cycle and its interactions with the Earth System. This study quantifies the DOD along with its monthly and year-to-year variability between 2003 and 2017 at global and regional levels based on the MIDAS (ModIs Dust AeroSol) dataset, which combines MODIS-Aqua retrievals and MERRA-2 reanalysis products. We also describe the annual and seasonal geographical distributions of DOD across the main dust source regions and transport pathways. MIDAS provides columnar mid-visible (550 nm) DOD at fine spatial resolution (0.1° × 0.1°), expanding the current observational capabilities for monitoring the highly variable spatiotemporal features of the dust burden. We obtain a global DOD of 0.032 ± 0.003 – approximately a quarter (23.4 % ± 2.4 %) of the global AOD – with about one order of magnitude more DOD in the northern hemisphere (0.056 ± 0.004; 31.8 % ± 2.7 %) than in the southern hemisphere (0.008 ± 0.001; 8.2 % ± 1.1 %) and about 3.5 times more DOD over land (0.070 ± 0.005) than over ocean (0.019 ± 0.002). The northern hemisphere monthly DOD is highly correlated with the corresponding monthly AOD (R2 = 0.94) and contributes 20 % to 48 % of it, both indicating a dominant dust contribution. In contrast, the contribution of dust to the monthly AOD does not exceed 17 % in the southern hemisphere, although the uncertainty in this region is larger. Among the major dust sources of the planet, the maximum DODs (~1.2) are recorded in the Bodélé Depression of the northern Lake Chad Basin, whereas moderate-to-high intensities are encountered in the Western Sahara (boreal summer), along the eastern parts of the Middle East (boreal summer) and in the Taklamakan Desert (spring). Over oceans, major long-range dust transport is observed primarily along the Tropical Atlantic (intensified during boreal summer) and secondarily in the North Pacific (intensified during boreal spring). Our calculated global and regional averages and associated uncertainties are consistent with some but not all recent observationally based studies. Our work provides a simple, yet flexible method to estimate consistent uncertainties across spatiotemporal scales, which will enhance the use of the MIDAS dataset in future studies.

Sand dunes and other active sands generally have a low content of fine grains and, therefore, are not considered to be major dust sources in current climate models. However, recent remote sensing studies have indicated that a surprisingly large fraction of dust storms are generated from regions covered by sand dunes, leading these studies to propose that sand dunes might be globally relevant sources of dust. To help understand dust emissions from sand dunes and other active sands, we present in situ field measurements of dust emission under natural saltation from a coastal sand sheet at Oceano Dunes in California. We find that saltation drives dust emissions from this setting that are on the low end of the range in emissions produced by non-sandy soils for similar wind speed. Laboratory analyses of sand samples suggest that these emissions are produced by aeolian abrasion of feldspars and removal of clay-mineral coatings on sand grain surfaces. We further find that this emitted dust is substantially finer than dust emitted from non-sandy soils, which could enhance its downwind impacts on human health, the hydrological cycle, and climate.

Mineral dust aerosols cool and warm the atmosphere by scattering and absorbing both solar (short-wave: SW) and thermal (long-wave: LW) radiation. However, large uncertainties remain in dust radiative effects, largely due to differences in the dust size distribution and optical properties simulated in Earth system models. Here, we improve the simulated dust properties with datasets that leverage measurements of size-resolved dust concentration and asphericity factor (improved simulation) in a coupled global chemical transport model (IMPACT) with a radiative transfer module (RRTMG) (default simulation). The global and annual average of dust aerosol optical depth at 550 nm (DAOD550) from the improved simulation (0.029) falls within the range of a semi-observation-based estimate (0.030 ± 0.005), in contrast to that (0.023) of the default simulation. Improved agreement against semi-observation-based estimate of the radiative effect efficiency was obtained using less absorptive SW and more absorptive LW dust refractive indices. Our sensitivity simulations reveal that the improved simulation leads to a similar net global dust radiative effect at the Top Of Atmosphere (TOA) on a global scale to the default simulation (−0.08 vs. −0.09 W ·m−2) but results in less cooling at the surface (−0.23 vs. −0.88 W ·m−2), because of enhanced LW warming by coarser aspherical dust. Our results thus suggest less atmospheric radiative heating due to aspherical dust with coarser size over the major source regions (0.15 vs. 0.79 W ·m−2 on a global scale).

A key challenge in accurate simulations of desert dust emission is the parameterization of the threshold wind speed above which dust emission occurs. However, the existing parameterizations yield a...

Even though desert dust is the most abundant aerosol by mass in Earth's atmosphere, the relative contributions of the world's major source regions to the global dust cycle remain poorly constrained. This problem hinders accounting for the potentially large impact of regional differences in dust properties on clouds, the Earth's energy balance, and terrestrial and marine biogeochemical cycles. Here, we constrain the contribution of each of the world's main dust source regions to the global dust cycle. We use an analytical framework that integrates an ensemble of global aerosol model simulations with observationally informed constraints on the dust size distribution, extinction efficiency, and regional dust aerosol optical depth (DAOD). We obtain a dataset that constrains the relative contribution of nine major source regions to size-resolved dust emission, atmospheric loading, DAOD, concentration, and deposition flux. We find that the 22–29 Tg (1 standard error range) global loading of dust with a geometric diameter up to 20 µm is partitioned as follows: North African source regions contribute ∼ 50 % (11–15 Tg), Asian source regions contribute ∼ 40 % (8–13 Tg), and North American and Southern Hemisphere regions contribute ∼ 10 % (1.8–3.2 Tg). These results suggest that current models on average overestimate the contribution of North African sources to atmospheric dust loading at ∼ 65 %, while underestimating the contribution of Asian dust at ∼ 30 %. Our results further show that each source region's dust loading peaks in local spring and summer, which is partially driven by increased dust lifetime in those seasons. We also quantify the dust deposition flux to the Amazon rainforest to be ∼ 10 Tg yr−1, which is a factor of 2–3 less than inferred from satellite data by previous work that likely overestimated dust deposition by underestimating the dust mass extinction efficiency. The data obtained in this paper can be used to obtain improved constraints on dust impacts on clouds, climate, biogeochemical cycles, and other parts of the Earth system.

Atmosphere has four times more coarse dust than in climate models, resulting in more radiative warming than previously estimated., Coarse mineral dust (diameter, ≥5 μm) is an important component of the Earth system that affects clouds, ocean ecosystems, and climate. Despite their significance, climate models consistently underestimate the amount of coarse dust in the atmosphere when compared to measurements. Here, we estimate the global load of coarse dust using a framework that leverages dozens of measurements of atmospheric dust size distributions. We find that the atmosphere contains 17 Tg of coarse dust, which is four times more than current climate models simulate. Our findings indicate that models deposit coarse dust out of the atmosphere too quickly. Accounting for this missing coarse dust adds a warming effect of 0.15 W·m−2 and increases the likelihood that dust net warms the climate system. We conclude that to properly represent the impact of dust on the Earth system, climate models must include an accurate treatment of coarse dust in the atmosphere.

The wind-driven saltation of sediments, such as snow and sand, is responsible for a wide range of geophysical processes. Blowing-snow, in particular, affects snow surface properties and drives snow redistribution in alpine terrain. As such, it is of fundamental importance for avalanche mechanics. One of the most important controls on initiation and development of snow saltation is the surface cohesion induced by ice particle sintering. Although inter-particle cohesion is known to limit the number of grains lifted from the surface through aerodynamic entrainment and granular splash, the role of cohesion in the development of saltation from onset to steady state is still poorly understood. Using a numerical model based on the discrete element method, we show that saltation over cohesive beds sustains itself at wind speeds one order of magnitude smaller than those necessary to initiate it, giving rise to hysteresis in which the occurrence of transport depends on the history of the wind. Our results further suggest that saltation over cohesive beds requires much longer distances to saturate, thereby increasing the size of the smallest stable bed forms.

This MATLAB scripts is used to plot Fig. 4 of Huang et al. (2020), in review. This scripts first uses Monte-Carlo method to sample a large number (10^8) of volume-equivalent dust particles from the globally-averaged shape distributions of AR and HWR (compiled in Supplementary Tables S2 and S3 of Huang et al., 2020). Second, for each of these generated particles, we used Eqs. (5) - (7) and Eq. (4), respectively, to obtain its the gravitational settling speed and that of its volume-equivalent spherical dust. Finally, for each dust particle, we calculated the reduction in terminal velocity and the enhancement of lifetime due to dust asphericity. Contact Yue Huang (hyue4@ucla.edu) if you are interested in our research!

Atmospheric deposition of dust aerosols is a significant source of exogenous iron (Fe) in marine ecosystems and is critical in setting primary marine productivity during summer. This dust-borne input of Fe is particularly important to the Southern Ocean, which is arguably the most biogeochemically important ocean because of its large spatial extent and its considerable influence on the global carbon cycle. However, there is large uncertainty in estimates of dust emissions in the Southern Hemisphere and thus of the deposition of Fe-containing aerosols onto oceans. Here we hypothesize that sparsely vegetated surfaces in arid and semiarid regions are important sources of Fe-containing aerosols to the Southern Ocean. We test this hypothesis using an improved dust emission scheme in conjunction with satellite products of vegetation cover and soil moisture in an atmospheric chemistry transport model. Our improved model shows a twofold increase of Fe input into the Southern Ocean in austral summer with respect to spring and estimates that the Fe input is more than double that simulated using a conventional dust emission scheme in summer. Our model results suggest that dust emissions from open shrublands contribute over 90% of total Fe deposition into the Southern Ocean. These findings have important implications for the projection of the Southern Ocean's carbon uptake.

Air pollution creates high Fe solubility in pyrogenic aerosols, raising the flux of biologically essential Fe to the oceans., Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity.

Desert dust aerosols affect Earth's global energy balance through direct interactions with radiation, and through indirect interactions with clouds and ecosystems. But the magnitudes of these effects are so uncertain that it remains unclear whether atmospheric dust has a net warming or cooling effect on global climate. Consequently, it is still uncertain whether large changes in atmospheric dust loading over the past century have slowed or accelerated anthropogenic climate change, or what the effects of potential future changes in dust loading will be. Here we present an analysis of the size and abundance of dust aerosols to constrain the direct radiative effect of dust. Using observational data on dust abundance, in situ measurements of dust optical properties and size distribution, and climate and atmospheric chemical transport model simulations of dust lifetime, we find that the dust found in the atmosphere is substantially coarser than represented in current global climate models. Since coarse dust warms climate, the global dust direct radiative effect is likely to be less cooling than the ~-0.4 W/m2 estimated by models in a current global aerosol model ensemble. Instead, we constrain the dust direct radiative effect to a range between -0.48 and +0.20 W/m2, which includes the possibility that dust causes a net warming of the planet., Comment: Published as article in Nature Geoscience

The global cycle of desert dust aerosols responds strongly to climate and human perturbations, and, in turn, impacts climate and biogeochemistry. Here we focus on desert dust size distributions, how these are characterized, emitted from the surface, evolve in the atmosphere, and impact climate and biogeochemistry. Observations, theory and global model results are synthesized to highlight the evolution and impact of dust sizes. Individual particles sizes are, to a large extent, set by the soil properties and the mobilization process. The lifetime of different particle sizes controls the evolution of the size distribution as the particles move downwind, as larger particles fall out more quickly. The dust size distribution strongly controls the radiative impact of the aerosols, as well as their interactions with clouds. The size of particles controls how far downwind they travel, and thus their ability to impact biogeochemistry downwind of the source region. © 2013 The Authors.

Predicting aeolian sediment transport is a long-standing and difficult challenge that is important to a variety of scientific disciplines, including geology, geomorphology, agriculture, meteorology, and climatology. Here, we argue that improvements in predictions of aeolian sediment transport are limited by incompatibilities between empirical measurements and mathematical models. We focus on the spatial and temporal variability in transport. Measurements indicate considerable variability on small time (second) and length (meter) scales, yet models are almost ubiquitously based on assumptions of time and space-invariant transport. Mismatches between measurements and models limit summative predictive capacity by reducing the ability to use measured data to test and drive models. We suggest: (i) revising model conceptualizations and evaluating the representativeness of steady state saltation to constrain the realism of existing models, (ii) improving and optimizing measurement technology to produce more reliable and accurate measurements, (iii) explicitly specifying the scale of measurements, and (iv) designing variable matching tests between models and measurements to work around measurement limitations. Continuing with the status quo, where measurements and models are dealt with separately, is likely to erode summative predictive capacity.

[1] To improve estimates of remote contributions of dust to fine particulate matter (PM2.5) in the western United States, new dust particle size distributions (PSDs) based upon scale-invariant fragmentation theory (Kok_PSD) with constraints from in situ measurements (IMP_PSD) are implemented in a chemical transport model (GEOS-Chem). Compared to initial simulations, this leads to reductions in the mass of emitted dust particles with radii

Aeolian transport of sand and dust is driven by turbulent winds that fluctuate over a broad range of temporal and spatial scales. However, commonly used aeolian transport models do not explicitly account for such fluctuations, likely contributing to substantial discrepancies between models and measurements. Underlying this problem is the absence of accurate sand flux measurements at the short time scales at which wind speed fluctuates. Here, we draw on extensive field measurements of aeolian saltation to develop a methodology for generating high-frequency (25 Hz) time series of total (vertically-integrated) saltation flux, namely by calibrating high-frequency (HF) particle counts to low-frequency (LF) flux measurements. The methodology follows four steps: (1) fit exponential curves to vertical profiles of saltation flux from LF saltation traps, (2) determine empirical calibration factors through comparison of LF exponential fits to HF number counts over concurrent time intervals, (3) apply these calibration factors to subsamples of the saltation count time series to obtain HF height-specific saltation fluxes, and (4) aggregate the calibrated HF height-specific saltation fluxes into estimates of total saltation fluxes. When coupled to high-frequency measurements of wind velocity, this methodology offers new opportunities for understanding how aeolian saltation dynamics respond to variability in driving winds over time scales from tens of milliseconds to days., 61 pages, 14 main text figures, 4 supporting figures, 2 tables

The present study investigates the dust emission and load over the Mediterranean basin using the coupled chemistry–aerosol–regional climate model RegCM-4. The first step of this work focuses on dust particle emission size distribution modeling. We compare a parameterization in which the emission is based on the individual kinetic energy of the aggregates striking the surface to a recent parameterization based on an analogy with the fragmentation of brittle materials. The main difference between the two dust schemes concerns the mass proportion of fine aerosol that is reduced in the case of the new dust parameterization, with consequences for optical properties. At the episodic scale, comparisons between RegCM-4 simulations, satellite and ground-based data show a clear improvement using the new dust distribution in terms of aerosol optical depth (AOD) values and geographic gradients. These results are confirmed at the seasonal scale for the investigated year 2008. This change of dust distribution has sensitive impacts on the simulated regional dust budget, notably dry dust deposition and the regional direct aerosol radiative forcing over the Mediterranean basin. In particular, we find that the new size distribution produces a higher dust deposition flux, and smaller top of atmosphere (TOA) dust radiative cooling. A multi-annual simulation is finally carried out using the new dust distribution over the period 2000–2009. The average SW radiative forcing over the Mediterranean Sea reaches −13.6 W m−2 at the surface, and −5.5 W m−2 at TOA. The LW radiative forcing is positive over the basin: 1.7 W m−2 on average over the Mediterranean Sea at the surface, and 0.6 W m−2 at TOA.

We review electrical activity in blowing sand and dusty phenomena on Earth, Mars, the Moon, and asteroids. On Earth and Mars, blowing sand and dusty phenomena such as dust devils and dust storms are important geological processes and the primary sources of atmospheric dust. Large electric fields have been measured in terrestrial dusty phenomena and are predicted to occur on Mars. We review the charging mechanisms that produce these electric fields and discuss the implications of electrical activity to dust lifting and atmospheric chemistry. In addition, we review theoretical ideas about electric discharges on Mars. Finally, we discuss the evidence that electrostatics is responsible for dust transport on the Moon and asteroids.

© Author(s) 2015. CC Attribution 3.0 License. Trace element deposition from desert dust has important impacts on ocean primary productivity, the quantification of which could be useful in determining the magnitude and sign of the biogeochemical feedback on radiative forcing. However, the impact of elemental deposition to remote ocean regions is not well understood and is not currently included in global climate models. In this study, emission inventories for eight elements primarily of soil origin, Mg, P, Ca, Mn, Fe, K, Al, and Si are determined based on a global mineral data set and a soil data set. The resulting elemental fractions are used to drive the desert dust model in the Community Earth System Model (CESM) in order to simulate the elemental concentrations of atmospheric dust. Spatial variability of mineral dust elemental fractions is evident on a global scale, particularly for Ca. Simulations of global variations in the Ca / Al ratio, which typically range from around 0.1 to 5.0 in soils, are consistent with observations, suggesting that this ratio is a good signature for dust source regions. The simulated variable fractions of chemical elements are sufficiently different; estimates of deposition should include elemental variations, especially for Ca, Al and Fe. The model results have been evaluated with observations of elemental aerosol concentrations from desert regions and dust events in non-dust regions, providing insights into uncertainties in the modeling approach. The ratios between modeled and observed elemental fractions range from 0.7 to 1.6, except for Mg and Mn (3.4 and 3.5, respectively). Using the soil database improves the correspondence of the spatial heterogeneity in the modeling of several elements (Ca, Al and Fe) compared to observations. Total and soluble dust element fluxes to different ocean basins and ice sheet regions have been estimated, based on the model results. The annual inputs of soluble Mg, P, Ca, Mn, Fe and K associated with dust using the mineral data set are 0.30 Tg, 16.89 Gg, 1.32 Tg, 22.84 Gg, 0.068 Tg, and 0.15 Tg to global oceans and ice sheets.

© 2014. The Authors. Aerosol-climate interactions constitute one of the major sources of uncertainty in assessing changes in aerosol forcing in the anthropocene as well as understanding glacial-interglacial cycles. Here we focus on improving the representation of mineral dust in the Community Atmosphere Model and assessing the impacts of the improvements in terms of direct effects on the radiative balance of the atmosphere. We simulated the dust cycle using different parameterization sets for dust emission, size distribution, and optical properties. Comparing the results of these simulations with observations of concentration, deposition, and aerosol optical depth allows us to refine the representation of the dust cycle and its climate impacts. We propose a tuning method for dust parameterizations to allow the dust module to work across the wide variety of parameter settings which can be used within the Community Atmosphere Model. Our results include a better representation of the dust cycle, most notably for the improved size distribution. The estimated net top of atmosphere direct dust radiative forcing is -0.23 ± 0.14 W/m2for present day and -0.32 ± 0.20 W/m2at the Last Glacial Maximum. From our study and sensitivity tests, we also derive some general relevant findings, supporting the concept that the magnitude of the modeled dust cycle is sensitive to the observational data sets and size distribution chosen to constrain the model as well as the meteorological forcing data, even within the same modeling framework, and that the direct radiative forcing of dust is strongly sensitive to the optical properties and size distribution used.

Simulations of the global dust cycle and its interactions with a changing Earth system are hindered by the empirical nature of dust emission parameterizations in climate models. Here we take a step towards improving global dust cycle simulations by presenting a physically-based dust emission model. The resulting dust flux parameterization depends only on the wind friction speed and the soil's threshold friction speed, and can therefore be readily implemented into climate models. We show that our parameterization's functional form is supported by a compilation of quality-controlled vertical dust flux measurements, and that it better reproduces these measurements than existing parameterizations. Both our theory and measurements indicate that many climate models underestimate the dust flux's sensitivity to soil erodibility. This finding can explain why dust cycle simulations in many models are improved by using an empirical preferential sources function that shifts dust emissions towards the most erodible regions. In fact, implementing our parameterization in a climate model produces even better agreement against aerosol optical depth measurements than simulations that use such a source function. These results indicate that the need to use a source function is at least partially eliminated by the additional physics accounted for by our parameterization. Since soil erodibility is affected by climate changes, our results further suggest that many models have underestimated the climate sensitivity of the global dust cycle.

The transport of sediment by a fluid along the surface is responsible for dune formation, dust entrainment, and a rich diversity of patterns on the bottom of oceans, rivers, and planetary surfaces. Most previous models of sediment transport have focused on the equilibrium (or saturated) particle flux. However, the morphodynamics of sediment landscapes emerging due to surface transport of sediment is controlled by situations out of equilibrium. In particular, it is controlled by the saturation length characterizing the distance it takes for the particle flux to reach a new equilibrium after a change in flow conditions. The saturation of mass density of particles entrained into transport and the relaxation of particle and fluid velocities constitute the main relevant relaxation mechanisms leading to saturation of the sediment flux. Here we present a theoretical model for sediment transport which, for the first time, accounts for both these relaxation mechanisms and for the different types of sediment entrainment prevailing under different environmental conditions. Our analytical treatment allows us to derive a closed expression for the saturation length of sediment flux, which is general and thus can be applied under different physical conditions. © 2014 American Physical Society.

[1] Global models have been used to deduce atmospheric iron supply to the ocean, but the uncertainty remains large. We used a global chemical transport model to investigate the effect of the estimated size distribution of dust on the bioavailable iron deposition. Simulations are performed with six different size distributions for dust aerosols at emission using similar aerosol optical depths (AODs) to constrain the total emission flux of dust. The global dust emission rate using a recent theoretical estimate for the dust size distribution at emission (2116 Tg yr−1) is about two times larger than the average of estimates using the other four empirical size distributions (1089 ± 469 Tg yr−1). In contrast to the large differences in total emissions, the emission of fine dust (diameter < 2.5 μm) is relatively robust (176 ± 34 Tg yr−1), due to the strong constraint of AOD on fine dust emission. Our model results indicate that soluble iron (SFe) deposition is relatively invariant to the dust size distribution at emission in regions where most soluble iron is provided by acid mobilization of fine dust. In contrast, the use of the theoretical size distribution suggests a larger deposition of SFe (by a factor of 1.2 to 5) in regions where the concentration of acidic gases is insufficient to promote iron dissolution in dust particles, such as the South Atlantic. These results could have important implications for the projection of marine ecosystem feedbacks to climate change and highlight the necessity to improve the dust size distribution.

The size distribution of mineral dust aerosols partially determines their interactions with clouds, radiation, ecosystems, and other components of the Earth system. Several theoretical models predict that the dust size distribution depends on the wind speed at emission, with larger wind speeds predicted to produce smaller aerosols. The present study investigates this prediction using a compilation of published measurements of the size-resolved vertical dust flux emitted by eroding soils. Surprisingly, these measurements indicate that the size distribution of naturally emitted dust aerosols is independent of the wind speed. The recently formulated brittle fragmentation theory of dust emission is consistent with this finding, whereas other theoretical dust emission models are not. The independence of the emitted dust size distribution with wind speed simplifies both the interpretation of geological records of dust deposition and the parameterization of dust emission in atmospheric circulation models., Comment: Published in Atmospheric Chemistry and Physics

[1] Saltation, the wind-driven hopping motion of sand grains, forms dunes and ripples, and ejects fine dust particles into the atmosphere on Mars. Although the wind speed at which saltation is initiated, the “fluid threshold,” has been studied extensively, the wind speed at which saltation is halted, the “impact threshold,” has been poorly quantified for Mars conditions. I present an analytical model of the impact threshold, which is in agreement with measurements and numerical simulations for Earth conditions. For Mars conditions, the impact threshold is approximately an order of magnitude below the fluid threshold, in agreement with previous studies. Saltation on Mars can thus be sustained at wind speeds an order of magnitude less than required to initiate it, leading to the occurrence of hysteresis. I include the effect of hysteresis into an improved parameterization of sand transport on Mars.

[1] The objective of the Phoenix mission is to determine if Mars' polar region can support life. Since liquid water is a basic ingredient for life, as we know it, an important goal of the mission is to determine if liquid water exists at the landing site. It is believed that a layer of Martian soil preserves ice by forming a barrier against high temperatures and sublimation, but that exposed ice sublimates without the formation of the liquid phase. Here we show possible independent physical and thermodynamical evidence that besides ice, liquid saline water exists in areas disturbed by the Phoenix Lander. Moreover, we show that the thermodynamics of freeze-thaw cycles can lead to the formation of saline solutions with freezing temperatures lower than current summer ground temperatures on the Phoenix landing site on Mars' Arctic. Thus, we hypothesize that liquid saline water might occur where ground ice exists near the Martian surface. The ideas and results presented in this article provide significant new insights into the behavior of water on Mars.

[1] We report evidence for the emission of non-thermal microwave radiation by a deep Martian dust storm. The observations were made using an innovative detector that can discriminate between radiation of thermal and non-thermal origin by measuring the high order moments of its electric field strength. Measurements with this detector, installed in a 34 m radio telescope of the Deep Space Network (DSN), were made for about 5 hours a day over a dozen days between 22 May and 16 June 2006. Non-thermal radiation was detected for a few hours only when a 35 km deep Martian dust storm was within the field of view of the radio telescope on 8 June 2006. The spectrum of the non-thermal radiation has significant peaks around predicted values of the lowest three modes of the Martian Schumann Resonance (SR). The SR results from electromagnetic standing waves formed in the concentric spherical cavity between the Martian surface and its ionosphere and forced by large-scale electric discharge. Thus, the non-thermal radiation was probably caused by electric discharge in the Martian dust storm.

Wind-blown sand, or 'saltation,' creates sand dunes, erodes geological features, and could be a significant source of dust aerosols on Mars. Moreover, the electrification of sand and dust in saltation, dust storms, and dust devils could produce electric discharges and affect atmospheric chemistry. We present the first calculations of electric fields in martian saltation, using a numerical model of saltation that includes sand electrification, plasma physics, and the adsorption of ions and electrons onto particulates. Our results indicate that electric discharges do not occur in martian saltation. Moreover, we find that the production of hydrogen peroxide and the dissociation of methane by electric fields are less significant than previously thought. Both these species are highly relevant to studies of past and present life on Mars., 5 journal pages, 3 figures, published in Geophysical Research Letters

Wind-blown sand, or "saltation," is an important geological process, and the primary source of atmospheric dust aerosols. Significant discrepancies exist between classical saltation theory and measurements. We show here that these discrepancies can be resolved by the inclusion of sand electrification in a physically based saltation model. Indeed, we find that electric forces enhance the concentration of saltating particles and cause them to travel closer to the surface, in agreement with measurements. Our results thus indicate that sand electrification plays an important role in saltation., 4 journal pages, 5 figures, and supplementary material. Article is in press at PRL

[1] Climate forcing by mineral dust aerosols is one of the most uncertain processes in our current understanding of climate change. The main natural sources of dust aerosols are blowing dust, dust devils, and dust storms. Electric fields larger than 100 kV/m have been measured in these phenomena. Theoretical calculations and laboratory experiments show that these electric fields produce electric forces that can reduce the critical wind speed necessary to initiate dust lifting and can even directly lift mineral particles from the surface. Thus, we conclude that electric forces enhance the natural lifting of mineral dust aerosols.

Particle lifting in dust devils on both Earth and Mars has been studied from many different perspectives, including how dust devils could influence the dust cycles of both planets. Here we review our current understanding of particle entrainment by dust devils by examining results from field observations on Earth and Mars, laboratory experiments (at terrestrial ambient and Mars-analog conditions), and analytical modeling. By combining insights obtained from these three methodologies, we provide a detailed overview on interactions between particle lifting processes due to mechanical, thermal, electrodynamical and pressure effects, and how these processes apply to dust devils on Earth and Mars. Experiments and observations have shown dust devils to be effective lifters of dust given the proper conditions on Earth and Mars. However, dust devil studies have yet to determine the individual roles of each of the component processes acting at any given time in dust devils.

This work reports on the current status of global modelling of iron (Fe) deposition fluxes and atmospheric concentrations and analyses of the differences between models, as well as between models and observations. A total of four global 3-D chemistry-transport (CTMs) and general circulation (GCMs) models have participated in this intercomparison, in the framework of the United Nations Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) Working Group 38, The Atmospheric Input of Chemicals to the Ocean. The global total Fe (TFe) emissions strength in the models is equal to ~ 72 Tg-Fe yr−1 (38–134 Tg-Fe yr−1) from mineral dust sources and around 2.1 Tg-Fe yr−1 (1.8–2.7 Tg-Fe yr−1) from combustion processes (sum of anthropogenic combustion/biomass burning and wildfires). The mean global labile Fe (LFe) source strength in the models, considering both the primary emissions and the atmospheric processing, is calculated to be 0.7 (±0.3) Tg-Fe yr−1, accounting for mineral dust and combustion aerosols together. The multi model ensemble global TFe and LFe deposition fluxes into the global ocean are calculated to be ~ 15 Tg-Fe yr−1 and ~ 0.3 Tg-Fe yr−1, respectively. The model intercomparison analysis indicates that the representation of the atmospheric Fe cycle varies among models, in terms of both the magnitude of natural and combustion Fe emissions as well as the complexity of atmospheric processing parametrizations of Fe-containing aerosols. The model comparison with aerosol Fe observations over oceanic regions indicate that most models overestimate surface level TFe mass concentrations near the dust source regions and tend to underestimate the low concentrations observed in remote ocean regions. All models are able to simulate the tendency of higher Fe loading near and downwind from the dust source regions, with the mean normalized bias for the Northern Hemisphere (~ 14), larger than the Southern Hemisphere (~ 2.4) for the ensemble model mean. This model intercomparison and model–observation comparison study reveals two critical issues in LFe simulations that require further exploration: 1) the Fe-containing aerosol size distribution and 2) the relative contribution of dust and combustion sources of Fe to labile Fe in atmospheric aerosols over the remote oceanic regions.