Volcanic ash deposited onto ice and snow in the Arctic has the potential to perturb the regional radiation balance by altering the surface reflectivity. In order to determine changes in surface albedo, loading fields of deposits must be quantified. Loading fields are defined as the total mass of ash deposited over an area. We assessed the areal extent and loading of ash deposits from the 2009 eruption of Mt. Redoubt using an Eulerian volcanic ash transport and dispersion model, Fall3D, combined with satellite and deposit observations. Because direct observations are often limited in remote Arctic regions, we devised a novel method for modeling ash deposit loading fields for the entire eruption based on best-fit parameters of a well-studied eruptive event. The major land-depositing eruptive events (events 2–6 on March 23–24) were chosen for analyses. Realistic ranges of model parameters were selected based on the well-described event 5 (March 23) and were estimated for other events. The model results were validated against NASA A-train satellite data and field measurements reported by the Alaska Volcano Observatory. Overall, good to moderate agreement was found. A total cumulative deposit area of 3.7 ? 106 km2 was produced, and loadings ranged from ~ 7000 ± 3000 g m? 2 near the vent to < 0.1 ± 0.002 g m? 2 on the outer margins of the deposits. Spatial gradients of total ash loading along a cross-section extending from Mt. Redoubt to ~ 600 km revealed event 5 contributed most to the total loading from 0 to 200 km, followed by events 3, 4, 6, and 2. Ash loading histories for total deposits showed that fallout time along the cross-section for each of events 2–6 ranged from ~ 5–17 h. Our deposit loading results suggest that ash from short-duration events can produce regionally significant deposits hundreds of kilometers from the volcano, with the potential of significantly modifying albedo over wide regions of ice and snow covered terrain.
We investigate the influence of wildfire smoke aerosols on cloud microphysics and precipitation using a coupled aerosol-cloud microphysics-meteorology model WRF-Chem-SMOKE. The Wildfire Automated Biomass Burning Algorithm (WF_ABBA) products are used to compute “online” hourly size- and composition-resolved smoke emission fluxes during Canadian boreal wildfires in the summer of 2007. Comparisons with MODIS aerosol optical depth, OMI aerosol index, and CALIPSO vertical feature mask demonstrate that WRF-Chem-SMOKE captures both the horizontal and vertical spatial distribution of smoke. However, estimated smoke emissions result in much lower AOD values than those of observations (by about ten-fold). Modeling experiments with varying amounts of smoke emissions of five to ten times as high as the original load reveal that low smoke load favors the collision-coalescence process at a certain stage, leading to either positive or negative changes in the cloud water path (CWP) relative to smoke-free conditions. For high smoke emissions, changes in CWP are positive, as large as 0.5 kg/m2. A domain-integrated increase in CWP is proportional to smoke loading. By contrast, both positive and negative changes in the rain water path (RWP) and the snow water path (SWP) are found. While domain-integrated changes in RWP are negative, those in SWP go from negative to positive under a high smoke load. Higher smoke loadings suppress precipitation initially, because of smoke-induced reduction of the collision-coalescence and riming processes, but ultimately cause an invigoration of precipitation. We found that precipitation is highly sensitive to 3D smoke fields and varies in a non-linear manner with smoke loads.
The goal of the chapter is to examine the changes that have occurred in dust aerosols, land cover and land use and regional climate in dryland East Asia, the major processes governing these changes, including coupling and feedbacks, and implications for the human-environment-climate systems. Comparison to drylands in Central Asia is provided to facilitate discussion of common features and regional specifics of the world’s drylands. The chapter starts by documenting the major historical changes in land cover and land use (LCLU) that have occurred in the region during the past decades (since the 1950s). These changes are explored in the context of variations and change in regional and global climate and human activities, including socio-economic transformations specific to the region. The main emphasis is on LCLU changes and climate variables that affect the production of wind-blown dust. Dust emission assessment is presented on the basis of integrative analyses of historical LCLU maps, climatological data and the regional coupled dust modeling system WRF-Chem-DuMo. The intensity and frequency of dust events are examined by using historical data of visibility and weather types provided by meteorological stations. Analysis also includes multi-satellite, multi-sensor records of atmospheric aerosols for the past decade. Building on observational data and modeling results, the interactions among LCLU, climate, and dust are addressed across the broad range of spatial and temporal scales and implications of the dust impacts for the human-environment-climate systems are explored.
We investigated the extent to which Asian dust can affect vegetation in dryland ecosystems through altering photosynthetically active radiation (PAR) and shortwave and longwave radiation components of the surface energy balance. Results show that dust decreases the surface radiative balance and total PAR. The diffuse component of PAR, however, increases with increasing dust load but then decreases after reaching a maximum at a certain optimum condition. The forcing efficiency ranges from -67.7 to -82.2 Wm-2τ0.5-1 in total PAR and from -68.8 to -122.1 Wm-2τ0.5-1 in surface radiative balance. The ratio of total PAR to downwelling shortwave flux remains nearly constant (0.45 ± 4%) similar to other aerosol types, while the ratio for the diffuse faction of PAR exhibits significant variations. The impact of dust on the gross photosynthetic rate varies among different types of crops. C4 plants such as corn tend to be less sensitive to the dust optical properties compared to C3 plants such as soybean and wheat.
This study examines how aerosols measured from the ground and space over the US Southeast change temporally over a regional scale during the past decade. PM2.5 data consist of two datasets that represent the measurements that are used for regulatory purposes by the US EPA and continuous measurements used for quickly disseminating air quality information. AOD data comes from three NASA sensors: the MODIS sensors onboard Terra and Aqua satellites and the MISR sensor onboard the Terra satellite. We analyze all available data over the state of Georgia from 2000–2009 of both types of aerosol data. The analysis reveals that during the summer the large metropolitan area of Atlanta has average PM2.5 concentrations that are 50 % more than the remainder of the state. Strong seasonality is detected in both the AOD and PM2.5 datasets; as evidenced by a threefold increase of AOD from mean winter values to mean summer values, and the increase in PM2.5 concentrations is almost twofold from over the same period. Additionally, there is good agreement between MODIS and MISR onboard the Terra satellite during the spring and summer having correlation coeffcients of 0.64 and 0.71, respectively. Monthly anomalies were used to determine the presence of a trend in all considered aerosol datasets. We found negative linear trends in both the monthly AOD anomalies from MODIS onboard Terra and the PM2.5 datasets, which are statistically signi?cant for ? = 0.05. Decreasing trends were also found for MISR onboard Terra and MODIS onboard Aqua, but those trends were not statistically signi?cant.
High northern latitude eruptions have the potential to release volcanic aerosol into the Arctic environment, perturbing the Arctic's climate system. In this study, we present assessments of shortwave (SW), longwave (LW) and net direct aerosol radiative forcings (DARFs) and atmospheric heating/cooling rates caused by volcanic aerosol from the 2009 eruption of Redoubt Volcano by performing radiative transfer modeling constrained by NASA A-Train satellite data. The Ozone Monitoring Instrument (OMI), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model for volcanic ash were used to characterize aerosol across the region. A representative range of aerosol optical depths (AODs) at 550 nm were obtained from MODIS, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) was used to determine the altitude and thickness of the plumes. The optical properties of volcanic aerosol were calculated using a compositionally resolved microphysical model developed for both ash and sulfates. Two compositions of volcanic aerosol were considered in order to examine a fresh, ash rich plume and an older, ash poor plume. Optical models were incorporated into a modified version of the Santa Barbara Disort Atmospheric Radiative Transfer (SBDART) model. Radiative transfer calculations were made for a range of surface albedos and solar zenith angles (SZA) representative of the region. We find that the total DARF caused by a fresh, thin plume (~2.5–7 km) at an AOD (550 nm) range of 0.16–0.58 and SZA = 55° is –46 W m?2AOD?1 at the top of the atmosphere (TOA), 110 W m?2AOD?1 in the aerosol layer, and – 150 W m?2AOD?1 at the surface over seawater. However, the total DARF for the same plume over snow and at the same SZA at TOA, in the layer, and at the surface is 170, 170, and ?2 W m?2AOD?1, respectively. We also see that the total DARF when SZA = 75° for the same layer over snow is 35 W m?2AOD?1 at TOA, 64 W m?2AOD?1 in the layer, and 11 W m?2AOD?1 at the surface. These results indicate that environmental conditions, such as surface albedo and SZA, control the sign of the radiative forcing at TOA and at the surface and the magnitude of the forcing in the aerosol layer. An older plume over snow at SZA = 55° would have total DARFs of 25, 31, and ?5 W m?2AOD?1 at TOA, in the layer, and at the surface, respectively. Our results demonstrate that plume aging can alter the magnitude of the radiative forcing. We also compare results for the thin plume to those for a thick plume (~3–20 km) with an AOD (550 nm) range of 1 to 3. The fresh, thin plume with AOD = 0.58, over seawater, and SZA = 55° will heat the atmosphere in the SW by ~2.5 K day?1 and cool the atmosphere in the LW by ~0.3 Kday?1. The fresh, thick plume with AOD = 3 under the same environmental conditions will produce SW heating in the atmosphere by ~31 Kday?1 and atmospheric LW cooling of ~6.7 K day?1. These calculations convey the importance of vertical plume structure in determining the magnitudes of the radiative effects. We compare our assessments with those reported for other aerosols typical to the Arctic environment (smoke from wildfires, Arctic haze, and dust) to demonstrate the importance of volcanic aerosols.
An ice nucleation parameterization accounting for the deliquescent heterogeneous freezing (DHF) mode was implemented into the Weather Research Forecast (WRF) model. The DHF mode refers to the freezing process for internally mixed aerosols with soluble and insoluble species that can serve as both cloud condensation nuclei (CCN) and ice nuclei (IN), such as dust. A modified version of WRF was used to examine the effect of Saharan dust on the early development of Hurricane Helene (2006) via acting as CCN and IN. The WRF simulations showed the tendency of DHF mode to promote ice formation at lower altitudes in strong updraft cores, increase the local latent heat release, and produce more low clouds and less high clouds. The inclusion of dust acting as CCN and IN through the DHF mode modified the storm intensity, track, hydrometeor distribution, cloud top temperature (hence the storm radiative energy budget), and precipitation and latent heat distribution. However, changes in storm intensity, latent heating rate, and total precipitation exhibit nonlinear dependence on the dust concentration. Improvement in the representation of atmospheric aerosols and cloud microphysics has the potential to contribute to better prediction of tropical cyclone development.
Aerosol-cloud interaction studies to date consider aerosol with a substantial fraction of soluble material as the sole source of cloud condensation nuclei (CCN). Emerging evidence suggests that mineral dust can act as good CCN through water adsorption onto the surface of particles. This study provides a first assessment of the contribution of insoluble dust to global CCN and cloud droplet number concentration (CDNC). Simulations are carried out with the NASA Global Modeling Initiative chemical transport model with an online aerosol simulation, considering emissions from fossil fuel, biomass burning, marine, and dust sources. CDNC is calculated online and explicitly considers the competition of soluble and insoluble CCN for water vapor. The predicted annual average contribution of insoluble mineral dust to CCN and CDNC in cloud-forming areas is up to 40 and 23.8%, respectively. Sensitivity tests suggest that uncertainties in dust size distribution and water adsorption parameters modulate the contribution of mineral dust to CDNC by 23 and 56%, respectively. Coating of dust by hygroscopic salts during the atmospheric aging causes a twofold enhancement of the dust contribution to CCN; the aged dust, however, can substantially deplete in-cloud supersaturation during the initial stages of cloud formation and can eventually reduce CDNC. Considering the hydrophilicity from adsorption and hygroscopicity from solute is required to comprehensively capture the dust-warm cloud interactions. The framework presented here addresses this need and can be easily integrated in atmospheric models.
This study reports laboratory measurements of particle size distributions, cloud condensation nuclei (CCN) activity, and droplet activation kinetics of wet generated aerosols from clays, calcite, quartz, and desert soil samples from Northern Africa, East Asia/China, and Northern America. The dependence of critical supersaturation, sc, on particle dry diameter, Ddry, is used to characterize particle-water interactions and assess the ability of Frenkel-Halsey-Hill adsorption activation theory (FHH-AT) and Kohler theory (KT) to describe the CCN activity of the considered samples. Regional dust samples produce unimodal size distributions with particle sizes as small as 40 nm, CCN activation consistent with KT, and exhibit hygroscopicity similar to inorganic salts. Clays and minerals produce a bimodal size distribution; the CCN activity of the smaller mode is consistent with KT, while the larger mode is less hydrophilic, follows activation by FHH-AT, and displays almost identical CCN activity to dry generated dust. Ion Chromatography (IC) analysis performed on regional dust samples indicates a soluble fraction that cannot explain the CCN activity of dry or wet generated dust. A mass balance and hygroscopicity closure suggests that the small amount of ions (of low solubility compounds like calcite) present in the dry dust dissolve in the aqueous suspension during the wet generation process and give rise to the observed small hygroscopic mode. Overall these results identify an artifact that may question the atmospheric relevance of dust CCN activity studies using the wet generation method. Based on a threshold droplet growth analysis, wet generated mineral aerosols display similar activation kinetics compared to ammonium sulfate calibration aerosol. Finally, a unified CCN activity framework that accounts for concurrent effects of solute and adsorption is developed to describe the CCN activity of aged or hygroscopic dusts.
Poor air quality episodes occur often in metropolitan Atlanta, GA. The primary focus of this research is to assess the capability of satellites as a tool in characterizing air quality in Atlanta. Results indicate that intracity PM2.5 (particulate matter <= 2.5 um in aerodynamic diameter) concentrations show similar patterns as other U.S. urban areas, with the highest concentrations occurring within the city. PM2.5 and MODIS (Moderate Resolution Imaging Spectroradiometer) aerosol optical depth (AOD) have higher values in the summer than spring, yet MODIS AOD doubles in the summer unlike PM2.5. Most (80%) of the Ozone Monitoring Instrument aerosol index (AI) is below 0.5 with little differences between spring and summer. Using this value as a constraint of the carbonaceous aerosol signal in the urban area, aerosol transport events such as wildfire smoke associated with higher positive AI values can be identified. The results indicate that MODIS AOD is well correlated with PM2.5 on a yearly and seasonal basis with correlation coefficients as high as 0.8 for Terra and 0.7 for Aqua. A possible alternative view of the PM2.5 and AOD relationship is seen through the use of AOD thresholds. These probabilistic thresholds provide a means to describe the air quality index (AQI) through the use of multiyear AOD records for a specific area. The National Ambient Air Quality Standards (NAAQS) are used to classify the AOD into different AQI codes and probabilistically determine thresholds of AOD that represent most of a specific AQI category. For example, 80% of cases of moderate AQI days have AOD values between 0.5 and 0.6. The development of AOD thresholds provides a useful tool for evaluating air quality from the use of satellites in regions where there are sparse ground-based measurements of PM2.5.
Limited observational data exists on the physical interactions between volcanic ash particles and water vapor; yet it is thought that these interactions can strongly impact the microphysical evolution of ash, with implications for its atmospheric lifetime and transport, as well as formation of water and ice clouds. In this study, we investigate for the first time, the hygroscopic properties of ultra-fine volcanic ash (<125 μm diameter) from the eruptions of Mt. St. Helens in 1980, El Chichon in 1982, Tungurahua in 2006, Chaiten in 2008, Mt. Redoubt in 2009, and Eyjafjallajokull in 2010. The hygroscopicity of the ash particles is quantified by their ability to uptake water and nucleate into cloud drops under controlled levels of water vapor supersaturation. Evidence presented strongly suggests that ash uptakes water efficiently via adsorption and a simple parameterization of ash hygroscopicity is developed for use in ash plume and atmospheric models.
This study reports laboratory measurements of cloud condensation nuclei (CCN) activity and droplet activation kinetics of aerosols dry-generated from clays, calcite, quartz, and desert soil samples from Northern Africa, East Asia/China, and Northern America. Based on the observed dependence of critical supersaturation, sc, with particle dry diameter, Ddry, we find that FHH adsorption activation theory is a far more suitable framework for describing fresh dust CCN activity than Kohler theory. set of FHH parameters (AFFH ~ 2.25 ± 0.75, BFFH ~ 1.20 ± 0.10) can adequately reproduce the measured CCN activity for all species considered, and also explains the large range of hygroscopicities reported in the literature. Based on threshold droplet growth analysis, mineral dust aerosols were found to display retarded activation kinetics compared to ammonium sulfate. Comprehensive simulations of mineral dust activation and growth in the CCN instrument suggest that this retardation is equivalent to a reduction of the water vapor uptake coefficient (relative to that for calibration ammonium sulfate aerosol) by 30–80%. These results suggest that dust particles do not require deliquescent material to act as CCN in the atmosphere.
Atmospheric aerosols have been hypothesized as playing an important role in signi?cant climate and environmental changes that have been occurring in the Arctic. This Chapter concentrates on the role of Arctic aerosols in the energy balance and the hydrological cycle by considering several major aerosol types (sulfates, black carbon and dust) that originate in Northern Eurasia. Aerosols can affect the energy balance directly by scattering, absorbing, and emitting atmospheric radiation as well as by changing the surface albedo. Furthermore, aerosols perturb the radiative energy balance indirectly by affecting the properties, lifetime, and coverage of clouds. Aerosol-induced changes in clouds are also important in the hydrological cycle. An additional complexity arises from the potential connection of aerosols to feedbacks that involve the physical climate, ecological, and human components of the Arctic system. The abundances, chemical composition and spatiotemporal distributions of natural and anthropogenic aerosols, which are controlled by sources and ageing processes occurring during atmospheric transport, are the major factors governing the aerosol climate forcing. Over the past decades, the warming climate, socio-economic changes and changes in land cover and land use occurring in Northern Eurasia have been affecting sources and properties of atmospheric aerosols. These changes were likely to affect not only aerosol burden in the Russian Arctic but through the whole Arctic. Understanding how changes in land cover and land use have been affecting the abundances and distributions of natural and anthropogenic aerosols and how the aerosol-induced varying forcing has been affecting the Arctic climate system is of great importance to understand the overall response of the Arctic region to global warming associated with steadily increasing greenhouse gases.
Atmospheric mineral dust is an important component of the climate system; however, representation of dust production in the climate models poses significant challenges. Satellite remote sensing has the potential to aid in determining the surface characteristics of active dust source regions that are of importance to dust emission modeling. This study uses data from the Moderate-resolution Imaging Spectroradiometer (MODIS) in conjunction with soil texture to investigate linkages of spatial distribution of surface characteristics related to dust emission, and their dynamics, at the seasonal time scale. In addition to standard MODIS land products such as surface albedo and NDVI which are strongly linked to dust emission, we introduce a roughness index (RI) and an arid soil surface index (ASSI) to aid in land surface characterization. Three regions of northwestern China known for dust emission, the Taklamakan, Badain Jaran, and Gurbantunggut Deserts, are examined for spatial and temporal changes of surface characteristics during winter–spring–summer transition February–July 2005. A new methodology is proposed by introducing regional masking derived from MODIS Band 10 surface albedo. The analysis demonstrates regional unique temporal and spatial characteristics in the 2005 seasonal transition for these areas. Seasonal modes of response are clearly present. The soil texture correlation results demonstrate that clay fraction has a consistently high negative correlation to albedo, as does vegetation. The analysis also demonstrates that RI is a dynamic characteristic changing both with season and on much shorter time scales.
This study uses published data on dust-water interactions to examine the importance of including water adsorption effects when describing the hygroscopic and cloud condensation nuclei (CCN) behavior of mineral dust aerosol. Adsorption activation theory (AT) better represents fresh dust-water interactions than Kohler theory (KT), as i) a consistent set of adsorption parameters can describe the hygroscopic behavior of dust (under both sub and supersaturated conditions), and ii) the dependence of critical supersaturation, sc, with particle dry diameter, Ddry, is closer to observations. The long adsorption timescale could also contribute to the large differences observed between dry and wet generated dust hygroscopicity. If KT and AT are consistently applied to the same dust size distribution, KT predicts up to tenfold higher CCN and 40% higher droplet number concentration than AT. This profoundly different behavior between the theories suggests that both may be required for a comprehensive description of atmospheric dust CCN activity.
Using data from MODIS for the 2000 - 2004 time period, we performed a statistical analysis of visible radiances observed in the presence of mineral dust in cloud-free and cloudy conditions over oceans. Spatial variability of the 555 nm radiances was examined by introducing two different measures: standard deviation (STD) and a local inhomogeneity parameter (LIP). We demonstrate that introducing the probability density function (PDF) of STD offers a new framework for probabilistic dust detection. We show that the probabilistic approach gives more accurate discrimination of dust from clouds compared with the MODIS fixed STD threshold method. Furthermore, the probabilistic approach enables one to determine the confidence level and skill of dust detection. In addition, we examined the capability of the probabilistic detection of dust-cloud mixed pixels currently not considered by operational algorithms. A low classification skill of dust-cloud pixels was found. Introducing multivariate PDFs by including multispectral data might help to overcome this problem.
Northern Eurasia, the largest land-mass in the northern extratropics, accounts for ∼20% of the global land area. However, little is known about how the biogeochemical cycles, energy and water cycles, and human activities specific to this carbon-rich, cold region interact with global climate. A major concern is that changes in the distribution of land-based life, as well as its interactions with the environment, may lead to a self-reinforcing cycle of accelerated regional and global warming. With this as its motivation, the Northern Eurasian Earth Science Partnership Initiative (NEESPI) was formed in 2004 to better understand and quantify feedbacks between northern Eurasian and global climates. The first group of NEESPI projects has mostly focused on assembling regional databases, organizing improved environmental monitoring of the region, and studying individual environmental processes. That was a starting point to addressing emerging challenges in the region related to rapidly and simultaneously changing climate, environmental, and societal systems. More recently, the NEESPI research focus has been moving toward integrative studies, including the development of modeling capabilities to project the future state of climate, environment, and societies in the NEESPI domain. This effort will require a high level of integration of observation programs, process studies, and modeling across disciplines
Significant problems with modeling dust emission are highlighted. Not only do dust emission schemes rely on various assumptions, but also their implementation within a regional or global model presents challenges. This paper provides an in-depth comparative analysis of two different physically based schemes that were originally developed by Marticorena and Bergametti (1995) and Shao et al. (1996) with some recent improvements. Both schemes were implemented in a dust module (DuMo) and coupled with the Weather Research and Forecasting (WRF) model. Here we examine the physical parameterizations employed by these schemes, identify the key input parameters, and establish linkages between them by developing a new data set for dust sources in Central and East Asia. The relative importance of the input parameters is assessed through partial derivatives. The major issues involved in implementing the physically based schemes within a regional model are also discussed. Consistent implementation of two state-of-the-art dust schemes within the same regional model enables us to bracket inherent uncertainties in simulated dust emission. The results of a case study based on WRF–DuMo simulations are presented to demonstrate associated biases in the magnitude and spatial patterns of emitted dust vertical fluxes. Also, recommendations on the selection of input parameters, including land and meteorological variables, to achieve an improved modeling of dust emission in Central and East Asia are provided.
Dust and black carbon aerosol have long been known to exert potentially important and diverse impacts on cloud droplet formation. Most studies to date focus on the soluble fraction of these particles, and overlook interactions of the insoluble fraction with water vapor (even if known to be hydrophilic). To address this gap, we developed a new parameterization that considers cloud droplet formation within an ascending air parcel containing insoluble (but wettable) particles externally mixed with aerosol containing an appreciable soluble fraction. Activation of particles with a soluble fraction is described through well-established Kohler theory, while the activation of hydrophilic insoluble particles is treated by "adsorption-activation" theory. In the latter, water vapor is adsorbed onto insoluble particles, the activity of which is described by a multilayer Frenkel-Halsey-Hill (FHH) adsorption isotherm modified to account for particle curvature. We further develop FHH activation theory to i) find combinations of the adsorption parameters AFHH, BFHH which yield atmospherically-relevant behavior, and, ii) express activation properties (critical supersaturation) that follow a simple power law with respect to dry particle diameter. The new parameterization is tested by comparing the parameterized cloud droplet number concentration against predictions with a detailed numerical cloud model, considering a wide range of particle populations, cloud updraft conditions, water vapor condensation coefficient and FHH adsorption isotherm characteristics. The agreement between parameterization and parcel model is excellent, with an average error of 10% and R2~0.98. A preliminary sensitivity study suggests that the sublinear response of droplet number to Kohler particle concentration is not as strong for FHH particles. [ABSTRACT FROM AUTHOR] Copyright of Atmospheric Chemistry & Physics is the property of European Geosciences Union and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
The optical and radiative properties of dust particles in solar and thermal infrared regions are investigated. Dust particles are assumed to be spheres and spheroids for a comparison aimed at understanding the nonsphericity effect of these particles on the radiation at the top of a dusty atmosphere. The classical Lorenz–Mie theory is employed to compute the optical properties of spherical dust particles. To compute the single-scattering properties of spheroidal dust particles, a combination of the T-matrix method and an approximate method is used in the present study. In the approximate method, applicable to large particles, the geometric optics method is applied to the computation of the scattering phase matrix. A combination of the solution from the geometric optics method and the contribution of the so-called edge effect is used to compute the extinction efficiency of a spheroidal particle whose absorption efficiency is computed by adding the so-called above- and below-edge effect (a term from the well-known complex angular momentum theory) to the geometric optics result. Numerical results show that the results from the T-matrix method and the present approximate approach converge at a size parameter of 50 for computing the integrated scattering properties (i.e., the extinction efficiency, single-scattering albedo, and asymmetry factor). Additionally, the phase functions computed from the two methods are quite similar for size parameters larger than 40 although some considerable differences may still be noticed for other phase matrix elements. Furthermore, the effect of surface roughness on the single-scattering properties of spheroidal particles is discussed. The present radiative transfer simulations illustrate the nonsphericity effect of dust particles is significant at short wavelengths, however, not at the thermal infrared wavelengths.
Northern Eurasia Land Surface Properties and Change and Its Role in the Global Earth System, Aspen, Colorado, 12–17 August 2007 Northern Eurasia is undergoing significant changes associated with warming climate and with socioeconomic changes during the entire twentieth century. Climatic changes over this vast landmass interact and affect the rate of global change through atmospheric circulation and through strong biogeophysical and biogeochemical couplings. Current and future interactions and feedbacks to the global system of this carbon-rich, cold-region component of the Earth system remain to a large extent unknown.
This study examines the importance of spectral optical properties of mineral dust in calculations of photolysis rates in clean and polluted marine environments. A set of optical characteristics was computed with Mie theory using data on the size distribution and composition of mineral dust from recent experimental and modeling studies. These models were incorporated into the National Center for Atmospheric Research tropospheric ultraviolet-visible radiation transfer code. The 13 analyzed photolysis reactions were classified into three groups according to their photolytic wavelengths and the vertical profile of J values in the aerosol-free atmosphere. The photolysis reactions of O3(O1 D), NO2, and NO3(NO) were selected as representative of groups I, II, and III, respectively. We find that depending on its properties, dust causes either a decrease or an increase in the spectral actinic fluxes relative to the aerosol-free condition. The wavelength range in which the changes in actinic fluxes are negative becomes broader as the amount of dust load increases, a dust size distribution is shifted to coarse size mode, and the iron oxide content in dust aggregates increases. Changes in actinic fluxes also depend on the sun position (time of the day) and an altitude considered. As a result, dust exerts the differing impact on J values of the three photolytic groups. The diurnal cycle of dust-affected J values of a given group is similar among the differing size distribution and dust compositions, but changes in J values vary by a factor of 1.5 - 2. For a given content of iron oxide, the largest changes are caused by the size distributions that are shifted to the fine size mode. A change in J values of groups I and II caused by the varying amount of iron oxide in dust aggregates (from 1% to 10%) is negative in and below the dust layer. In contrast, J values of group III increase in the presence of low absorbing dust (with 1% of iron oxide), but they decrease with increasing dust absorption. Our results indicate that the dust composition not only will be needed to accurately model a decrease in J values of groups I and II but also to determine a correct sign and value of changes of J values of group III. In the case of dust mixed with pollution, we find that the external mixing of dust and black carbon causes somewhat larger changes in J values compared to the internal mixing of these species. We conclude that regional differences in size and composition of mineral dust as well as related changes in the vertical profile and diurnal cycle of actinic fluxes and J values should be taken into account if more realistic assessments of the dust impact on photochemistry through photolysis are to be achieved.
It has been suggested that simultaneous satellite measurements of mid-visible fine mode aerosol optical depth τ f and CO concentrations can aid in improving the characterization of biomass burning in chemical transport models. However different approaches for retrieving τ f have recently been proposed. Using MODIS and MOPITT data, we examine the impact these have on the regression slope between enhancements of τ f and CO (Δτ f/ΔCO) for representative biomass burning cases, including savanna and extratropical forests. Both MODIS Collection 4 and recent Collection 5 aerosol products are used in our study. We find that τ f varies systematically with retrieval method causing systematic differences in the slope of regression. Regardless of method used, noticeable differences in regression slope are observed for different types of biomass burning. Our results point out the need for consistency in defining τ f between satellite measurements and models if the Δτ f/ΔCO ratio is to be used as a constraint.
We examine the performance of several mixing rules that are commonly used in modeling optical constants of aerosol mixtures either in remote sensing or radiation transfer/climate studies employing the new refractive index data reported in Part I. We demonstrate that the optical constants of the considered mixtures are not accurately modeled using pure solute optical constants (e.g., ammonium sulfate optical constants and the optical constants of pure water) due to the complex ion-ion and ion-water interactions. On the other hand, we do find that ternary and quaternary mixtures can be well modeled by applying the mixing rules to lower order multi-component optical constants data, e.g., binary data to determine ternary optical constants, or binary and ternary data to determine quaternary optical constants. By using lower order optical constants data sets, much of the ion-ion and ion-water effects are captured. Both mass-fraction and volume-fraction weighting of the "component" optical constants yield satisfactory results, performing as well or better than the more complicated mixing rules. These findings will be of practical use in remote sensing and radiation transfer/climate studies as well as help guide the decision on what optical constants measurements will be required.
In this paper, the first part of two, we present new high-spectral-resolution infrared (IR) optical constants for multi-component aqueous solutions composed of ammonium sulfate, ammonium nitrate, sulfuric acid and nitric acid over a range of compositions and temperatures representative of tropospheric conditions and atmospheric aerosols. The optical constants were determined from ATR measurements via a Kramers-Kronig transformation. To accomplish this, we adapted an existing technique for estimating the real index of refraction of aqueous sulfate and nitrate solutions at multiple visible frequencies as a function of concentration and temperature. An approximation of the low-frequency behavior of the ATR spectrum was also used to reduce the error associated with using ATR data of finite frequency range. This paper also provides a brief examination of absorption spectra for analyzed mixtures in relation to their composition and temperature and discusses possible implications. The new optical constants will be of great utility to high-spectral-resolution IR remote sensing as well as radiative balance analysis in climate studies because they will enable researchers for the first time to model the impacts of tropospheric aqueous sulfate-nitrate-ammonium multi-component aerosols, including their mixtures with other important species such as dust or soot.
This study investigates how the choice of the planetary boundary layer (PBL) parameterization and dust emission scheme affects the prediction of dust entrainment simulated with a regional mesoscale model. For this analysis we consider a representative dust episode which occurred on April 2001 in the Aral Sea region. The meteorological fields were simulated using the PSU/NCAR MM5 modeling system considering two different boundary layer parameterizations. In each case, emitted dust fluxes were computed off-line by incorporating MM5 meteorological fields into the dust module DuMo. Several dust emission schemes with a prescribed erodible fraction and fixed threshold wind speed were the subject of our analysis. Implications to assessment of the anthropogenic fraction of dust emitted in the Aral region were investigated by conducting the full, half, and no lake modeling experiments. Our results show that the discrepancies in dust fluxes between the two different PBLs are much higher compared to the discrepancy associated with the use of considered dust production schemes. Furthermore, the choice of the PBL affects the timing and duration of a modeled dust event. We demonstrate that different combinations of the PBL parameterization and wind- or friction velocity-driven dust emission schemes can result in up to about a 50% difference in predicted dust mass caused by the Aral Sea desiccation. We found that the drying-up of the Aral cannot only affect the dust emission by expanding the source area, but also by affecting atmospheric characteristics, especially winds. These competitive factors add further complexity to quantification of the anthropogenic dust fraction in the region.
The first direct in situ measurements of HO2NO2 in the upper troposphere were performed from the NASA DC-8 during the Intercontinental Chemical Transport Experiment–North America 2004 with a chemical ionization mass spectrometer (CIMS). These measurements provide an independent diagnostic of HOx chemistry in the free troposphere and complement direct observations of HOx, because of the dual dependency of HO2NO2 on HOx and NOx. On average, the highest HO2NO2 mixing ratio of 76 pptv (median = 77 pptv, σ = 39 pptv) was observed at altitudes of 8–9 km. Simple steady state calculations of HO2NO2, constrained by measurements of HOx, NOx, and J values, are in good agreement (slope = 0.90, R2 = 0.60, and z = 5.5–7.5 km) with measurements in the midtroposphere where thermal decomposition is the major loss process. Above 8 km the calculated steady state HO2NO2 is in poor agreement with observed values (R2 = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model-derived HOx show reasonable agreement with the observed HO2NO2 in both the midtroposphere (slope = 0.96, intercept = 7.0, and R2 = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R2 = 0.58). These results indicate that observed HO2 and HO2NO2 are in poor agreement in the upper troposphere but that HO2NO2 levels are consistent with current photochemical theory.
We report on measurements that were specifically designed to determine iron oxides in mineral dust aerosols needed for improved optical modeling. Atmospheric dust samples as well as samples generated in a wind tunnel from soils were analyzed by a number of analytical techniques for their total and free iron content (bulk and size resolved), hematite and goethite, mineralogy, and size distribution. These samples are representative of several important dust sources in East Asia and northern Africa. A novel data set generated from these measurements enables us to perform an in-depth modeling study of dust optical properties in the solar spectrum. We modeled the iron oxide–clay aggregates, which are the key light-absorbing species, as well as their mixtures with nonabsorbing minerals. A volume fraction of iron oxide in aggregates was determined from measurements. Significant differences in the single-scattering albedo, ω 0, were found between hematite- and goethite-clay aggregates, although these calculations involved several important assumptions about the partition of hematite and goethite in size-resolved aggregates. Furthermore, we found that variability of the free iron content is large enough to cause important differences in ω 0 of mineral dust originating from different sources. In contrast, this variability has little effect on the extinction coefficient and optical depth. We demonstrate that for the same size distribution, ω 0 calculated from data obtained for Chinese and Tunisian samples show higher values and more distinct wavelength dependence than those of Niger dust. All the above ω 0 differ from ones calculated using the refractive indices of Patterson et al. (1977) or the OPAC model (Hess et al., 1998), which are often used in radiative transfer studies. We conclude that information on a size-resolved content of free iron and a fraction of hematite and goethite in aggregates will need to be known on a regional basis to improve the prediction of the single-scattering albedo at solar wavelengths and hence the radiative impact of atmospheric mineral dust.
ACE-Asia was a multi-national collaboration organized to investigate and understand the chemistry, radiative properties, and climatic effects of mineral dust and other aerosol particles in the East Asia/Northwest Pacific region. Studies conducted at the Gosan and Zhenbeitai surface supersites show striking variations in aerosol concentrations and properties that were affected by the occurrence and origins of the Asian dust storms, air mass pathways, and mixing during the transport. Investigations conducted on the R/V Ronald H. Brown (RHB) showed that dust had a pervasive influence on the chemical composition, size distribution, and optical properties of the aerosol. Analyses using an aerosol time-of-flight mass spectrometer on the RHB showed that most of the coarse-particle nitrate and sulfate in post-frontal air was associated with dust, and more remarkably, that competitive or exclusionary processes evidently are involved in the uptake or production of these substances. Studies conducted onboard the NCAR C-130 aircraft showed that coarse-mode dust was less absorbing and less hygroscopic than pollution aerosol and that there was little correlation in light scattering and absorption by the sub- vs. super-micrometer aerosol. Below ?2 km, dust was commonly mixed with pollutants, and this had a stronger influence on the optical properties of the submicrometer particles than the coarse-mode dust; at higher altitudes, the dust was less affected by pollution. Single particle analyses of C-130 samples showed that the mixing of black carbon (BC) with dust was common, but only certain types of BC particles were aggregated. Models were used in the planning, execution and interpretative phases of ACE-Asia; and summaries of modeling results are presented to illustrate the progress being made in identifying new dust sources; in depicting the time-varying, three-dimensional structure of dust plumes; and in quantifying the production, transport, and deposition of Asian dust.
Numerical simulations have been carried out to understand the effects of overlapping cirrus clouds and mineral dust on the high-spectral-resolution infrared spectrum in the 600 - 2400 cm-1 region. A combination of the discrete ordinates radiative transfer and line-by-line models that account for the multiple scattering and monochromatic molecular absorption in the atmosphere is utilized to simulate the down-looking infrared spectrum at the top of the atmosphere with a resolution of 0.2 cm-1. It is demonstrated that the spectral slope of the infrared radiance in the 800–1000 cm−1 region can be used to discriminate coexisting cirrus-dust scenes from those associated only with cirrus clouds or dust alone. In a case for a cirrus cloud overlapping with a dust layer, the spectral features in the 1100 - 1200 cm-1 and 1400 - 1850 cm-1 regions can be potentially useful for retrieving the optical thicknesses of the dust layer and cirrus cloud, respectively
We use MODIS Terra Level 1B data containing calibrated and geolocated radiances to investigate a regional signature of wind-blown mineral dust in the thermal-IR. MODIS data over oceans for the 2000 - 2004 time period were examined for the presence of dust plumes that originate from the main dust sources located in East and South Asia, Middle East, Northern Africa, and Australia. A number of representative cases for different source regions were selected and analyzed in terms of brightness temperatures at three IR channels centered at 8.55, 11.03, and 12.02 μm. The distinct differences in the thermal-IR signature of atmospheric dust for the considered regions were found. Our analysis indicates that these differences are likely due to different mineralogical composition, although other factors (e.g., multilayered vertical distribution) may be also involved. Implications of our findings to the detection of dust based on the techniques using brightness temperature differences are discussed.
Linearly polarized radiation is sensitive to the microphysical properties of aerosols, namely, to the particle-size distribution and refractive index. The discriminating power of polarized radiation increases strongly with the increasing range of scattering angles and the addition of multiple wavelengths. The polarization and directionality of the Earth's reflectances (POLDER) missions demonstrate that some aerosol properties can be successfully derived from spaceborne polarimetric, multiangular measurements at two visible wavelengths. We extend the concept to analyze the retrieval capabilities of a spaceborne instrument with six polarimetric channels at 412, 445, 555, 865, 1250, and2250 nm,measuring approximately 100 scattering angles covering a range between 50 and150 deg. Our focus is development of an analysis methodology that can help quantify the benefits of such multiangular and multispectral polarimetric measurements. To that goal we employ a sensitivity metric approach in a framework of the principal-component analysis. The radiances and noise used to construct the sensitivity metric are calculated with the realistic solar flux for representative orbital viewing geometries, accounting for surface reflection from the ground, and statistical and calibration errors of a notional instrument. Spherical aerosol particles covering a range of representative microphysical properties (effective radius, effective variance, real and imaginary parts of the refractive index, single-scattering albedo) are considered in the calculations. We find that there is a limiting threshold for the effective size (approximately0.7 µm), below which the weak scattering intensity results in a decreased signal-to-noise ratio and minimal polarization sensitivity, precluding reliable aerosol retrievals. For such small particles, close to the Rayleigh scattering limit, the total intensity provides a much stronger aerosol signature than the linear polarization, inspiring retrieval when the combined signals of intensities and the polarization fraction are used. We also find a strong correlation between aerosol parameters, in particular between the effective size and the variance, which forces one to simultaneously retrieve at least these two parameters.
We present a systematic theoretical study of atmospheric mineral dust radiative properties, focusing on implications for multiangle and multispectral remote sensing. We model optical properties of complex, nonspherical mineral dust mixtures in three visible-near-infrared satellite channels: 0.550, 0.672, and 0.866 μm, accounting for recent field and laboratory data on mineral dust morphology and mineralogy. To model the optical properties of mineral dust, we employ the discrete dipole approximation technique for particles up to 2 μm diameter and the T matrix method for particles up to 12 μm. We investigate the impact of particle irregularity, composition, and size distribution on particle optical properties, and we develop optical models for representative natural mineral dust composition-size-shape types. Sensitivity studies with these models indicate that Multiangle Imaging Spectroradiometer (MISR) data should be able to distinguish plate-like from grain-like dust particles, weakly from strongly absorbing compositional types, and monomodal from bimodal size distributions. Models containing grain-like, weakly absorbing, bimodal distributions of dust particles were favored for optically thick Saharan and Asian dust plume examples, whereas strongly absorbing and plate-like particles were rejected. We will present detailed, systematic MISR sensitivity studies and analysis of more complex field cases using the optical models derived here in a future paper
This study presents a detailed examination of east Asian dust events during March - April of 2001, by combining satellite multisensor observation (Total Ozone Mapping Spectrometer (TOMS), Moderate-Resolution Imaging Spectroradiometer (MODIS), and Sea-Viewing Wide Field-of-View Sensor (SeaWiFS)) meteorological data from weather stations in China and Mongolia and the Pennsylania State University/National Center for Atmospheric Research Mesoscale Modeling System (MM5) driven by the National Centers for Environmental Prediction Reanalysis data. The main goal is to determine the extent to which the routine surface meteorological observations (including visibility) and satellite data can be used to characterize the spatiotemporal distribution of dust plumes at a range of scales. We also examine the potential of meteorological time series for constraining the dust emission schemes used in aerosol transport models. Thirty-five dust events were identified in the source region during March and April of 2001 and characterized on a case-by-case basis. The midrange transport routes were reconstructed on the basis of visibility observations and observed and MM5-predicted winds with further validation against satellite data. We demonstrate that the combination of visibility data, TOMS aerosol index, MODIS aerosol optical depth over the land, and a qualitative analysis of MODIS and SeaWiFS imagery enables us to constrain the regions of origin of dust outbreaks and midrange transport, though various limitations of individual data sets were revealed in detecting dust over the land. Only two long-range transport episodes were found. The transport routes and coverage of these dust episodes were reconstructed by using MODIS aerosol optical depth and TOMS aerosol index. Our analysis reveals that over the oceans the presence of persistent clouds poses a main problem in identifying the regions affected by dust transport, so only partial reconstruction of dust transport routes reaching the west coast of the United States was possible
The dust source region in East Asia consists of deserts and gobi-deserts in Northern China and Southern Mongolia. First part of this paper discusses the identification criteria of dust sources. A dust emission mechanism serves as the basis of our analyses and the dust storm frequency is considered the principal criterion. The second part studies Mongolian dust sources. Three types of dust sources in Southern Mongolia are identified and characterized, whose dust emission rates are calculated with US EPA (Environmental Protection Agency) formulas. The dust emission rate increases from north to south across the country by five orders with the strongest dust emission area in the south gobi-area. The third part reveals that the united dust source region in East Asia is comprised of two parts or systems: (a) the Mongolian Plateau dust source system, and (b) the Tarim Basin dust source system. The two systems are related and distinguished not only by topography and the distribution pattern of gobis, deserts and loess lands, but also by soil texture, climate, and dust storm meteorology. For example, M (moving) type dust storms are typical for the Mongolian Plateau system while S (stationary) type dust storms are typical for the Tarim Basin system. Total dust emission from the source region in East Asia is estimated at 10.4?106 ton yr?1 for PM10 (dust particles smaller than 10 ?m in diameter), 27.6?106 ton yr?1 for PM30, and 51.3?106 ton yr?1 for PM50.
Mineral dust aerosols have complex nonspherical shapes and varying composition. This study utilizes data on morphology (size and shape) and composition of dust particles to determine the extent to which the optical properties of real particles differ from those of spheres. A method for modeling the optical properties of complex particle mixtures is proposed. The method combines dust particle composition-shape-size (CSS) distributions reconstructed from the electron microscopy data, effective medium approximations and discrete dipole approximation. The method is used to compute optical characteristics of realistic dust mixtures representative of Saharan and Asian dust. We demonstrate that considered CSS distributions result in various differences in the extinction coefficient, single scattering albedo, asymmetry parameter and the scattering phase function relative to the volume-equivalent spheres and the mixtures of the randomly oriented oblate and prolate spheroids. Implications of these differences for radiation/climate modeling and remote sensing are discussed
This study investigates how the loading and composition of atmospheric dust affect IR radiances observed by satellite narrowband and high-resolution sensors. To compute monochromatic radiances accounting for multiple scattering and absorption by aerosols and atmospheric gases, we employed a new radiative transfer code which combines the line-by-line algorithm and discrete ordinate technique. New dust optical models required for such computations were developed for the representative mineral mixtures. We demonstrate that dust decreases the brightness temperature observed by satellite sensors depending mainly on the dust burden and composition, though the sensitivity to the composition differs between the satellite sensors. We found that mineral dust has a unique radiative signature (termed here a “negative slope”) which separates the effect of dust from that of clouds and gases. We conclude that dust must be accounted for in atmospheric correction algorithms if the retrievals of the sea surface temperature and atmospheric gaseous species from the thermal IR radiances are to be of high accuracy
Northern China, covering many deserts, gobi-deserts and arid loess-lands, is one of the world's largest sources of atmospheric dust. In this study, we estimate the dust annual mean emission rates and perform comparative characterization of dust sources in Northern China by combining the geographical, pedological and 30-year (1951–1980) climatological data. Multi-year averaged emission rates of PM50, PM30 and PM10 (i.e., dust particulates smaller than 0.05, 0.03 and 0.01 mm in diameter, respectively) were calculated using the modified US EPA empirical formulas. We demonstrate that the main dust sources in Northern China are the Taklimakan Desert (the annual mean PM10 emission rate, Q10, is some 0.38 ton/ha yr), the Central gobi-desert (Q10=0.24 ton/ha yr), and the deserts located on the Alxa Plateau (Q10=0.05 ton/ha yr). The Loess Plateau appears to be a weak dust source. We identify and characterize three broad types of dust sources in Northern China: Type 1. Deserts in dry-agricultural areas, Type 2. Gobi-deserts and deserts located on the plateaus, and Type 3. Deserts and gobi-deserts located in topographical lows. Types 1–3 sources contribute 1%, 35% and 64%, respectively, to the total annual mean emission of PM10 dust. Although the maximum of dust emission occurs in spring, each source type has a distinct seasonal cycle. The analysis of both the seasonal cycle pattern and spatial distribution of dust emission rates demonstrates that a combination of extreme aridity and strong winds is a key factor governing the dust emission in Northern China.
The quality of satellite aerosol retrievals depends critically upon the modeling accuracy of the aerosol optical properties. The optical properties of mineral aerosols depend on particle morphology, mineralogy, and state of mixing. Here we investigate how the realistic morphology and composition of dust particles affect the optical properties of dust mixtures by utilizing microscopy data that recently become available of dust samples collected in the atmosphere. The data were used to reconstruct the representative composition-shape-size (CSS) distributions and then the discrete dipole approximation technique was applied to calculate the optical properties. We demonstrate that the presence of sharp-edge, angular-type particles results in various differences in the scattering phase function, asymmetry parameter, optical depth and single scattering albedo compared to those of the volume-equivalent spheres or ellipses. These differences are sufficiently large as to affect the retrievals of aerosol optical properties from satellite and ground-based remote sensing observations at the solar wavelengths
This paper provides an introduction to the special section of the Journal of Geophysical Research on mineral dust. We briefly review the current experimental and theoretical approaches used to quantify the dust radiative impacts, highlight the outstanding issues, and discuss possible strategies to overcome the emerging problems. We also introduce the contributing papers of this special section. Despite the recent notable advances in dust studies, we demonstrate that the radiative effects of dust remain poorly quantified due to both limited data and incomplete understanding of relative physical and chemical processes. The foremost needs are (1) to quantify the spatial and temporal variations of dust burden in the atmosphere and develop a predictive capability for the size- and composition-resolved dust particle distribution; (2) to develop a quantitative description of the processes that control the spatial and temporal variabilities of dust physical and chemical properties and radiative effects; (3) to develop new instrumentation (especially to measure the dust particle size distribution in a wide range from about 0.01 μm to 100 μm, scattering phase function and light absorption by dust particles); and (4) to develop new techniques for interpreting and merging the diverse information from satellite remote sensing, in situ and ground-based measurements, laboratory studies, and model simulations. Because dust distribution and effects are heterogeneous, both spatially and temporally, a promising strategy to advance our knowledge is to perform comprehensive studies at the targeted regions affected by mineral dust of both natural and anthropogenic origin.
We investigate the importance of the layered vertical distribution of absorbing and non‐absorbing tropospheric aerosols for the retrieval of the aerosol optical depth from satellite radiances measured at visible wavelengths at a single viewing angle. We employ lidar and in‐situ measurements of aerosol extinction coefficients and optical depths to model radiances which would have been observed by a satellite. Then, we determine the aerosol optical depth that would produce the observed radiance under various sets of assumptions which are often used in current retrieval algorithms. We demonstrate that, in the presence of dust or other absorbing aerosols, the retrieved aerosol optical depth can underestimate or overestimate the observed optical depth by a factor of two or more depending on the choice of an aerosol optical model and the relative position of different aerosol layers. The presence of undetected clouds provides a further complication.
We explore several issues relevant to assessments of solar and infrared radiative effects due to mineral aerosols. One issue is the importance of the vertical distribution of dust for calculations of dust radiative heating rates. Another issue is the role that clouds may play in augmenting the radiative forcing by dust. We also explore the importance of the composition of mineral aerosols by employing spectral optical properties for dust that comes from two different regions of the globe, the Saharan and Afghan deserts. A combined longwave and shortwave radiative transfer model was used to determine the instantaneous radiative forcing in the atmosphere, radiative fluxes at the surface, and radiative heating rates by airborne mineral aerosols for clear-sky and cloudy atmospheric conditions. Extensive calculations with our model show that increasing dust loading results in increasing both solar heating rates and infrared cooling rates. However, the net instantaneous rates during the day are always positive, yielding net radiative heating of the dust layer. With similar atmospheric conditions and dust loading, Saharan dust causes larger heating rates than Afghan dust. The magnitudes of the Saharan dust heating rates can easily be 25% larger than Afghan dust heating rates at high Sun angles and over bright surfaces. Also, Saharan dust yields more positive values of TOA (top of the atmosphere) radiative forcing than Afghan dust; and for a diurnal average, this can lead to a change of sign of the TOA radiative forcing from negative to positive just due to mineralogical composition. Clouds significantly influence the direct radiative impact of dust depending on cloud altitude and optical depth. Moreover, this influence is strongly dependent on Sun position and surface albedo.
We describe a technique to model the radiative properties of mineral aerosols which accounts for their composition. We compile a data set of refractive indices of major minerals and employ it, along with data on mineralogical composition of dust from various locations, to calculate spectral optical and radiative properties of mineral aerosol mixtures. Such radiative properties are needed for climate modeling and remote sensing applications. We consider external mixtures of individual minerals, as well as mixtures of aggregates. We demonstrate that an external mixture of individual minerals must contain unrealistically high amounts of hematite to have a single scattering albedo lower than 0.9 at 500 nm wavelength. In contrast, aggregation of hematite with quartz or clays can strongly enhance absorption by dust at solar wavelengths. We also simulate the daily mean net (solar + infrared) forcing by dust of varying compositions. We found that, for a given composition and under similar atmospheric conditions, a mixture of aggregates can cause the positive radiative forcing while a mixture of individual minerals gives the negative forcing.
We present results of modeling the radiative properties of airborne mineral aerosols in a wide range of wavelengths from 0.2 to 40 micrometers. We focus on dust radiative properties which are needed both for climate change assessments and for remote sensing applications. There are some differences in dust properties required for climate change studies and remote sensing studies (Sokolik and Toon, 1996; Kaufman et al., 1997). However, the major uncertainties, poor understanding of space and time variability of aerosol radiative properties, are similar. In particular, absorption by dust is a main concern.
We explore the importance of the composition of airborne mineral aerosols for assessments of their direct radiative forcing at infrared wavelengths. Our calculations employing Mie theory and data on spectral refractive indices show that the existing variations in refractive indices can cause large changes in the major aerosol optical characteristics. Calculations of IR radiative forcings at the top of the atmosphere and IR downward and upward fluxes, based on an one-dimensional radiation transfer code, give a wide range of results for varying optical models of the mineral aerosols. We estimate that for a “low dust loading” scenario the changes in IR downward flux at the surface relative to dust free conditions are in the range from 7 to 14 W/m2 depending upon the mineral aerosol selected. Under “dry tropics” atmospheric conditions the IR forcing at the top of the atmosphere is in the range from 2 to 7 W/m2. In turn, for a “high dust loading” scenario the calculated changes, relative to dust free conditions, in IR downward flux at the surface vary from 50 to 80 W/m2, and the IR forcing at the top of the atmosphere varies from 15 to 25 W/m2. Therefore, we conclude that incorporation of regionally and temporally varying dust mineralogical composition into general circulation models could be beneficial for decreasing the currently large uncertainties in the assessment of radiative forcing by the natural and anthropogenic components of the airborne mineral aerosols. Also the use of appropriate mineralogical data is required for remote sensing of the atmospheric aerosols using satellite infrared observations
Several recent studies focused on estimation of the direct forcing by natural and anthropogenic components of the mineral aerosols have pointed out that there are large uncertainties in the assessments of both solar and infrared forcings (Sokolik and Toon, 1996a; Sokolik and Toon, 1996b; Tegen et a1.1996). In this paper we explore the importance of varying mineralogical composition of airborne mineral aerosols for an assessment of dust regional and global radiative forcings. We examined the available measurements of the microphysical and optical properties of dust aerosols from various geographical locations, and related them to the dust mineralogical composition. Based on empirical data and our modeling we estimated the uncertainties in solar and infrared forcings due to varying regional optical properties of the mineral aerosols in specific geographical regions.
AIRBORNE mineral dust can have a significant effect on the Earth's radiation budget, as it can both scatter sunlight back to space (leading to negative radiative forcing), and absorb solar and infrared radiation (leading to positive forcing)1,2. The effects of mineral aerosols on the radiation budget are important relative to those of other types of aerosols—such as sulphate and smoke particles—due to the widespread distribution and large optical depth of mineral dust. Various human activities, such as land use practices, can result in additional loading of dust, increasing the radiative forcing. Previous studies have attempted to estimate the radiative effects of both the natural and anthropogenic components of the dust3,4. Here we use estimates of anthropogenic dust inputs and observations of dust optical properties to show that although the key quantities contributing to the evaluation of the direct solar radiative forcing by dust generated through human activities have a wide range of uncertainty, the forcing by anthropogenically generated mineral aerosols may be comparable to the forcing by other anthropogenic aerosols. On a regional scale the forcing due to mineral aerosols can greatly exceed that due to sulphate aerosols and can be comparable to that of clouds. Our analysis enables us to highlight the key quantities that need to be better characterized to reduce the (currently large) uncertainties in these estimates.
We estimate the direct radiative forcing by airborne mineral aerosols by using published data on microphysical and optical properties of dust (Ackerman and Cox, 1982; Carlson and Benjamin. 1980; D'Almeida, 1987; Fouquart et al, 1987; W.M.O. 1983; Sokolik and Golitsyn, 1993; Sokolik et al, 1993). Examination of spectra of refractive indices, single scattering albedo and asymmetry parameter for dust models reveals large discrepancies. These discrepancies are caused by inability to model the physical processes responsible for dust optical properties, measurement uncertainties, as well as by actual differences in properties of mineral aerosols originating from various geographical regions. which can be a serious complication in assessment of the regional dust effects.
A dust aerosol optical model is proposed as a result of analysis of the data obtained during the U.S.S.R.-U.S. experiment in Tadzhikistan, 1989. The model consists of spectral dependencies of the single scattering albedo, ?, the asymmetry parameter of the scattering phase function, g, and of possible values of the aerosol optical thickness, ?, for background conditions and for dust storms. This model and available published optical models of dust aerosols were used to perform simulations of radiative forcing in the dusty atmosphere. Results obtained are compared with corresponding results for Saharan aerosol. It is shown that atmospheric dust strongly decreases the total radiative balance of the underlying surface and at the same time induces general warming of the underlying surface-atmosphere system due to a decrease in the system albedo over the arid zones. The calculated heating/cooling rates in the atmosphere are analysed together with data from aerological soundings and radiation measurements.
The results of studies on dust aerosol optical properties obtained during the U.S.S.R.-U.S. experiment are discussed. The ground-based and aircraft measurements carried out during the experiment allow the estimation of characteristic values of aerosol optical depth, aerosol light-scattering coefficients, the degree of linear polarization, and aureole phase functions for different atmospheric conditions in Central Soviet Asia. Two dust storms were observed for which the recorded aerosol optical depth at ? = 0.55 ?m {?a(0.55)} reached 1.5 on 16 and 17 September 1989, and 3.5 on 20 and 21 September 1989. The optical characteristics (spectral dependence of the optical depth, degree of linear polarization) were similar for two dust episodes.
Size distribution data obtained during the U.S.S.R.-U.S. dust experiment make it possible to propose a general conception about the size distribution of dust aerosols within the size range 0.005–100 ?m. The microstructure for the optically active fraction of arid aerosol is approximated in the form of a log-normal distribution with parameters D = 3.5?6 ?m, ?2 = 0.5?0.8, which can be used when estimating radiative calculations.
This paper compares published data on the complex refractive index of atmospheric dust aerosols, for different geographical regions, with data obtained for dust aerosol samples collected during the joint U.S.S.R.—U.S. experiment in Tadzhikistan, 1989. The disadvantages of methods used for estimation of the imaginary part, ?, of the refractive index are discussed. There is a considerable range of values of ? for dust aerosols, which is crucial for optical characteristic simulation. The existing discrepancy in ? can be due to uncertainties in methods used as well as due to the specific chemical composition of dust aerosols from various geographic locations.