Study of the strongest dust storm occurred in Uzbekistan in November 2021

We studied and reconstructed a severe Central Asian dust storm of November 4, 2021, through high-resolution TROPOMI UVAI spaceborne observations, ground-based aerosol measurements, and Lagrangian particle modeling. The dust storm was caused by the front part of a cold polar anticyclone front from the Ural-Volga regions, which struck the central and eastern parts of Uzbekistan under favorable atmospheric conditions. Two plumes spread out, causing a thick haze to blanket the region. The most severe dust storm effects hit the capital of Uzbekistan (Tashkent) and the Fergana Valley, where the thick atmospheric dust layer dropped the visibility to 200 m. PM10 concentrations reached 18,000 µg/m3 (260-fold exceedance of the local long-term average). The PM2.5 concentrations remained above 300 µg/m3 for nearly ten days, indicating an extremely long-lasting event. The dust storm was caused by an extremely strong summer heatwave of 2021 in Kazakhstan with unprecedentedly high temperatures reaching 46.5 °C. The long-lasting drought dried up the soil down to 50 cm depth, triggering the soil cover denudation due to drying out vegetation and losing its moisture. This event was the worst since 1871 and considering the increasing aridity of Central Asia, the onset of potentially recurring severe dust storms is alarming.

Aerosols' variability and their relationship with climatic parameters over West Africa

Aerosols' influences on Earth's climate have been documented by several authors. This ranges from scattering and reflecting of shortwave radiation (direct effect) which is also regarded as the "Whitehouse Effect," to the ability to act as condensation nuclei (indirect effect) which results in cloud droplet formation. This broad summary of aerosol's effect on earth's climate has in turn affected some other weather variables either positively or negatively depending on people's perspectives. This work was done in a view to ascertaining some of these claims by determining the statistical significance of some certain aerosol's relationships with some selected weather variables. This was done over six (6) stations across the West African region to represent the climatic zones from the rainforest around the coasts to the desert of the Sahel. Data used consist of aerosol types (biomass burning, carbonaceous, dust, and PM_2.5) and climatic types (convective precipitation, wind speed, and water vapor) over a period of 30 years, with the python and ferret programs explicitly used for the graphical analyses. Climatologically, locations close to the point source seem to record more of the presence of the pollutants than the farthest ones. Results indicated that aerosols were more pronounced in the dry months of NDJF over the rainforest region depending on the latitudinal position of the location. The relationship result showed a negative correlation between convective precipitation and aerosols, except carbonaceous. But the strongest relationship can be found between water vapor and the selected aerosol types.

A Global-Scale Mineral Dust Equation

A robust method to estimate mineral dust mass in ambient particulate matter (PM) is essential, as the dust fraction cannot be directly measured but is needed to understand dust impacts on the environment and human health. In this study, a global-scale dust equation is developed that builds on the widely used Interagency Monitoring of Protected Visual Environments (IMPROVE) network's "soil" formula that is based on five measured elements (Al, Si, Ca, Fe, and Ti). We incorporate K, Mg, and Na into the equation using the mineral-to-aluminum (MAL) mass ratio of (K₂O + MgO + Na₂O)/Al₂O₃ and apply a correction factor (CF) to account for other missing compounds. We obtain region-specific MAL ratios and CFs by investigating the variation in dust composition across desert regions. To calculate reference dust mass for equation evaluation, we use total-mineral-mass (summing all oxides of crustal elements) and residual-mass (subtracting non-dust species from total PM) approaches. For desert dust in source regions, the normalized mean bias (NMB) of the global equation (within ±1%) is significantly smaller than the NMB of the IMPROVE equation (-6% to 10%). For PM_2.5 with high dust content measured by the IMPROVE network, the global equation estimates dust mass well (NMB within ±5%) at most sites. For desert dust transported to non-source regions, the global equation still performs well (NMB within ±2%). The global equation can also represent paved road, unpaved road, and agricultural soil dust (NMB within ±5%). This global equation provides a promising approach for calculating dust mass based on elemental analysis of dust.

Variations in Aerosol Optical Properties over East Asian Dust Storm Source Regions and Their Climatic Factors during 2000–2021

The East Asian dust storms occur in western and northern China, and southern Mongolia every year, particularly in spring. In this study, we use satellite aerosol products to demonstrate the spatial and temporal variation in aerosol optical depth (AOD) from MODIS, and the absorbing aerosol index (AAI) from TOMS and OMI, over the main dust storm source regions (MDSR), and to investigate their relationship to vegetation coverage (NDVI), soil properties (surface soil moisture content and soil temperature 0–10 cm underground), and climatic factors (surface wind speed, air temperature at 2 m above the ground, and precipitation) in spring for the period of 2000–2021. Compared with dust storm occurrence frequency (DSF) observed at surface stations, MODIS AOD, TOMS AAI, and OMI AAI showed consistent spatial distributions and seasonal variations with DSF in the MDSR, with correlation coefficients of 0.88, 0.55, and 0.88, respectively. The results showed that AOD and AAI over the MDSR decreased during 2000–2005, 2006–2017, and 2000–2021, but increased during 2017–2021.The improvements in vegetation coverage and soil moisture together with favorable climatic factors (the increase in temperature and precipitation and the decrease in surface wind speed) resulted in the decreasing trend of AOD and AAI during 2000–2005, 2006–2017, and the entire period of 2000–2021. Conversely, the increase in surface wind speed, the decrease in temperature and the low soil moisture in 2018 and 2020 were the reasons for the increases in AOD and AAI over the MDSR during 2017–2021. The combination effects of surface wind, temperature, soil moisture, and vegetation coverage would determine DSF, AOD, and AAI, in the end, under global climate change.

Dust Radiative Effect Characteristics during a Typical Springtime Dust Storm with Persistent Floating Dust in the Tarim Basin, Northwest China

A special topography and ultra-high atmospheric boundary layer conditions in the Tarim Basin (TB) lead to the unique spatial–temporal distribution characteristics of dust aerosols. A typical dust storm with persistent floating dust over the TB from 27 April to 1 May 2015 was used to investigate the characteristics of the dust radiative effect using the Weather Research and Forecasting Model with Chemistry (WRF-Chem). Based on reasonable evaluations involving in situ sounding observations, as well as remotely sensed MODIS observations of meteorology, dust aerosols, and the ultra-high atmospheric boundary layer, the simulation characterized the complete characteristics of the dust direct radiative effect (DDRE) during the dust storm outbreak stage and persistent floating dust stage over the TB. During the daytime, the shortwave (SW) radiative effect heated the atmosphere and cooled the land surface (SUR), whereas the longwave (LW) radiative effect had the opposite effect on the TB. Regarding low-level dust, the LW radiative effect was greater than the SW DDRE in the atmosphere, while for high-level dust the situation was reversed. During the nighttime, the LW DDRE at the top of the atmosphere (TOA), at the SUR, and in the atmosphere was less than that during the daytime, when the DDRE at the SUR was the most significant. In contrast to the daytime, the near-surface dust aerosols exerted an LW warming effect in the atmosphere during the nighttime; however, the dust LW radiative effect had a cooling effect from above a 100 m altitude until the top of the dust layer. In contrast, the DDRE heating rate peaked at the top of the dust layer within the TB. The event-averaged net DDRE was 0.53, −5.90, and 6.43 W m−2 at the TOA, at the SUR, and in the atmosphere over the TB, respectively. The dust SW radiative effect was stronger than the dust L4W radiative effect over the TB at the SUR and in the atmosphere. Moreover, the DDRE at the TOA was weaker than that at the SUR. Overall, the study revealed noteworthy radiative effect features of dust aerosols during typical dust storms with persistent floating dust over the TB.

Quantitatively Assessing the Contributions of Dust Aerosols to Direct Radiative Forcing Based on Remote Sensing and Numerical Simulation

Dust aerosols substantially impinge on the Earth’s climate by altering its energy balance, particularly over Northwest China, where dust storms occur frequently. However, the quantitative contributions of dust aerosols to direct radiative forcing (DRF) are not fully understood and warrant in-depth investigations. Taking a typical dust storm that happened during 9–12 April 2020 over Northwest China as an example, four simulation experiments based on the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) were designed, including a real scenario with dust emissions and three hypothetical scenarios without dust emissions, with dust emissions doubled, and with dust emissions reduced by half, to quantitatively evaluate the contributions of dust aerosols to DRF and then to surface temperature, with particular attention to the differences between daytime and nighttime. Moreover, multi-satellite observations were used to reveal the behavior of dust events and to evaluate the model performance. During the daytime, the net dust radiative forcing induced by dust aerosols was –3.76 W/m2 at the surface (SFC), 3.00 W/m2 in the atmosphere (ATM), and –0.76 W/m2 at the top of the atmosphere (TOA), and thus led to surface air temperature cooling by an average of –0.023 ℃ over Northwest China. During the nighttime, the net dust radiative forcing was 2.20 W/m2 at the SFC, –2.65 W/m2 in the ATM, and –0.45 W/m2 at the TOA, which then resulted in surface temperature warming by an average of 0.093 ℃ over Northwest China. These results highlight that the contribution of dust aerosols to DRF is greater during the daytime than that during the nighttime, while exhibiting the opposite impact on surface temperature, as dust can slow down the rate of surface temperature increases (decreases) by reducing (increasing) the surface energy during the daytime (nighttime). Our findings are critical to improving the understanding of the climate effects related to dust aerosols and provide scientific insights for coping with the corresponding disasters induced by dust storms in Northwest China.

Analysis of Mineral Aerosol in the Surface Layer over the Caspian Lowland Desert by the Data of 12 Summer Field Campaigns in 2002–2020

In-situ knowledge on characteristics of mineral aerosols is important for weather and climate prediction models, particularly for modeling such processes as the entrainment, transport and deposition of aerosols. However, field measurements of the dust emission flux, dust size distribution and its chemical composition under realistic wind conditions remain rare. In this study, we present experimental data over annual expeditions in the arid and semi-arid zones of the Caspian Lowland Desert (Kalmykia, south of Russia); we evaluate characteristics of mineral aerosol concentration and fluxes, estimate its chemical composition and calculate its long-distance transport characteristics. The mass concentration in different years ranges from several tens to several hundred of μg m−3. The significant influence of wind velocity on the value of mass and counting concentration and on the proposed entrainment mechanisms is confirmed. An increased content of anthropogenic elements (S, Sn, Pb, Bi, Mo, Ag, Cd, Hg, etc.), which is characteristic for all observation points in the south of the European Russia, is found. The trajectory analysis show that long-range air particles transport from the Caspian Lowland Desert to the central regions of European Russia tends to increase in the recent decades.

Impact of hematite on dust absorption at wavelengths ranging from 0.2 to 1.0 µm: an evaluation of literature data using the T-matrix method

Hematite is the absorbing mineral component of dust aerosols in the shortwave spectral region. However, dust shortwave absorption related to hematite suffers from significant uncertainties. In this study, we evaluated available hematite complex refractive index data in the literature on determining the dust effective refractive index at wavelengths ranging from 0.2 to 1.0 µm using rigorous T-matrix methods. Both spherical and super-spheroidal dust with hematite inclusions were examined to compute the dust optical properties and associated effective refractive indices. We found that the imaginary part of the effective refractive index retrieved from all available hematite complex refractive index data is larger than the measured effective values from Di Biagio et al. [Atmos. Chem. Phys.19, 15503, (2019)10.5194/acp-19-15503-2019]. The result obtained using the hematite refractive index from Hsu and Matijevic [Appl. Opt.241623 (1985)10.1364/AO.24.001623] is closest to but approximately two times larger than Di Biagio et al. [Atmos. Chem. Phys.19, 15503, (2019)10.5194/acp-19-15503-2019]. Our results emphasize the importance of accurate measurements of mineral refractive indices to clarify the dust absorption enigma.

Quantify the Contribution of Dust and Anthropogenic Sources to Aerosols in North China by Lidar and Validated with CALIPSO

Persistent heavy haze episodes have repeatedly shrouded North China in recent years. Besides anthropogenic emissions, natural dust also contributes to the aerosols in this region. Through continuous observation by a dual-wavelength Raman lidar, the primary aerosol types and their contributions to air pollution in North China were determined. The following three aerosol types can be classified: natural dust, anthropogenic aerosols, and the mixture of anthropogenic aerosols and dust (polluted dust). The classification results are basically consistent with the classification results from the cloud–aerosol lidar and infrared pathfinder satellite observations (CALIPSO) satellite measurements. The relative bias of the lidar ratio between the Raman lidar and CALIPSO is less than 25% over 90% of the cases, indicating that the CALIPSO lidar ratio selection algorithm is reasonable. The classification results show that approximately 45% of aerosols below 1.8 km are contributed by polluted dust during our one year observations. The contribution of dust increased with height, from 6% at 500 m to 28% at 1,800 m, while the contribution of anthropogenic aerosols decreased from 49% to 25%. In addition, polluted dust is the major aerosol subtype below 1.0 km in spring (over 60%) and autumn (over 70%). Anthropogenic aerosols contribute more than 75% of air pollution in summer. In winter, anthropogenic aerosols prevailed (over 80%) in the lower layer, while polluted dust (around 60%) dominated the upper layer. Our results identified the primarily aerosol types to assess the contributions of anthropogenic and natural sources to air pollution in North China, and highlight that natural dust plays a crucial role in lower-layer air pollution in spring and autumn, while controlling anthropogenic aerosols will significantly improve air quality in winter.

Improved Algorithms for Remote Sensing-Based Aerosol Retrieval during Extreme Biomass Burning Events

This study proposed an aerosol characterization process using satellites for severe biomass burning events. In general, these severely hazy cases are labeled as “undecided” or “hazy.” Because atmospheric aerosols are significantly affected by factors such as air quality, global climate change, local environmental risk, and human and biological health, efficient and accurate algorithms for aerosol retrieval are required for global satellite data processing. Our previous classification of aerosol types was based primarily on near-ultraviolet (UV) data, which facilitated subsequent aerosol retrieval. In this study, algorithms for aerosol classification were expanded to events with serious biomass burning aerosols (SBBAs). Once a biomass burning event is identified, the appropriate radiation simulation method can be applied to characterize the SBBAs. The second-generation global imager (SGLI) on board the Japanese mission JAXA/Global Change Observation Mission-Climate contains 19 channels, including red (674 nm) and near-infrared (869 nm) polarization channels with a high resolution of 1 km. Using the large-scale wildfires in Kalimantan, Indonesia in 2019 as an example, the complementarity between the polarization information and the nonpolarized radiance measurements from the SGLI was demonstrated to be effective in radiation simulations for biomass burning aerosol retrieval. The retrieved results were verified using NASA/AERONET ground-based measurements, and then compared against JAXA/SGLI/L2-version-1 products, and JMA/Himawari-8/AHI observations.

Imaging atmospheric aerosol particles from a UAV with digital holography

The lack of quantitative characterization of aerosol particles and their loading in the atmosphere is one of the greatest uncertainties in climate-change science. Improved instrumentation capable of determining the size and shape of aerosol particles is needed in efforts to reduce this uncertainty. We describe a new instrument carried by an unmanned aerial vehicle (UAV) that images free-floating aerosol particles in the atmosphere. Using digital holography, the instrument obtains the images in a non-contact manner, resolving particles larger than ten micrometers in size in a sensing volume of approximately three cubic centimeters. The instrument, called the holographic aerosol particle imager (HAPI), has the unique ability to image multiple particles freely entering its sensing volume from any direction via a single measurement. The construction of HAPI consists of 3D printed polymer structures that enable a sufficiently low size and weight that it may be flown on a commercial-grade UAV. Examples from field trials of HAPI show images of freshly emitted tree pollen and mineral dust.

Dust radiative forcing and its impact on surface energy budget over West Africa

Dust is the dominant aerosol type over West Africa (WA), and therefore accurate simulation of dust impact is critical for better prediction of weather and climate change. The dust radiative forcing (DRF) is estimated using two sets of experiments in this study: one without and the other with dust aerosol and its feedbacks with the Weather Research and Forecasting with Chemistry model (WRF-Chem). Results show that DRF presents a net warming effect at the top-of-atmosphere (TOA) and in the atmosphere (ATM), and cooling at the surface (SFC). The net DRF over WA is estimated to be 9 W/m2 at the TOA, 23 W/m2 in the ATM, and - 13 W/m2 at the SFC. Furthermore, dust-induced a reduction of sensible heat up to 24 W/m2 and SFC temperature up to 2 °C cooling over WA, an increase of latent heat up to 12 W/m2 over Sahara, a decrease up to 24 W/m2 over the vegetated surfaces and an increase in the surface energy balance up to 12 W/m2 over WA. The presence of dust significantly influences the surface energy budget over WA, suggesting that dust effects should be considered in more climate studies to improve the accuracy of climate predictions.

Five-year observation of aerosol optical properties and its radiative effects to planetary boundary layer during air pollution episodes in North China: Intercomparison of a plain site and a mountainous site in Beijing

The aerosol microphysical, optical and radiative properties of the whole column and upper planetary boundary layer (PBL) were investigated during 2013 to 2018 based on long-term sun-photometer observations at a surface site (~106 m a.s.l.) and a mountainous site (~1225 m a.s.l.) in Beijing. Raman-Mie lidar data combined with radiosonde data were used to explore the aerosol radiative effects to PBL during dust and haze episodes. The results showed size distribution exhibited mostly bimodal pattern for the whole column and the upper PBL throughout the year, except in July for the upper PBL, when a trimodal distribution occurred due to the coagulation and hygroscopic growth of fine particles. The seasonal mean values of aerosol optical depth at 440 nm for the upper PBL were 0.31 ± 0.34, 0.30 ± 0.37, 0.17 ± 0.30 and 0.14 ± 0.09 in spring, summer, autumn and winter, respectively. The single-scattering albedo at 440 nm of the upper PBL varied oppositely to that of the whole column, with the monthly mean value between 0.91 and 0.96, indicating weakly to slightly strong absorptive ability at visible spectrum. The monthly mean direct aerosol radiative forcing at the Earth's surface and the top of the atmosphere varied from -40 ± 7 to -105 ± 25 and from -18 ± 4 to -49 ± 17 W m^-2, respectively, and the maximum atmospheric heating was found in summer (~66 ± 12 W m^-2). From a radiative point of view, during dust episode, the presence of mineral dust heated the lower atmosphere, thus promoting vertical turbulence, causing more air pollutants being transported to the upper air by the increasing PBLH. In contrast, during haze episode, a large quantity of absorbing aerosols (such as black carbon) had a cooling effect on the surface and a heating effect on the upper atmosphere, which favored the stabilization of PBL and occurrence of inversion layer, contributing to the depression of the PBLH.

An Overview of Dynamic Heterogeneous Oxidations in the Troposphere

Due to the adverse effect of atmospheric aerosols on public health and their ability to affect climate, extensive research has been undertaken in recent decades to understand their sources and sinks, as well as to study their physical and chemical properties. Atmospheric aerosols are important players in the Earth’s radiative budget, affecting incoming and outgoing solar radiation through absorption and scattering by direct and indirect means. While the cooling properties of pure inorganic aerosols are relatively well understood, the impact of organic aerosols on the radiative budget is unclear. Additionally, organic aerosols are transformed through chemical reactions during atmospheric transport. The resulting complex mixture of organic aerosol has variable physical and chemical properties that contribute further to the uncertainty of these species modifying the radiative budget. Correlations between oxidative processing and increased absorptivity, hygroscopicity, and cloud condensation nuclei activity have been observed, but the mechanisms behind these phenomena have remained unexplored. Herein, we review environmentally relevant heterogeneous mechanisms occurring on interfaces that contribute to the processing of aerosols. Recent laboratory studies exploring processes at the aerosol–air interface are highlighted as capable of generating the complexity observed in the environment. Furthermore, a variety of laboratory methods developed specifically to study these processes under environmentally relevant conditions are introduced. Remarkably, the heterogeneous mechanisms presented might neither be feasible in the gas phase nor in the bulk particle phase of aerosols at the fast rates enabled on interfaces. In conclusion, these surface mechanisms are important to better understand how organic aerosols are transformed in the atmosphere affecting the environment.

Modeling the single and multiple scattering properties of soot-laden mineral dust aerosols

Fractal particle morphologies are employed to study the light scattering properties of soot-laden mineral dust aerosols. The applicability of these models is assessed in comparison with measurements and other numerical studies. To quantify the dust-soot mixing effects on the single and multiple scattering properties, a parameterization of the effective bulk properties is developed. Based on the parameterized bulk properties, polarized one-dimensional radiative transfer simulations are performed. The results indicate that small uncertainties in conjunction with soot contamination parameters may lead to large uncertainties in both forward and inverse modeling involving mineral dust contaminated with soot.

Light Absorption of Secondary Organic Aerosol: Composition and Contribution of Nitroaromatic Compounds

Secondary organic aerosol (SOA) can affect the atmospheric radiation balance through absorbing light at shorter visible and UV wavelengths. However, the composition and optical properties of light-absorbing SOA is poorly understood. In this work, SOA filter samples were collected during individual chamber experiments conducted with three biogenic and eight aromatic volatile organic compound (VOC) precursors in the presence of NO_X and H₂O₂. Compared with the SOA generated using the aromatic precursors, biogenic SOA generally exhibits negligible light absorption above 350 nm; the aromatic SOA generated in the presence of NO_X shows stronger light absorption than that generated with H₂O₂. Fifteen nitroaromatic compound (NAC) chemical formulas were identified and quantified in SOA samples. Their contributions to the light absorption of sample extracts were also estimated. On average, the m-cresol/NO_X SOA sample has the highest mass contribution from NACs (10.4 ± 6.74%, w/w), followed by naphthalene/NO_X (6.41 ± 2.08%) and benzene/NO_X (5.81 ± 3.82%) SOA. The average contributions of NACs to total light absorption were at least two times greater than their average mass contributions at 365 and 400 nm, revealing the potential use of chromophoric NACs as brown carbon (BrC) tracers in source apportionment and air quality modeling studies.

Heterogeneous Photo-oxidation of SO2 in the Presence of Two Different Mineral Dust Particles: Gobi and Arizona Dust

The impact of authentic mineral dust particles sourced from the Gobi Desert (GDD) on the kinetic uptake coefficient of SO₂ was studied under varying environments (humidity, O₃, and NO_x) using both an indoor chamber and an outdoor chamber. There was a significant increase in the kinetic uptake coefficient of SO₂ (γ_SO4^2-_,light) for GDD particles under UV light compared to the value (γ_SO4^2-_,dark) under dark conditions at various relative humidities (RH) ranging from 20% to 80%. In both the presence and the absence of O₃ and NO_x, γ_SO4^2-_,light and γ_SO4^2-_,dark greatly increased with increasing RH. The resulting γ_SO4^2-_,light of GDD particles was also compared to that of Arizona Test Dust (ATD) particles. The γ_SO4^2-_,light values of GDD were 2 to 2.5 times greater than those of ATD for all RH levels. To understand the photocatalytic act of dust particles, both GDD and ATD were characterized for the metal element composition of fresh particles, the aerosol acidity of aged particles, and the hygroscopic properties of both fresh and aged particles. We conclude that the difference in the formation of sulfate between GDD and ATD particles is regulated mainly by the quantity of the semiconductive metals in dust particles and partially by hygroscopic properties.

Solving the inverse problem for coarse-mode aerosol particle morphology with digital holography

Coarse mode atmospheric aerosol particles are abundant in agricultural, desert, and urban environments. Accurate characterisation of these particles' morphology is an important need in scientific and applied contexts, especially to advance our understanding for how such aerosols influence solar radiative forcing of the atmosphere. Elastic light scattering is a standard method to study aerosol particles in a contact-free manner, wherein measured scattering patterns are interpreted to infer particle morphology. Due in part to the absence of wave-phase information in these measurements, the inference is not unique, a difficulty generally known as the inverse problem. An alternative approach is digital holography where wave-phase information is encoded in the measurements. We show that digital holography and spatial filtering can solve the inverse problem for free-flowing aerosol particles in the sense that a measured scattering pattern can be uniquely associated with the particle size, shape, and orientation producing it.

A 20-year simulated climatology of global dust aerosol deposition

Based on a 20-year (1991-2010) simulation of dust aerosol deposition with the global climate model CAM5.1 (Community Atmosphere Model, version 5.1), the spatial and temporal variations of dust aerosol deposition were analyzed using climate statistical methods. The results indicated that the annual amount of global dust aerosol deposition was approximately 1161±31Mt, with a decreasing trend, and its interannual variation range of 2.70% over 1991-2010. The 20-year average ratio of global dust dry to wet depositions was 1.12, with interannual variation of 2.24%, showing the quantity of dry deposition of dust aerosol was greater than dust wet deposition. High dry deposition was centered over continental deserts and surrounding regions, while wet deposition was a dominant deposition process over the North Atlantic, North Pacific and northern Indian Ocean. Furthermore, both dry and wet deposition presented a zonal distribution. To examine the regional changes of dust aerosol deposition on land and sea areas, we chose the North Atlantic, Eurasia, northern Indian Ocean, North Pacific and Australia to analyze the interannual and seasonal variations of dust deposition and dry-to-wet deposition ratio. The deposition amounts of each region showed interannual fluctuations with the largest variation range at around 26.96% in the northern Indian Ocean area, followed by the North Pacific (16.47%), Australia (9.76%), North Atlantic (9.43%) and Eurasia (6.03%). The northern Indian Ocean also had the greatest amplitude of interannual variation in dry-to-wet deposition ratio, at 22.41%, followed by the North Atlantic (9.69%), Australia (6.82%), North Pacific (6.31%) and Eurasia (4.36%). Dust aerosol presented a seasonal cycle, with typically strong deposition in spring and summer and weak deposition in autumn and winter. The dust deposition over the northern Indian Ocean exhibited the greatest seasonal change range at about 118.00%, while the North Atlantic showed the lowest seasonal change at around 30.23%. The northern Indian Ocean had the greatest seasonal variation range of dry-to-wet deposition ratio, at around 74.57%, while Eurasia had the lowest, at around 12.14%.

Urban dust in the Guanzhong basin of China, part II: A case study of urban dust pollution using the WRF-Dust model

We developed a regional dust dynamical model (WRF-Dust) to simulate surface dust concentrations in the Guanzhong (GZ) basin of China during two typical dust cases (19th Aug. and 26th Nov., 2013), and compared model results with the surface measurements at 17 urban and rural sites. The important improvement of the model is to employ multiple high-resolution (0.5-500 m) remote sensing data to construct dust sources. The new data include the geographic information of constructions, croplands, and barrens over the GZ basin in summer and winter of 2013. For the first time, detailed construction dust emissions have been introduced in a regional dust model in large cities of China. Our results show that by including the detailed dust sources, model performance at simulating dust pollutions in the GZ basin is significantly improved. For example, the simulated dust concentration average for the 17 sites increases from 28 μg m(-3) to 59 μg m(-3), closing to the measured concentration of 66 μg m(-3). In addition, the correlation coefficient (r) between the calculated and measured dust concentrations is also improved from 0.17 to 0.57, suggesting that our model better presents the spatial variation. Further analysis shows that urban construction activities are the crucial source in controlling urban dust pollutions. It should be considered by policy makers for mitigating particulate air pollution in many Chinese cities.

Heterogeneous photooxidation of sulfur dioxide in the presence of airborne mineral dust particles

Heterogeneous photocatalytic oxidation of SO₂on the surface of Arizona dust particles was investigated in the absence and the presence of NOxand O₃under varying humidity using a 2-m³indoor photoirradiation chamber.

Geochemical behaviour of PM10 aerosol constituents under the influence of succeeding anticyclonic/cyclonic situations: case of Sfax City, southern Tunisia

The present study investigates the geochemical behaviour of PM10 aerosol constituents (Cl, Na, Si, Al, Ca, Fe, Mg, Mn, Pb, Zn, S) at Sfax City (Tunisia) under succeeding meteorological conditions, including short-lived anticyclonic, cyclonic and prolonged anticyclonic situations. The results revealed daily total concentrations fluctuating between 4.07 and 88.51 μg/m(3). The highest level recorded was noted to occur under the effect of the short-lived anticyclonic situation characterized by low wind speeds. It was 1.5 times higher than those recorded during cyclonic and long-lived anticyclonic situations characterized by moderate to high wind speeds. During the cyclonic situation, the marked increase of (Na and Cl) concentrations is associated with relatively high sea wind speeds (6 to 9 m/s), which are in turn responsible for a slight increase of crustal elements such as Al, Ca, Si, Fe and Mg, by the entrainment in the air of dust from roads and undeveloped areas. During the two anticyclonic situations, the simultaneous increase (due to communal transport) of crustal (Ca, Si, Al, Fe, Mg) and man-made (Mn, S, Pb, Zn) elements was noted to be associated with the dominance of terrigenious wind flows with speeds varying between 1.5 and 4 m/s. However, the significant contribution rates observed for Cl under the prevalence of such winds as compared to other crustal elements such as Fe suggested the influence of the sebkhas of Southern Tunisia.

Seasonal dynamics of threshold friction velocity and dust emission in Central Asia

An improved model representation of mineral dust cycle is critical to reducing the uncertainty of dust-induced environmental and climatic impact. Here we present a mesoscale model study of the seasonal dust activity in the semiarid drylands of Central Asia, focusing on the effects of wind speed, soil moisture, surface roughness heterogeneity, and vegetation phenology on the threshold friction velocity (u_*t ) and dust emission during the dust season of 1 March to 31 October 2001. The dust model WRF-Chem-DuMo allows us to examine the uncertainties in seasonal dust emissions due to the selection of dust emission scheme and soil grain size distribution data. To account for the vegetation effects on the u_*t , we use the Moderate Resolution Imaging Spectroradiometer monthly normalized difference vegetation index to derive the dynamic surface roughness parameters required by the physically based dust schemes of Marticorena and Bergametti (1995, hereinafter MB) and Shao et al. (1996, hereinafter Shao). We find the springtime u_*t is strongly enhanced by the roughness effects of temperate steppe and desert ephemeral plants and, to less extent, the binding effects of increased soil moisture. The u_*t decreases as the aboveground biomass dies back and soil moisture depletes during summer. The u_*t dynamics determines the dust seasonality by causing more summer dust emission, despite a higher frequency of strong winds during spring. Due to the presence of more erodible materials in the saltation diameter range of 60-200 µm, the dry-sieved soil size distribution data lead to eight times more season-total dust emission than the soil texture data, but with minor differences in the temporal distribution. On the other hand, the Shao scheme produces almost the same amount of season-total dust emission as the MB scheme, but with a strong shift toward summer due to the strong sensitivity of the u_*t to vegetation. By simply averaging the MB and Shao model experiments, we obtain a mean estimate (Exp_mean) of season-total dust emission of 255.6 Mt (megaton), of which 26.8%, 50.4%, and 22.8% are produced in spring (March-April-May), summer (June-July-August), and autumn (September-October), respectively. The Exp_mean estimate identifies the Ustyurt Plateau, dried seabed of Aral Sea (called Aralkum), Caspian Sea coast, and loess deserts as the strongest dust source areas in Central Asia. The spatial distribution and seasonality of the Exp_mean estimate are in general agreement with ground station dusty weather observations and satellite aerosol optical depth and absorbing aerosol index products. Compared to Cakmur et al. (2006), the Exp_mean estimate suggests Central Asia contributes 10-17% to the global dust emission in 2001.

KEY POINTS

The WRF-Chem-DuMo model is used to study dust seasonality in Central Asia An accurate representation of u_*t is critical for dust seasonality Multiexperiment mean dust emission estimate agrees with observations.

Scanning electron microanalysis and analytical challenges of mapping elements in urban atmospheric particles

Elemental mapping with energy-dispersive X-ray spectroscopy (EDX) associated with scanning electron microscopy is highly useful for studying internally mixed atmospheric particles. Presented is a study of individual particles from urban airsheds and the analytical challenges in qualitatively determining the composition and origin of heterogeneous urban-air particles from high-resolution elemental maps. Coarse-mode particles were taken from samples collected in three U.S. cities: Atlanta, Los Angeles, and Seattle. Elemental maps distinguished particles with heterogeneously mixed phases from those with homogeneously mixed phases that also contained inclusions or surface adducts. Elemental mapping at low and high beam energies, along with imaging at an oblique angle helped to classify particles by origin. The impact of particle shape on X-ray microanalysis was demonstrated by having the beam enter the particle at ≥ 52° from normal. Potential misinterpretations of particle composition due to artifacts in the elemental maps were minimized by tilt imaging to reveal particle surface roughness and depth, mapping at low beam energies, noting the position of the EDX detector in the map field, and assessing differences in the mass absorption coefficients of the particle's major elements to anticipate X-ray self-absorption.

Size-differentiated chemical characteristics of Asian paleo dust: records from aeolian deposition on Chinese Loess Plateau

The Chinese Loess Plateau (CLP) receives and potentially contributes to Asian dust storms that affect particulate matter (PM) concentrations, visibility, and climate. Loess on the CLP has experienced little weathering effect and is regarded as an ideal record to represent geochemical characteristics of Asian paleo dust. Samples were taken from 2-, 9-, and 15-m depths (representing deposition periods from approximately 12,000 to approximately 200,000 yr ago) in the Xi Feng loess profile on the CLP. The samples were resuspended and then sampled through total suspended particulates (TSP), PM10, PM2.5, and PM1 (PM with aerodynamic diameters < approximately 30, 10, 2.5, and 1 microm, respectively) inlets onto filters for mass, elemental, ionic, and carbon analyses using a Desert Research Institute resuspension chamber. The elements Si, Ca, Al, Fe, K, Mg, water-soluble Ca (Ca2+), organic carbon, and carbonate carbon are the major constituents (> 1%) in loess among the four PM fractions (i.e., TSP, PM10, PM2.5, and PM1). Much of Ca is water soluble and corresponds with measures of carbonate, indicating that most of the calcium is in the form of calcium carbonate rather than other calcium minerals. Most of the K is insoluble, indicating that loess can be separated from biomass burning contributions when K+ is measured. The loess has elemental abundances similar to those of the upper continental crust (UCC) for Mg, Fe, Ti, Mn, V, Cr, and Ni, but substantially different ratios for other elements such as Ca, Co, Cu, As, and Pb. These suggest that the use of UCC as a reference to represent pure or paleo Asian dust needs to be further evaluated. The aerosol samples from the source regions have similar ratios to loess for crustal elements, but substantially different ratios for species from anthropogenic sources (e.g., K, P, V, Cr, Cu, Zn, Ni, and Pb), indicating that the aerosol samples from the geological-source-dominated environment are not a "pure" soil product as compared with loess.

Reflectance spectroscopy is an effective tool for monitoring soot pollution in an urban suburb

This study examines whether converting the fossil fuel of the Tel Aviv power station from oil to gas influences air pollution in the local urban environment. To this end, the spectral properties of accumulated dust on tree leaves and paper bags were assessed before (2004) and after (2006) the conversion. The sampling site was a garden in a neighborhood located 2700m downwind of the power station. In addition, air pollution concentrations and particulate matter parameters recorded by a local meteorological station were analyzed (PM(10), NO(x), NO(2), NO, and SO(2)). Although differences in the average monthly concentration of pollution parameters are mostly insignificant between the two periods, the accumulated particulate matter exhibits considerably different spectral patterns. All first period samples exhibit a distinctly concave slope in the spectral region between 400 and 1400nm, indicative of high amounts of soot, most likely due to the combustion products of fuel oil exhausted by the power plant. In contrast, the second period samples exhibit spectra that indicate reduced soot content and even appear slightly convex, evidencing the presence of dust of mineral origin, a feature likely masked by the soot in the first period. Thus, the spectral data support that the power plant conversion results in less pollution. More generally, this study corroborates that VIS-NIR-SWIR spectroscopy characterizes key properties of the particulate layer accumulating on sampled surfaces and thus, is a powerful method for monitoring the urban environment.

Modeling optical properties of mineral aerosol particles by using nonsymmetric hexahedra

We explore the use of nonsymmetric geometries to simulate the single-scattering properties of airborne dust particles with complicated morphologies. Specifically, the shapes of irregular dust particles are assumed to be nonsymmetric hexahedra defined by using the Monte Carlo method. A combination of the discrete dipole approximation method and an improved geometric optics method is employed to compute the single-scattering properties of dust particles for size parameters ranging from 0.5 to 3000. The primary optical effect of eliminating the geometric symmetry of regular hexahedra is to smooth the scattering features in the phase function and to decrease the backscatter. The optical properties of the nonsymmetric hexahedra are used to mimic the laboratory measurements. It is demonstrated that a relatively close agreement can be achieved by using only one shape of nonsymmetric hexahedra. The agreement between the theoretical results and their measurement counterparts can be further improved by using a mixture of nonsymmetric hexahedra. It is also shown that the hexahedron model is much more appropriate than the "equivalent sphere" model for simulating the optical properties of dust particles, particularly, in the case of the elements of the phase matrix that associated with the polarization state of scattered light.