Dust Optical Properties Research Articles

Intercomparison of WRF-chem aerosol schemes during a dry Saharan dust outbreak in Southern Iberian Peninsula

The Iberian Peninsula (IP), where this study is conducted, has experienced an increase of the frequency and intensity of Saharan aerosol dust outbreaks over the latest decades, which may have an impact on its regional climate. The Weather Research and Forecasting model coupled with chemistry (WRF-chem) has been used worldwide to simulate dust outbreaks and can support the analysis of such potential impacts. However, it includes multiple alternative aerosol parameterization choices that have not been conveniently evaluated in the study region yet. Here, three of the most popular WRF-chem aerosol parameterization schemes, namely, the Goddard Chemistry Aerosol Radiation and Transport (GOCART), the Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) and the Modal Aerosol Dynamics Model for Europe (MADE) schemes, are inter-compared during a strong and dry dust outbreak on July 2021 in southern IP. The results show that the three schemes predict qualitatively similar dust intrusion patterns that are consistent with ground observations and have inter-model dust loading differences smaller than 4%. However, their average dust size distributions differ notably. While GOCART is reasonably consistent with observations, MOSAIC underpredicts the amount of dust particles with sub-micron diameters and overpredicts that of large particles and MADE does the opposite. This is found to have a strong detrimental impact on the prediction performance of dust optical properties in MOSAIC and MADE, which is related, at least partially, with issues in the required inter-sectional redistribution of dust parameters during the dust emission and calculation of optical properties. Overall, GOCART generally appears a better choice for strong and dry dust outbreak events in southern IP. It remains to be evaluated during wet dust outbreaks, which is a work underway.

Large-Scale Network-Based Observations of a Saharan Dust Event across the European Continent in Spring 2022

Between 14 March and 21 April 2022, an extensive investigation of an extraordinary Saharan dust intrusion over Europe was performed based on lidar measurements obtained by the European Aerosol Research Lidar Network (EARLINET). The dust episode was divided into two distinct periods, one in March and one in April, characterized by different dust transport paths. The dust aerosol layers were studied over 18 EARLINET stations, examining aerosol characteristics during March and April in four different regions (M-I, M-II, M-III, and M-IV and A-I, A-II, A-III, and A-IV, respectively), focusing on parameters such as aerosol layer thickness, center of mass (CoM), lidar ratio (LR), particle linear depolarization ratio (PLDR), and Ångström exponents (ÅE). In March, regions exhibited varying dust geometrical and optical properties, with mean CoM values ranging from approximately 3.5 to 4.8 km, and mean LR values typically between 36 and 54 sr. PLDR values indicated the presence of both pure and mixed dust aerosols, with values ranging from 0.20 to 0.32 at 355 nm and 0.24 to 0.31 at 532 nm. ÅE values suggested a range of particle sizes, with some regions showing a predominance of coarse particles. Aerosol Optical Depth (AOD) simulations from the NAAPS model indicated significant dust activity across Europe, with AOD values reaching up to 1.60. In April, dust aerosol layers were observed between 3.2 to 5.2 km. Mean LR values typically ranged from 35 to 51 sr at both 355 nm and 532 nm, while PLDR values confirmed the presence of dust aerosols, with mean values between 0.22 and 0.31 at 355 nm and 0.25 to 0.31 at 532 nm. The ÅE values suggested a mixture of particle sizes. The AOD values in April were generally lower, not exceeding 0.8, indicating a less intense dust presence compared to March. The findings highlight spatial and temporal variations in aerosol characteristics across the regions, during the distinctive periods. From 15 to 16 March 2022, Saharan dust significantly reduced UV-B radiation by approximately 14% over the ATZ station (Athens, GR). Backward air mass trajectories showed that the dust originated from the Western and Central Sahara when, during this specific case, the air mass trajectories passed over GRA (Granada, ES) and PAY (Payerne, CH) before reaching ATZ, maintaining high relative humidity and almost stable aerosol properties throughout its transport. Lidar data revealed elevated aerosol backscatter (baer) and PLDR values, combined with low LR and ÅE values, indicative of pure dust aerosols.

Seeing the Unseen: A Method to Detect Unresolved Rings in Protoplanetary Disks

While high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations reveal a wealth of substructure in protoplanetary disks, they remain incapable of resolving the types of small-scale dust structures predicted, for example, by numerical simulations of the streaming instability. In this article, we propose a method to find evidence for unresolved, optically thick dusty rings in protoplanetary disks. We demonstrate that, in presence of unresolved rings, the brightness of an inclined disk exhibits a distinctive emission peak at the minor axis. Furthermore, the azimuthal brightness depends on both the geometry of the rings and the dust optical properties; we can therefore use the azimuthal brightness variations to both detect unresolved rings and probe their properties. By analyzing the azimuthal brightness in the test case of ringlike substructures formed by streaming instability, we show that the resulting peak is likely detectable by ALMA for typical disk parameters. Moreover, we present an analytic model that not only qualitatively but also quantitatively reproduces the peak found in the simulations, validating its applicability to infer the presence of unresolved rings in observations and characterize their optical properties and shape. This will contribute to the identification of disk regions where streaming instability (and thus planet formation) is occurring.

Sensitivities of Spectral Optical Properties of Dust Aerosols to Their Mineralogical and Microphysical Properties

AbstractDust aerosol is a key component in global radiative forcing and climate modeling. We developed a mineralogy‐resolved spectral optical property model for dust aerosols. The model incorporates nine mineral groups and applies the effective medium methods to obtain the spectral complex refractive indices of inhomogeneous dust particles. This study considers 5,000 sets of dust mineral composition mixtures along with the use of the bimodal particle size distribution. We calculate the bulk properties of these samples in a wide spectral range from 0.2 to 50 μm using a comprehensive database of hexahedral dust single‐scattering properties. Through sensitivity analyses, this study reveals that the illite and hematite components substantially affect the optical properties of dust. Furthermore, the effective radii of both the fine‐mode and the coarse‐mode dust particles have significant impacts on the bulk optical properties, albeit to varying degrees.

A sensitivity study on radiative effects due to the parameterization of dust optical properties in models

Abstract. Most of the dust models underestimate the load of the large dust particles, consider spherical shapes instead of irregular ones, and have to deal with a wide range of the dust refractive index (RI) to be used. This leads to an incomplete assessment of the dust radiative effects and dust-related impacts on climate and weather. The current work aims to provide an assessment, through a sensitivity study, of the limitations of models to calculate the dust direct radiative effect (DRE) due to the underrepresentation of its size, RI, and shape. We show that the main limitations stem from the size and RI, while using a more realistic shape plays only a minor role, with our results agreeing with recent findings in the literature. At the top of the atmosphere (TOA) close to dust sources, the underestimation of size issues an underestimation of the direct warming effect of dust of ∼ 18–25 W m−2, for DOD = 1 (dust optical depth) at 0.5 µm, depending on the solar zenith angle (SZA) and RI. The underestimation of the dust size in models is less above the ocean than above dust sources, resulting in an underestimation of the direct cooling effect of dust above the ocean by up to 3 W m−2, for aerosol optical depth (AOD) of 1 at 0.5 µm. We also show that the RI of dust may change its DRE by 80 W m−2 above the dust sources and by 50 W m−2 at downwind oceanic areas for DOD = 1 at 0.5 µm at TOA. These results indicate the necessity of including more realistic sizes and RIs for dust particles in dust models, in order to derive better estimations of the dust DRE, especially near the dust sources and mostly for studies dealing with local radiation effects of dust aerosols.

Dust optical properties observed by high-spectral-resolution lidar in western Japan

Dust optical properties observed by high-spectral-resolution lidar in western Japan

Flexible implementation of the particle shape and internal inhomogeneity in the invariant imbedding T-matrix method.

We report a new implementation of the invariant imbedding T-matrix (IITM) method based on a discrete spherical grid approach for representing the particle shape and internal inhomogeneity. The new version of the IITM (referred to as the IITM-discrete) improves the flexibility of the IITM-especially for inhomogeneous particles. It is much more convenient for specifying the particle morphology in the electromagnetic wave scattering simulations. Particle shape is represented by a series of discrete spherical layers ranging from the inscribed sphere to the circumscribed sphere. Spherical layers are discretized by the centroidal Voronoi tessellation (CVT) approach. The procedure of computing the U-matrix (the only shape-dependent module in the T-matrix program) is simplified upon using the gridded particle shape and refractive index information saved in an external file. The grid resolution is a key factor that determines the numerical accuracy and computational cost. Numerical tests of IITM-discrete show its compatibility with other light scattering methods. Using IITM-discrete, we found that the internal inhomogeneity could have large impact on dust optical properties.

Synthetic photometry for carbon-rich giants

Context. The properties and the evolution of asymptotic giant branch (AGB) stars are strongly influenced by their mass loss through a stellar wind. This, in turn, is believed to be caused by radiation pressure due to the absorption and scattering of the stellar radiation by the dust grains formed in the atmosphere. The optical properties of dust are often estimated using the small particle limit (SPL) approximation, and it has been used frequently in modelling AGB stellar winds when performing radiation-hydrodynamics (RHD) simulations. Aims. We aim to investigate the effects of replacing the SPL approximation by detailed Mie calculations of the size-dependent opacities for grains of amorphous carbon forming in C-rich AGB star atmospheres. Methods. We performed RHD simulations for a large grid of carbon star atmosphere+wind models with different effective temperatures, luminosities, stellar masses, carbon excesses, and pulsation properties. Also, a posteriori radiative transfer calculations for many radial structures (snapshots) of these models were done, resulting in spectra and filter magnitudes. Results. We find that, when giving up the SPL approximation, the wind models become more strongly variable and more dominated by gusts, although the average mass-loss rates and outflow speeds do not change significantly; the increased radiative pressure on the dust throughout its formation zone does, however, result in smaller grains and lower condensation fractions (and thus higher gas-to-dust ratios). The photometric K magnitudes are generally brighter, but at V the effects of using size-dependent dust opacities are more complex: brighter for low mass-loss rates and dimmer for massive stellar winds. Conclusions. Given the large effects on spectra and photometric properties, it is necessary to use the detailed dust optical data instead of the simple SPL approximation in stellar atmosphere+wind modelling where dust is formed.

The impact of using assimilated Aeolus wind data on regional WRF-Chem dust simulations

Abstract. Land–atmosphere interactions govern the process of dust emission and transport. An accurate depiction of these physical processes within numerical weather prediction models allows for better estimating the spatial and temporal distribution of the dust burden and the characterisation of source and recipient areas. In the presented study, the ECMWF-IFS (European Centre for Medium-Range Weather Forecast – Integrated Forecasting System) outputs, produced with and without the assimilation of Aeolus quality-assured Rayleigh–clear and Mie–cloudy horizontal line-of-sight wind profiles, are used as initial or boundary conditions in the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate 2-month periods in the spring and autumn of 2020, focusing on a case study in October. The experiments have been performed over the broader eastern Mediterranean and Middle East (EMME) region, which is frequently subjected to dust transport, as it encompasses some of the most active erodible dust sources. Aerosol- and dust-related model outputs (extinction coefficient, optical depth and concentrations) are qualitatively and quantitatively evaluated against ground- and satellite-based observations. Ground-based columnar and vertically resolved aerosol optical properties are acquired through AERONET sun photometers and PollyXT lidar, while near-surface concentrations are taken from EMEP. Satellite-derived vertical dust and columnar aerosol optical properties are acquired through LIVAS (LIdar climatology of Vertical Aerosol Structure) and MIDAS (ModIs Dust AeroSol), respectively. Overall, in cases of either high or low aerosol loadings, the model predictive skill is improved when WRF-Chem simulations are initialised with the meteorological fields of Aeolus wind profiles assimilated by the IFS. The improvement varies in space and time, with the most significant impact observed during the autumn months in the study region. Comparison with observation datasets saw a remarkable improvement in columnar aerosol optical depths, vertically resolved dust mass concentrations and near-surface particulate concentrations in the assimilated run against the control run. Reductions in model biases, either positive or negative, and an increase in the correlation between simulated and observed values was achieved for October 2020.

Simultaneous profiling of dust aerosol mass concentration and optical properties with polarized high-spectral-resolution lidar

Dust particles originating from arid desert regions can be transported over long distances, presenting severe risks to climate, environment, social economics, and human health at the source and downwind regions. However, there has been a dearth of continuous diurnal observations of vertically resolved mass concentration and optical properties of dust aerosols, which hinders our understanding of aerosol mixing, stratification, aerosol-cloud interactions, and their impacts on the environment. To fill the gap of the insufficient observations, to the best of our knowledge, this work presents the first high-spectral-resolution lidar (HSRL) observation providing days of continuous profiles of the mass concentration, along with particle linear depolarization ratio (PLDR), backscattering coefficient, extinction coefficient and lidar ratio (LR), simultaneously. We present the results of two strong dust events observed by HSRL over Beijing in 2021. The maximum particle mass concentrations reached (1.52 ± 3.5) x103 μg/m3 and (19.48 ± 0.36) x103 μg/m3 for the two dust events, respectively. The retrieved particle mass concentrations and aerosol optical depth (AOD) agree well with the observation from the surface PM10 concentrations and sun photometer with correlation coefficients of 0.90 and 0.95, respectively. The intensive properties of PLDR and LR of the dust aerosols are 0.31 ± 0.02 and 39 ± 7 sr at 532 nm, respectively, which are generally close to those obtained from observations in the downwind areas. Moreover, inspired by the observations from HSRL, a universal analytical relationship is discovered to evaluate the proportion of dust aerosol backscattering, extinction, AOD, and mass concentration using PLDR. The universal analytical relationship reveals that PLDR can directly quantify dust aerosol contribution, which is expected to further expand the application of polarization technology in dust detection. These valuable observations and findings further our understanding of the contribution of dust aerosol to the environment and help supplement dust aerosol databases.

Size-resolved dust direct radiative effect efficiency derived from satellite observations

Abstract. The role of mineral dust aerosol in the global radiative energy budget is often quantified by the dust direct radiative effect (DRE). The dust DRE strongly depends on dust aerosol optical depth (DAOD), therefore, DRE efficiency (DREE = DRE / DAOD) is widely compared across different studies to eliminate differences due to the various dust loads. Nevertheless, DREE is still influenced by the uncertainties associated with dust particle size distribution (PSD) and optical properties. In this study, we derive a global clear-sky size-resolved DREE dataset in both shortwave (SW) and longwave (LW) at top of the atmosphere (TOA) and surface based on satellite observations (i.e., satellite-retrieved dust extinction spatial and vertical distributions). In the DREE dataset, dust geometric diameter from 0.1 to 100 µm is divided into 10 bins and the corresponding monthly mean DREE (with respect to DAOD at 532 nm) for each size bin is derived by using the Rapid Radiative Transfer Model (RRTM). Three sets of state of the art dust refractive indices (RI) and two sets of dust shape models (sphere vs. spheroid) are adopted to investigate the sensitivity of dust DREE to dust absorption and shape. As a result, the size-resolved dust DREE dataset contains globally distributed monthly mean dust DREE at TOA and surface for each of 10 size bins with 5∘ (longitude) ×2∘ (latitude) resolution as well as for each dust RI and shape combination. The size-resolved dust DREE dataset can be used to readily calculate global dust DRE for any DAOD and dust PSD, including the uncertainty in the DRE induced by dust microphysical properties, (e.g., dust PSD, RI and shape). By calculating dust DRE based on DAOD climatology retrieved from different satellite sensors and based on different dust PSD, we find that uncertainty in the spatial pattern of DAOD induces more than 10 % of the uncertainty in SW dust DRE at TOA. The observation-based dust PSD induces around 15–20 % uncertainty in dust DRE at TOA and in the atmosphere. The sensitivity assessments of dust DRE to dust RI and shape further suggest that dust nonsphericity induces a negligible effect on dust DRE estimations, while dust RI turns out to be the most important factor in determining dust DRE, particularly in SW.

Dust transport and advection measurement with spaceborne lidars ALADIN and CALIOP and model reanalysis data

Abstract. In this paper, a long-term large-scale Saharan dust transport event which occurred between 14 and 27 June 2020 is tracked with the spaceborne lidars ALADIN (Atmospheric Laser Doppler Instrument) and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) together with ECMWF (European Centre for Medium-Range Forecasts) and HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory model) analysis. We evaluate the performance of ALADIN and CALIOP on the observations of dust optical properties and wind fields and explore the possibility of tracking the dust events and calculating the dust mass advection with the combination of satellite and model data. The dust plumes are identified with the AIRS/Aqua Dust Score Index and with the vertical feature mask product from CALIOP. The emission, dispersion, transport and deposition of the dust event are monitored using the data from AIRS/Aqua, CALIOP and HYSPLIT. With the quasi-synchronized observations by ALADIN and CALIOP, combined with the wind field and relative humidity, the dust advection values are calculated. From this study, it is found that the dust event generated on 14 and 15 June 2020 from the Sahara in North Africa dispersed and moved westward over the Atlantic Ocean, finally being deposited in the western Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped in the northeasterly trade-wind zone between latitudes of 5∘ and 30∘ N and altitudes of 0 and 6 km. Aeolus provided the observations of the dynamics of this dust transport event in the Saharan Air Layer (SAL). From the measurement results on 19 June 2020, the dust plumes are captured quasi-simultaneously over the emission region (Western Sahara), the transport region (middle Atlantic) and the deposition region (western Atlantic) individually, which indicates that the dust plume area over the Atlantic on the morning of this day is quite enormous and that this dust transport event is massive and extensive. The quasi-synchronization observation results of 15, 16, 19, 24 and 27 June by ALADIN and CALIOP during the entire transport process show good agreement with the Dust Score Index data and the HYSPLIT trajectories, which indicates that the transport process of the same dust event is tracked by ALADIN and CALIOP, verifies that the dust transport spent around 2 weeks from the emission to the deposition and achieved the respective observations of this dust event's emission phase, development phase, transport phase, descent phase and deposition phase. Finally, the advection values for different dust parts and heights on 19 June and on the entire transport routine during transportation are computed. On 19 June, the mean dust advection values are about 1.91±1.21 mg m−2 s−1 over the emission region, 1.38±1.28 mg m−2 s−1 over the transport region and 0.75±0.68mgm-2s-1 over the deposition region. In the whole lifetime of the dust event, the mean dust advection values were about 1.51±1.03mgm-2s-1 on 15 June 2020, 2.19±1.72mgm-2s-1 on 16 June 2020, 1.38±1.28mgm-2s-1 on 19 June 2020, 1.60±1.08mgm-2s-1 on 24 June 2020 and 1.03±0.60mgm-2s-1 on 27 June 2020. During the dust development stage, the mean advection values gradually increased and reached their maximum on 16 June with the enhancement of the dust event. Then, the mean advection values decreased during the transport and the deposition of the dust over the Atlantic Ocean, the Americas and the Caribbean Sea.

Vertical Structure of Dust Aerosols Observed by a Ground-Based Raman Lidar with Polarization Capabilities in the Center of the Taklimakan Desert

The vertical structure of dust properties in desert sources is crucial for evaluating their long-range transportation and radiative forcing. To investigate vertical profiles of dust optical properties in the Taklimakan Desert, we conducted ground-based polarization Raman lidar measurements in Tazhong (83.39°E, 38.58°N, 1103 m above sea level), located at the center of the Taklimakan Desert in the summer of 2019. The lidar system developed by Lanzhou University for continuous network observation is capable of measuring polarization at 532 and 355 nm and detecting Raman signals at 387, 407, and 607 nm. The results indicate that dust aerosols in the central Taklimakan Desert were regularly lifted over 6 km during the summer with a mass concentration of 400–1000 µg m−3, while the majority of the dust remained restricted within 2 km. Moreover, the height of the boundary layer can reach 5–6 km in the afternoon under the strong convention. Above 3 km, dust is composed of finer particles with an effective radius (Reff.) less than 3 μm and a Ångström exponent (AE) related to the extinction coefficient (AEE)532,355 greater than 4; below 3 km, however, dust is dominated by coarser particles. In addition, the particle depolarization ratios (PDR) of Taklimakan dust are 0.32 ± 0.06 at 532 nm and 0.27 ± 0.04 at 355 nm, while the lidar ratios (LRs) are 49 ± 19 sr at 532 nm and 43 ± 12 sr at 355 nm. This study firstly provides information on dust vertical structure and its optical properties in the center of the desert, which may aid in further evaluating their associated impacts on the climate and ecosystem.

15-Year Analysis of Direct Effects of Total and Dust Aerosols in Solar Radiation/Energy over the Mediterranean Basin

The direct radiative effects of atmospheric aerosols are essential for climate, as well as for other societal areas, such as the energy sector. The goal of the present study is to exploit the newly developed ModIs Dust AeroSol (MIDAS) dataset for quantifying the direct effects on the downwelling surface solar irradiance (DSSI), induced by the total and dust aerosol amounts, under clear-sky conditions and the associated impacts on solar energy for the broader Mediterranean Basin, over the period 2003–2017. Aerosol optical depth (AOD) and dust optical depth (DOD) derived by the MIDAS dataset, along with additional aerosol and dust optical properties and atmospheric variables, were used as inputs to radiative transfer modeling to simulate DSSI components. A 15-year climatology of AOD, DOD and clear-sky global horizontal irradiation (GHI) and direct normal irradiation (DNI) was derived. The spatial and temporal variability of the aerosol and dust effects on the different DSSI components was assessed. Aerosol attenuation of annual GHI and DNI were 1–13% and 5–47%, respectively. Over North Africa and the Middle East, attenuation by dust was found to contribute 45–90% to the overall attenuation by aerosols. The GHI and DNI attenuation during extreme dust episodes reached 12% and 44%, respectively, over particular areas. After 2008, attenuation of DSSI by aerosols became weaker mainly because of changes in the amount of dust. Sensitivity analysis using different AOD/DOD inputs from Copernicus Atmosphere Monitoring Service (CAMS) reanalysis dataset revealed that using CAMS products leads to underestimation of the aerosol and dust radiative effects compared to MIDAS, mainly because the former underestimates DOD.

Polarization Lidar Measurements of Dust Optical Properties at the Junction of the Taklimakan Desert–Tibetan Plateau

Previous studies have shown that dust aerosols may accelerate the melting of snow and glaciers over the Tibetan Plateau. To investigate the vertical structure of dust aerosols, we conducted a ground-based observation by using multi-wavelength polarization lidar which is designed for continuous network measurements. In this study, we used the lidar observation from September to October 2020 at the Ruoqiang site (39.0°N, 88.2°E; 894 m ASL), located at the junction of the Taklimakan Desert–Tibetan Plateau. Our results showed that dust aerosols can be lifted up to 5 km from the ground, which is comparable with the elevation of the Tibetan Plateau in autumn with a mass concentration of 400–900 μg m−3. Moreover, the particle depolarization ratio (PDR) of the lifted dust aerosols at 532 nm and 355 nm are 0.34 ± 0.03 and 0.25 ± 0.04, respectively, indicating the high degree of non-sphericity in shape. In addition, extinction-related Ångström exponents are very small (0.11 ± 0.24), implying the large values in size. Based on ground-based lidar observation, this study proved that coarse non-spherical Taklimakan dust with high concentration can be transported to the Tibetan Plateau, suggesting its possible impacts on the regional climate and ecosystem.

Radiative Transfer Modeling of an SN 1987A Light Echo—AT 2019xis

Abstract We use a Monte Carlo radiative transfer model (MCRTM) to simulate the UBVRI light curves, images, and linear polarization of a light echo from supernova SN 1987A in the Large Magellanic Cloud (LMC) using various dust cloud shapes, sizes, and optical properties. We compare the theoretical simulations to the observations of AT 2019xis, a light echo detected at a large angular distance (4.′05) from SN 1987A. We estimate the size and optical thickness of the dust cloud based on the simulation results and the observations of the Optical Gravitational Lensing Experiment (OGLE-IV) Transient Detection System (OTDS) I-band light curve. The mass of the dust cloud is calculated using the estimated size, optical thickness, and extinction coefficient. If the dust cloud is assumed to correspond to a gas-to-dust ratio of 300, the total mass of the dust cloud is approximately 7.8–9.3 M ⊙. Based on these theoretical models, we show that the morphological shapes of the light echoes in the wavelength range of or shorter than the U band to be very different from those in the longer wavelength bands, and the difference carries important information on the early UV radiation of SN 1987A.

First retrieval of absorbing aerosol height over dark target using TROPOMI oxygen B band: Algorithm development and application for surface particulate matter estimates

Constraint of the vertical distribution of aerosol particles is crucial for the study of aerosol plume structure, aerosol radiative effects, and ultimately monitoring surface air pollution. We developed an algorithm to retrieve the aerosol optical central height (AOCH) of absorbing aerosols by using, for the first time, the oxygen (O2) A and B absorption band measurements from the TROPOspheric Monitoring Instrument (TROPOMI) over dark targets. For the retrieval, narrow band radiance at seven channels ranging from ultraviolet (UV) to shortwave infrared (SWIR) are convolved from TROPOMI hyperspectral measurements. Subsequently, cloudy pixels are screened out by using the slope of spectral reflectance, while aerosol types (dust and smoke) are classified by the wavelength dependence of aerosol path radiance in conjunction with UV aerosol index. Surface reflectance over land is derived from the MODIS surface bi-directional reflectance climatology, and over water from the GOME-2 surface Lambert-equivalent reflectivity (LER) database. The aerosol optical depth (AOD) and AOCH are retrieved through an approach of look-up-table accounting for AERONET-based dust and smoke optical properties. For multiple smoke and dust plume events around the world, our retrieved AOCH values agree with space-borne lidar CALIOP counterparts, with a mean bias of <0.15 km and a correlation coefficient of 0.85–0.87. Due in part to adding the O2 B band, our retrieval represents an aerosol extinction peak height better than the TROPOMI operational Level 2 aerosol layer height retrieved from only the O2 A band. The latter shows 0.5–2 km low bias, especially over land. Finally, the high potential of AOCH for improving surface PM2.5 estimates is also illustrated with a case study in which the high bias of surface PM2.5 in MERRA-2 data is corrected after being scaled by the retrieved AOCH.

Vertical assessment of the mineral dust optical and microphysical properties as retrieved from the synergy between polarized micro-pulse lidar and sun/sky photometer observations using GRASP code

The potential of the synergetic use of both polarized Micro-Pulse Lidar (P-MPL) and Sun/sky photometer observations by using the GRASP (Generalized Retrieval of Atmosphere and Surface Properties) code for retrieving dust properties has been analysed in this work. Both the columnar and height-resolved variables have been evaluated in comparison, respectively, with AERONET data and vertical P-MPL-derived profiles of the dust optical and microphysical properties over a mid-latitude site. The linear regression analysis together with the mean fractional bias values and relative differences between datasets have been used as the statistical proxies for assessing the degree of agreement by examining 24 selected dust cases. Regarding columnar variables, the microphysical properties (both the particle volume concentration and effective radius) presented a high confidence level between GRASP and AERONET retrievals by taking into account all the proxies. In addition, the single scattering albedo and the real refractive index presented a good confidence level at all the wavelengths, as well, unlike the imaginary refractive index, which was confident at 440 nm only. By analysing the height-resolved properties of the total particle backscatter coefficient and the total volume concentration, the degree of agreement shows a high confidence between GRASP retrievals and P-MPL-derived variables, which were retrieved by using an alternative, well-validated, polarization-based methodology. Indeed, these results can be essential for application with no-polarized systems (e.g., ceilometers, lidars …). Overall, the synergy between P-MPL and Sun/sky photometer measurements regarding the usual co-location of both MPLNET and AERONET networks in combination with GRASP code represent an improved approach to derive the height-resolved optical and microphysical properties for mineral dust particles with a high degree of confidence.

AzTEC survey of the central molecular zone: increasing spectral index of dust with density

ABSTRACT The central molecular zone (CMZ) of our Galaxy hosts an extreme environment analogous to that found in typical starburst galaxies in the distant Universe. In order to understand dust properties in environments like our CMZ, we present results from a joint spectral energy distribution analysis of our AzTEC/Large Millimeter Telescope survey, together with existing Herschel far-IR data on the CMZ, from a wavelength range of 160 $\mu {\rm m}$ to 1.1 mm. We include global foreground and background contributions in a novel Bayesian modelling that incorporates the point spread functions of the different maps, which enables the full utilization of our high-resolution (10.5 arcsec) map at 1.1 mm and reveals unprecedentedly detailed information on the spatial distribution of dusty gas across the CMZ. There is a remarkable trend of increasing dust spectral index β, from 2.0 to 2.4, towards dense peaks in the CMZ, indicating a deficiency of large grains or a fundamental change in dust optical properties. This environmental dependence of β could have a significant impact on the determination of dust temperature in other studies. Depending on how the optical properties of dust deviate from the conventional model, dust temperatures could be underestimated by $10\!-\!50{{\ \rm per\ cent}}$ in particularly dense regions.

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.