Aerosol optical depth (τ) is routinely reported at wavelengths that are not directly measured by ground-based sun photometers, in particular at 550 nm for satellite validation and at longer wavelengths for short-wave infrared applications. These values are typically obtained by spectral interpolation or extrapolation, most often using linear or quadratic regressions in logarithmic space. However, the uncertainty structure of such regressions is frequently treated incorrectly, because measurement uncertainties in τ are absolute and approximately wavelength-independent in linear space, and therefore become wavelength-dependent in logarithmic space. As a result, the measurement covariance matrix must be explicitly accounted for in log–log regression, although this is rarely done in practice. This study provides both a formal and a practical framework for estimating τ at non-measured wavelengths together with its associated uncertainty. A rigorous formulation is presented for linear and quadratic regression in logarithmic space, including the propagation of random and systematic (bias-related) errors from the original spectral measurements to the interpolated or extrapolated wavelength.Sensitivity analyses based on synthetic aerosol optical depth spectra generated with the GRASP forward model are used to compare six different approaches for deriving τ(550) and τ(2000), including linear and quadratic regressions over different spectral ranges as well as the GRASP-AOD method. When the covariance matrix is treated correctly, quadratic log–log regression is found to be the most robust method for estimating τ(550), and its results become essentially independent of the chosen spectral range. In contrast, when the covariance matrix is neglected, the same regression becomes highly sensitive to the selected wavelengths, and artificially improved performance is obtained when restricting the fit to the central AERONET channels. These findings are confirmed using real AERONET observations. When the full covariance treatment is applied, differences between estimates obtained using different spectral ranges remain below 0.002 at all sites analysed. When it is ignored, root-mean-square differences exceeding 0.01 are observed at sites dominated by fine-mode aerosols.Finally, the uncertainty propagation framework is applied to real data and shows that the uncertainty of interpolated τ follows the expected Beer–Lambert law governing sun-photometer measurements, scaling with optical air mass. This provides an independent validation of the formal error model. Overall, this work establishes a consistent methodology for spectral interpolation and extrapolation of τ, ensuring both accurate values and physically meaningful uncertainties for satellite validation and related applications.

The TRANSAMA campaign (Transit to AMARYLLIS-AMAGAS oceanographic cruise), conducted aboard the research vessel Marion Dufresne II assessed instrument performance and investigated aerosol properties during its transit from La Reunion Island to Barbados (April-May 2023). A set of remote sensing instruments, including two CE318-T Sun-sky-lunar photometers and a CE370 single-wavelength elastic lidar, was deployed under the MAP-IO (Marion Dufresne Atmospheric Program-Indian Ocean) framework. Performance assessments of the deployed instrumentation support the development of coupled lidar-photometer systems for shipborne atmospheric observations, while acknowledging current detection limits. Synergistic observations provided vertically resolved aerosol properties, such as extinction coefficients, alongside atmospheric structure, highlighting the marine boundary layer (MBL) top at 800 +/- 300 m. While the photometer observations revealed clean atmospheric conditions over the South Atlantic (AOD440=0.08 +/- 0.04), thin aerosol layers above the MBL were identified as long-range transported residual biomass-burning-urban aerosols from Southern Africa with effective LR of 33 +/- 12 sr. Cloud layers covering a large range of altitudes (up to 16 km) were observed in 53 % of the lidar profiles, with a higher frequency at lower altitudes, where aerosol layers were more frequently detected. These findings emphasize the impact of continental aerosols on remote oceanic regions and demonstrate the capabilities of synergistic lidar-photometer measurements for advancing our understanding of aerosol variability, cloud formation, and climate processes over the oceans.

Atmospheric Research Expedition to Abu Dhabi (AREAD) is a ship campaign that sailed from Vigo, Spain to Abu Dhabi, UAE between 26/11/2022 and 19/12/2022, aimed at the characterization of the atmospheric composition and at identifying pollutants transport over the Mediterranean, Red Sea and Arabian Sea. In this study, we present preliminary results from the remote sensing observations acquired during the cruise. The area of interest, surrounded by deserts and anthropogenic sources, has been recognized as a climate change hotspot due to extreme temperature increases and an important contribution to greenhouse gas emissions. Limited studies focus on the area because of limited and few observational data are available.AREAD’s main objective was to contribute to the knowledge of trace gases and aerosol concentrations in the region and to complement with wintertime observations the AQABA campaign (24/06/2017-03/09/2017) performed in the same region during the summer season. The research vessel’s voyage included observations in the Suez Canal, one of the most heavily used navigational hubs in global trade routes. The on-board instrumentation included in-situ observations for trace gases (NO, NO2, O3, SO2, CO2, CH4) and aerosol optical, physical and chemical properties (PM1, PM10, particle sizes, aerosol spectral absorption and scattering). In addition, remote sensing of aerosol, clouds and boundary layer height (BL) was obtained with a VAISALA CL51 ceilometer and an automatic ship-photometer (CIMEL CE318T, modified for marine applications) included in AERONET.The preliminary results suggest a change of regime between the Mediterranean and the Suez Canal. AOD levels remained below 0.1 for the first part of the cruise and increased to more than 0.2 after the entrance of the ship into the Suez Canal. Even though there was no significant variation in BL height which remained below 1km for most of the cruise, increased particle backscatter is observed within the BL and in elevated layers after the Suez Canal. Desert dust, trade ship emissions and pollution from Middle East fossil energy production plants could be some of the species contributing to the higher aerosol loading observed in the latter leg of the cruise. Acknowledgment: The datasets presented in this research were acquired during the Atmospheric Research Expedition to Abu Dhabi (AREAD) on-board research vessel Jaywun, operated by the Environment Agency of Abu Dhabi (EAD), which is gratefully acknowledged. The ship-photometer is developed in the frame of AGORA-Lab joint laboratory (Laboratoire d’Optique Atmospherique from CNRS/University of Lille and CIMEL Electronique company).

The presentation discusses the approaches to model aerosol properties realized in GRASP (Generalized Retrieval of Aerosol and Surface Properties) algorithm (Dubovik et al., 2021). GRASP algorithm is developed based heritage of earlier efforts on the AERONET retrieval development (Dubovik and King, 2000, Dubovik et al., 2000) with idea to use the same algorithm for different applications. Thus, at present, GRASP is a versatile algorithm that could be applied diverse observations including laboratory, passive and active remote sensing measurements from ground and space. All those observations have different sensitivities to details of aerosol properties and, therefore, analysis of each type of observations requires adequate approach to model aerosol properties. For example, aerosol model used for interpretation of AERONET observations includes many more paraments than aerosol model used for interpretation of satellite observations from single view satellite imager such a MODIS or OLCI. At the same time, the aerosol models used for different observations should be consistent and compatible. Following this concept, GRASP aerosol forward model can be adequately adjusted for applications to very different observations ranging from in situ nephelometers, ground-based AERONET radiometers to satellite polarimetric and radiometric imagers, as well as, to ground-based and satellite lidar observations. The aerosol model describes all aerosol properties: size distribution, complex index of refraction, or composition, particle shape, rules of for several component mixing, vertical profile description, etc. The presentation overviews historical evolution of all details of aerosol model, the assumptions made for adaptation to different observations of different types of and the rational used for the current specific design od aerosol model. Finaly, the comparative analysis will be done to outline the differences and agreements with other common approaches used in aerosol remote sensing. Dubovik, O., A. Smirnov, B. N. Holben, etc., “Accuracy assessments of aerosol optical properties retrieved from AERONET Sun and sky-radiance measurements”, J. Geophys. Res.,105, 9791-9806, https://doi.org/10.1029/2000JD900040, 2000.Dubovik, O. and M. D. King, “A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements”, J. Geophys. Res., 105, 20,673-20,696, https://doi.org/10.1029/2000JD900282, 2000.Dubovik, O., D. Fuertes, P. Litvinov, et al. , “A Comprehensive Description of Multi-Term LSM for Applying Multiple a Priori Constraints in Problems of Atmospheric Remote Sensing: GRASP Algorithm, Concept, and Applications”, Front. Remote Sens. 2:706851. doi: 10.3389/frsen.2021.706851, 2021.

Abstract. The Earth's oceans play a critical role in regulating the global climate and atmospheric processes, with marine aerosols significantly influencing weather patterns, air quality, and climate dynamics. Despite extensive land-based aerosol monitoring through networks like AERONET (AErosol RObotic NETwork), marine aerosol characterization remains a critical gap, due in part to the logistical challenges of conducting measurements in remote oceanic environments. To address this, robust, automated, and precise monitoring systems adapted for research vessels are essential. This study reports on the first three years (July 2021 to June 2024) of continuous aerosol optical depth (AOD) measurements collected aboard the R.V. Marion Dufresne using a ship-adapted CIMEL 318-T automatic photometer in the frame of MAP-IO (Marion Dufresne Atmospheric Program – Indian Ocean) program. The dataset comprises over 25,000 quality-assured AOD measurements, primarily from the South-West Indian Ocean region, revealing mid-range AOD and Angström exponent values consistent with previous studies. The reliability and precision of the system were validated through dual-instrument comparisons conducted during the Amaryllis-Amagas/Transama campaign, yielding strong correlations (R > 0.96 for different wavelengths) and low root-mean-squared errors (RMSE < 0.01), within the expected error margins for AERONET ground-based sites, and benefiting from the continue tracking system implemented for ship-adapted version. Additionally, recurrent comparisons with the ground-based AERONET site at Saint-Denis (La Réunion) further confirm the system's accuracy, presenting good correlations despite differences in altitude and the greater influence of local urban aerosols in Saint-Denis. Retrievals from spectral AOD and sky radiance data collected over the Indian Ocean during a biomass burning event (October 2023) demonstrate the feasibility of deriving detailed aerosol properties, including size distribution and optical characteristics, from shipborne platforms adapted for marine conditions, following the protocols of the AERONET standard algorithm. Observed SSA values, ranging from 0.88 to 0.95 with higher absorption at longer wavelengths, align with those recorded at the Saint-Denis site during the event and are consistent with expectations for a mixture of biomass burning (at the end of the dry season) and sea salt aerosols. These preliminary results underscore the potential of shipborne systems to provide comprehensive aerosol characterization in remote marine environments.

Several sun photometer networks worldwide include instruments for aerosol optical depth (AOD) observations, such as Global Atmospheric Atmosphere Watch-Precision Filter Radiometer (GAW-PFR) and Aerosol Robotic Network (AERONET). AERONET provides additional aerosol properties such as the detailed volume size distribution and the single scattering albedo through inversion modelling of sky radiance measurements. However, the data availability for such properties is limited due to the limited number of daily almucantar sky radiance scans and cloudiness. AOD is measured significantly more frequently as there can be one measurement even every minute. Also, the AOD measurements are affected only by clouds being too close or covering the solar disk. The Generalized Retrieval of Atmosphere and Surface Properties (GRASP) is a flexible inversion model to retrieve aerosol properties from various observations. One of its capabilities is the retrieval of the volume concentration, the volume median radius and geometric standard deviation for each aerosol size distribution mode and the separation of AOD to each mode using only spectral AOD as an input parameter (known as the GRASP-AOD application). Such properties are important for various applications, as the size of aerosols affects their interaction with solar radiation, clouds and radiative forcing modelling. Particle size also shows significant differences depending on the aerosol type such as dust or biomass burning. In this study, we selected four common stations of GAW-PFR and AERONET, used GRASP to retrieve the bimodal size distribution parameters from AOD measured by GAW-PFR instruments (PFRs) and validated the results for different conditions using AERONET data as reference. One of those sites includes a multi-year parallel timeseries from two different BTS spectroradiometers that combined can provide direct spectral irradiance (and as a result AOD) in the 300–2150 nm range. Using this dataset, we were able to investigate the effect and potential benefits of the increased spectral range on GRASP-AOD retrievals. This is mostly focused on the retrieval of the coarse mode volume median radius, which is particularly challenging with the filter radiometers measuring up to 862 or 1020 nm. We also assessed the performance for certain dust and biomass burning cases. Our results showed good agreement between PFR AOD-based and AERONET sky radiance inversions for AOD modal separation and volume concentrations. Significant improvement of the PFR-AERONET intercomparison was also possible for the fine mode volume and effective radius when restricting the datasets to AOD at 500 nm > 0.1 and Ångström Exponent (AE) > 1. Also, the results showed consistency with previous study regarding the validation of such retrievals using AERONET AOD. Focusing on conditions with high proportion of dust particles, we found consistent results with the general cases Using AOD with a larger spectral range (from BTS spectroradiometer), we found that the wavelength selection may affect the results and that using longer wavelengths can increase the sensitivity of coarse mode volume median radius to AOD and improve the correlation of the GRASP BTS AOD-based and AERONET datasets. However, the available data were limited, so it is not clear under what conditions the inclusion of such wavelengths will result in more accurate retrievals or to what extent. Finally, we were able to reproduce with GRASP the aerosol size characteristics of unusual biomass burning cases from the Canadian wildfires during 2023, but the results showed systematically increased fine mode radius and concentration compared to the AERONET output.

The Earth's oceans play a critical role in regulating the global climate and atmospheric processes, with marine aerosols significantly influencing weather patterns, air quality, and climate dynamics. Despite extensive land-based aerosol monitoring through networks like AERONET (AErosol RObotic NETwork), marine aerosol characterization remains a critical gap, due in part to the logistical challenges of conducting measurements in remote oceanic environments. To address this, robust, automated, and precise monitoring systems adapted for research vessels are essential. This study reports on the first 3 years (July 2021-June 2024) of continuous aerosol optical depth (AOD) measurements collected aboard the research vessel Marion Dufresne using a ship-adapted CIMEL CE318-T automatic photometer in the frame of the MAP-IO (Marion Dufresne Atmospheric Program - Indian Ocean) program. The dataset comprises over 25 000 quality-assured AOD measurements, primarily from the southwest Indian Ocean region, revealing mid-range AOD and & Aring;ngstr & ouml;m exponent values consistent with previous studies. The reliability and precision of the system were validated through dual-instrument comparisons conducted during the Amaryllis-Amagas/Transama campaign, yielding strong correlations (R>0.96 for different wavelengths) and low root-mean-square error (RMSE<0.01), within the expected error margins for AERONET ground-based sites and benefiting from the continue tracking system implemented for ship-adapted version. Additionally, recurrent comparisons with the ground-based AERONET site at Saint-Denis (La R & eacute;union) further confirm the system's accuracy, presenting good correlations despite differences in altitude and the greater influence of local urban aerosols in Saint-Denis. Retrievals from spectral AOD and sky radiance data collected over the Indian Ocean during a biomass burning event (October 2023) demonstrate the feasibility of deriving detailed aerosol properties, including size distribution and optical characteristics, from shipborne platforms adapted for marine conditions, following the protocols of the AERONET standard algorithm. Observed single scattering albedo (SSA) values, ranging from 0.88 to 0.95 with higher absorption at longer wavelengths, align with those recorded at the Saint-Denis site during the event and are consistent with expectations for a mixture of biomass burning (at the end of the dry season) and sea salt aerosols. These preliminary results underscore the potential of shipborne systems to provide comprehensive aerosol characterization in remote marine environments.

We present the capabilities of a compact dual-wavelength depolarization lidar to assess the spatiotemporal variations in aerosol properties aboard moving vectors. Our approach involves coupling the lightweight Cimel CE376 lidar, which provides measurements at 532 and 808 nm and depolarization at 532 nm, with a photometer to monitor aerosol properties. The assessments, both algorithmic and instrumental, were conducted at ATOLL (ATmospheric Observatory of LiLle) platform operated by the Laboratoire d'Optique Atmosphérique (LOA), in Lille, France. An early version of the CE376 lidar co-located with the CE318-T photometer and with a multi-wavelength Raman lidar were considered for comparisons and validation. We developed a modified Klett inversion method for simultaneous two-wavelength elastic lidar and photometer measurements. Using this setup, we characterized aerosols during two distinct events of Saharan dust and dust smoke aerosols transported over Lille in spring 2021 and summer 2022. For validation purposes, comparisons against the Raman lidar were performed, demonstrating good agreement in aerosol properties with relative differences of up to 12 % in the depolarization measurements. Moreover, a first dataset of CE376 lidar and photometer performing on-road measurements was obtained during the FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality) field campaign deployed in summer 2019 over the northwestern USA. By lidar and photometer mapping in 3D, we investigated the transport of released smoke from active fire spots at William Flats (northeast WA, USA). Despite extreme environmental conditions, our study enabled the investigation of aerosol optical properties near the fire source, distinguishing the influence of diffuse, convective, and residual smoke. Backscatter, extinction profiles, and column-integrated lidar ratios at 532 and 808 nm were derived for a quality-assured dataset. Additionally, the extinction Ångström exponent (EAE), color ratio (CR), attenuated color ratio (ACR), and particle linear depolarization ratio (PLDR) were derived. In this study, we discuss the capabilities (and limitations) of the CE376 lidar in bridging observational gaps in aerosol monitoring, providing valuable insights for future research in this field.

This article is devoted to the presentation of the MAP-IO observation program. This program, launched in early 2021, has enabled the observation of nearly 700 d of measurements over the Indian and Southern Ocean with the equipment of 17 meteorological and oceanographic scientific instruments on board the ship Marion Dufresne. Several observational techniques have been developed to respond to the difficulties of observations on board the ship, in particular for passive remote sensing data, as well as for quasi-autonomous data acquisition and transfer. The first measurements made it possible to draw up unprecedented climatological data of the Southern Ocean regarding the size distribution and optical thickness of aerosols, the concentration of trace gases and greenhouse gases, UV, and integrated water vapor. High-resolution observations of phytoplankton in surface waters have also shown a great variability in latitude in terms of abundance and community structure (diversity). The operational success of this program and these unique scientific results together establish a proof of concept and underline the need to transform this program into a permanent observatory. The multi-year rotations over the Indian Ocean will enable us to assess the trends and seasonal variability of phytoplankton, greenhouse gases, ozone, and marine aerosols in a sensitive and poorly documented climatic region. Without being exhaustive, MAP-IO should make it possible to better understand and assess the biological carbon pump, to study the variability of gases and aerosols in a region that is remote in relation to the main anthropogenic sources, and to monitor the transport of stratospheric ozone by the Brewer–Dobson circulation. The meteorological MAP-IO data set is publicly available at https://www.aeris-data.fr/catalogue-map-io/ (last access: 26 August 2024) (atmospheric data) and at https://doi.org/10.17882/89505 (Thyssen et al., 2022a) (phytoplankton data).

Several sun photometer networks (e.g. the Aerosol Robotic Network (AERONET), the Global Atmospheric Watch-Precision Filter Radiometer (GAW-PFR) retrieve aerosol optical depth (AOD). AERONET provides more aerosol properties such as the effective radius and the single scattering albedo through inversion modelling using sky radiance measurements. However, the data availability for such properties is limited due to the low number of sky radiance scans and cloudiness. In contrast, AOD measurements are significantly more frequent (every minute for GAW-PFR). In the past, methods were developed to retrieve aerosol properties using only direct sun scans. Kazadzis et al., 2014 used linear estimation technique to retrieve the effective radius and volume concentration using only AOD measurements. The Generalized Retrieval of Aerosol and Surface Properties (GRASP) uses multi-term least square method to retrieve the AOD, the volume concentration, the median radius and geometric standard deviation separately for fine and coarse mode. Previous studies concern the application of GRASP-AOD to CIMEL sun photometers [Torres & Fuertes, 2021]. In this study, we aim to apply it to GAW-PFR instruments (PFRs) at common locations with AERONET, assess the retrieval homogeneity with AERONET products and investigate the effect of the wavelength differences and AOD uncertainties. Also, we aim to apply the method to the Precision Spectroradiometers (PSR) and investigate the potential improvements using higher spectral range and resolution. Preliminary retrievals in a common GAW-PFR and ARONET station Hohenpeissenberg) show good results for the fine and coarse mode AOD separation and the fine mode median radius (correlation factor with AERONET standard inversions >0.9 for AOD and >0.8 for fine mode radius). For the coarse mode the retrievals are less accurate, mainly for the median radius in which GRASP-AOD shows low sensitivity to its fluctuations. ReferencesKazadzis, S., Veselovskii, I., Amiridis, V., Gröbner, J., Suvorina, A., Nyeki, S., Gerasopoulos, E., Kouremeti, N., Taylor, M., Tsekeri, A., and Wehrli, C. (2014): Aerosol microphysical retrievals from precision filter radiometer direct solar radiation measurements and comparison with AERONET, Atmos. Meas. Tech., 7, 2013–2025, https://doi.org/10.5194/amt-7-2013-2014.Torres, B. and Fuertes, D. (2021): Characterization of aerosol size properties from measurements of spectral optical depth: a global validation of the GRASP-AOD code using long-term AERONET data, Atmos. Meas. Tech., 14, 4471–4506, https://doi.org/10.5194/amt-14-4471-2021.

Stratospheric sulfate aerosols play a key role on atmospheric chemistry and Earth’s radiation budget, but their size distribution, a critical parameter in climate models, is generally poorly-known. We address such gap for the 2022 Hunga Tonga-Hunga Ha’apai (HT-HH) volcanic eruption by exhaustively analyzing a set of satellite observations (TROPOMI, IASI, AHI, CALIOP) together with photometric ground observations from the worldwide open-access AERONET network. We document a rapid growth of HT-HH sulfate aerosols in the days following eruption, faster than observed for 1991 Pinatubo eruption, likely due to the exceptional hydration of the stratosphere by this phreatomagmatic eruption. An unusual aerosol fine mode (peak radius in 0.3-0.5 µm) is identified at 20 stations of the southern hemisphere until May 2023 (time of writing). Nevertheless, 1.4 years after eruption, HT-HH sulfate aerosols remain smaller than Pinatubo particles. Smaller aerosols backscatter more efficiently visible light and sediment more slowly than larger particles, implying stronger and longer-lasting negative radiative forcing.

This paper explores the potential of all-sky cameras to retrieve aerosol properties with the GRASP code (Generalized Retrieval of Atmosphere and Surface Properties). To this end, normalized sky radiances (NSRs) extracted from an all-sky camera at three effective wavelengths (467, 536 and 605 nm) are used in this study. NSR observations are a set of relative (uncalibrated) sky radiances in arbitrary units. NSR observations have been simulated for different aerosol loads and types with the forward radiative transfer module of GRASP, indicating that NSR observations contain information about the aerosol type, as well as about the aerosol optical depth (AOD), at least for low and moderate aerosol loads. An additional sensitivity study with synthetic data has been carried out to quantify the theoretical accuracy and precision of the aerosol properties (AOD, size distribution parameters, etc.) retrieved by GRASP using NSR observations as input. As a result, the theoretical accuracy of AOD is within ±0.02 for AOD values lower than or equal to 0.4, while the theoretical precision goes from 0.01 to 0.05 when AOD at 467 nm varies from 0.1 to 0.5. NSR measurements recorded at Valladolid (Spain) with an all-sky camera for more than 2 years have been inverted with GRASP. The retrieved aerosol properties are compared with independent values provided by co-located AERONET (AErosol RObotic NETwork) measurements. AODs from both data sets correlate with determination coefficient (r2) values of about 0.87. Finally, the novel multi-pixel approach of GRASP is applied to daily camera radiances together by constraining the temporal variation in certain aerosol properties. This temporal linkage (multi-pixel approach) provides promising results, reducing the highly temporal variation in some aerosol properties retrieved with the standard (one by one or single-pixel) approach. This work implies an advance in the use of all-sky cameras for the retrieval of aerosol properties.

We present the mapping at fine spatial scale of aerosol optical properties using a mobile laboratory equipped with LIDAR (Light Detection And Ranging), sun photometer and in situ instruments for performing on-road measurements. The mobile campaign was conducted from 9 May to 19 May 2017 and had the main objective of mapping the distribution of pollutants in the Beijing and North China Plain (NCP) region. The highest AOD (Aerosol Optical Depth) at 440 nm of 1.34 and 1.9 were recorded during two heavy pollution episodes on 18 May and 19 May 2017, respectively. The lowest Planetary Boundary Layer (PBL) heights (0.5–1.5 km) were recorded during the heavy pollution events, correlating with the highest AOD and southern winds. The transport of desert dust from the Gobi Desert was captured during the mobile measurements, impacting Beijing during 9–13 May 2017. Exploring the NCP outside Beijing provided datasets for regions with scarce ground measurements and allowed the mapping of high aerosol concentrations when passing polluted cities in the NCP (Baoding, Tianjin and Tangshan) and along the Binhai New Area. For the first time, we provide mass concentration profiles from the synergy of LIDAR, sun photometer and in situ measurements. The case study along the Binhai New Area revealed mean extinction coefficients of 0.14 ± 0.10 km−1 at 532 nm and a mass concentration of 80 ± 62 μg/m3 in the PBL (<2 km). The highest extinction (0.56 km−1) and mass concentrations (404 μg/m3) were found in the industrial Binhai New Area. The PM10 and PM2.5 fractions of the total mass concentration profiles were separated using the columnar size distribution, derived from the sun photometer measurements. This study offers unique mobile datasets of the aerosol optical properties in the NCP for future applications, such as satellite validation and air quality studies.

Understanding the uncertainties in the retrieval of aerosol and surface properties is very important for an adequate characterization of the processes that occur in the atmosphere. However, the reliable characterization of the error budget of the retrieval products is a very challenging aspect that currently remains not fully resolved in most remote sensing approaches. The level of uncertainties for the majority of the remote sensing products relies mostly on post-processing validations and intercomparisons with other data, while the dynamic errors are rarely provided. Therefore, implementations of fundamental approaches for generating dynamic retrieval errors and the evaluation of their practical efficiency remains of high importance. This study describes and analyses the dynamic estimates of uncertainties in aerosol-retrieved properties by the GRASP (Generalized Retrieval of Atmosphere and Surface Properties) algorithm. The GRASP inversion algorithm, described by Dubovik et al. (2011, 2014, 2021), is designed based on the concept of statistical optimization and provides dynamic error estimates for all retrieved aerosol and surface properties. The approach takes into account the effect of both random and systematic uncertainties propagations. The algorithm provides error estimates both for directly retrieved parameters included in the retrieval state vector and for the characteristics derived from these parameters. For ex ample, in the case of the aerosol properties, GRASP directly retrieves the size distribution and the refractive index that are used afterwards to provide phase function, scattering, extinction, single scattering albedo, etc. Moreover, the GRASP algorithm provides full covariance matrices, i.e. not only variances of the retrieval errors but also correlations coefficients of these errors. The analysis of the correlation matrix structure can be very useful for identifying less than obvious retrieval tendencies. This appears to be a useful approach for optimizing observation schemes and retrieval set-ups. In this study, we analyse the efficiency of the GRASP error estimation approach for applications to ground-based observations by a sun/sky photometer and lidar. Specifically, diverse aspects of the error generations and their evaluations are discussed and illustrated. The studies rely on a series of comprehensive sensitivity tests when simulated sun/sky photometer measurements and lidar data are perturbed by random and systematic errors and inverted. Then, the results of the retrievals and their error estimations are analysed and evaluated. The tests are conducted for different observations of diverse aerosol types, including biomass burning, urban, dust and their mixtures. The study considers observations of AErosol RObotic NETwork (AERONET) sun/sky photometer measurements at 440, 675, 870 and 1020 nm and multiwavelength elastic lidar measurements at 355, 532 and 1064 nm. The sun/sky photometer data are inverted alone or together with lidar data. The analysis shows overall successful retrievals and error estimations for different aerosol characteristics, including aerosol size distribution, complex refractive index, single scattering albedo, lidar ratios, aerosol vertical profiles, etc. Also, the main observed tendencies in the error dynamic agree with known retrieval experience. For example, the main accuracy limitations for retrievals of all aerosol types relate to the situations with low optical depth. Also, in situations with multicomponent aerosol mixtures, the reliable characterization of each component is possible only in limited situations, for example, from radiometric data obtained for low solar zenith angle observations or from a combination of radiometric and lidar data. At the same time, the total optical properties of aerosol mixtures are always retrieved satisfactorily. In addition, the study includes an analysis of the detailed structure of the correlation matrices for the retrieval errors in mono- and multicomponent aerosols. The conducted analysis of error correlation appears to be a useful approach for optimizing observation schemes and retrieval set-ups. The application of the approach to real data is provided.

The study presents a climatology of aerosol composition concentrations obtained by a recently developed algorithm approach, namely the Generalized Retrieval of Atmosphere and Surface Properties (GRASP)/Component. It is applied to the whole archive of observations from the POLarization and Directionality of the Earth's Reflectances (POLDER-3). The conceptual specifics of the GRASP/Component approach is in the direct retrieval of aerosol speciation (component fraction) without intermediate retrievals of aerosol optical characteristics. Although a global validation of the derived aerosol component product is challenging, the results obtained are in line with general knowledge about aerosol types in different regions. In addition, we compare the GRASP-derived black carbon (BC) and dust components with those of the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) product. Quite a reasonable general agreement was found between the spatial and temporal distribution of the species provided by GRASP and MERRA-2. The differences, however, appeared in regions known for strong biomass burning and dust emissions; the reasons for the discrepancies are discussed. The other derived components, such as concentrations of absorbing (BC, brown carbon (BrC), iron-oxide content in mineral dust) and scattering (ammonium sulfate and nitrate, organic carbon, non-absorbing dust) aerosols, represent scarce but imperative information for validation and potential adjustment of chemical transport models. The aerosol optical properties (e.g., aerosol optical depth (AOD), Ångström exponent (AE), single-scattering albedo (SSA), fine- and coarse-mode aerosol optical depth (AODF AND AODC)) derived from GRASP/Component were found to agree well with the Aerosol Robotic Network (AERONET) ground reference data, and were fully consistent with the previous GRASP Optimized, High Precision (HP) and Models retrieval versions applied to POLDER-3 data. Thus, the presented extensive climatology product provides an opportunity for understanding variabilities and trends in global and regional distributions of aerosol species. The climatology of the aerosol components obtained in addition to the aerosol optical properties provides additional valuable, qualitatively new insight about aerosol distributions and, therefore, demonstrates advantages of multi-angular polarimetric (MAP) satellite observations as the next frontier for aerosol inversion from advanced satellite observations. The extensive satellite-based aerosol component dataset is expected to be useful for improving global aerosol emissions and component-resolved radiative forcing estimations. The GRASP/Component products are publicly available (https://www.grasp-open.com/products/, last access: 15 March 2022) and the dataset used in the current study is registered under https://doi.org/10.5281/zenodo.6395384 (Li et al., 2022b).

Two layers of Saharan dust observed over Leipzig, Germany, in February and March 2021 were used to provide the first-ever lidar measurements of the dust lidar ratio (extinction-to-backscatter ratio) and linear depolarization ratio at all three classical lidar wavelengths (355, 532 and 1064 nm). The pure-dust conditions during the first event exhibit lidar ratios of 47 ± 8, 50 ± 5 and 69 ± 14 sr and particle linear depolarization ratios of 0.242 ± 0.024, 0.299 ± 0.018 and 0.206 ± 0.010 at wavelengths of 355, 532 and 1064 nm, respectively. The second, slightly polluted-dust case shows a similar spectral behavior of the lidar and depolarization ratio with values of the lidar ratio of 49 ± 4, 46 ± 5 and 57 ± 9 sr and the depolarization ratio of 0.174 ± 0.041, 0.298 ± 0.016 and 0.242 ± 0.007 at 355, 532 and 1064 nm, respectively. The results were compared with Aerosol Robotic Network (AERONET) version 3 (v3) inversion solutions and the Generalized Retrieval of Aerosol and Surface Properties (GRASP) at six and seven wavelengths. Both retrieval schemes make use of a spheroid shape model for mineral dust. The spectral slope of the lidar ratio from 532 to 1064 nm could be well reproduced by the AERONET and GRASP retrieval schemes. Higher lidar ratios in the UV were retrieved by AERONET and GRASP. The enhancement was probably caused by the influence of fine-mode pollution particles in the boundary layer which are included in the columnar photometer measurements. Significant differences between the measured and retrieved wavelength dependence of the particle linear depolarization ratio were found. The potential sources for these uncertainties are discussed.

We analyse the airborne measurements of above-cloud aerosols from the AErosol, RadiatiOn, and CLOuds in southern Africa (AEROCLO-sA) field campaign performed in Namibia during August and September 2017. The study aims to retrieve the aerosol above-cloud direct radiative effect (DRE) with well-defined uncertainties. To improve the retrieval of the aerosol and cloud properties, the airborne demonstrator of the Multi-Viewing, Multi-Channel, Multi-Polarization (3MI) satellite instrument, called the Observing System Including PolaRisation in the Solar Infrared Spectrum (OSIRIS), was deployed on-board the SAFIRE (Service des Avions Français Instrumentés pour la Rechercheen Environnement) Falcon 20 aircraft during 10 flights performed over land, over the ocean, and along the Namibian coast. The airborne instrument OSIRIS provides observations at high temporal and spatial resolutions for aerosol above clouds (AACs) and cloud properties. OSIRIS was supplemented with the Photomètre Léger Aéroporté pour la surveillance des Masses d'Air version 2 (PLASMA2). The combined airborne measurements allow, for the first time, the validation of AAC algorithms previously developed for satellite measurements. The variations in the aerosol properties are consistent with the different atmospheric circulation regimes observed during the deployment. Airborne observations typically show strong aerosol optical depth (AOD; up to 1.2 at 550 nm) of fine-mode particles from biomass burning (extinction Ångström exponent varying between 1.6 and 2.2), transported above bright stratocumulus decks (mean cloud top around 1 km above mean sea level), with cloud optical thickness (COT) up to 35 at 550 nm. The above-cloud visible AOD retrieved with OSIRIS agrees within 10 % of the PLASMA2 sun photometer measurements in the same environment. The single scattering albedo (SSA) is one of the most influential parameters on the AAC DRE calculation that remains largely uncertain in models. During the AEROCLO-sA campaign, the average SSA obtained by OSIRIS at 550 nm is 0.87, which is in agreement within 3 %, on average, with previous polarimetric-based satellite and airborne retrievals. The strong absorption of the biomass burning plumes in the visible range is generally consistent with the observations from the Aerosol Robotic Network (AERONET) ground-based sun photometers. This, however, shows a significant increase in the particles' absorption at 440 nm in northern Namibia and Angola, which indicates more absorbing organic species within the observed smoke plumes. Biomass burning aerosols are also vertically collocated, with significant amounts of water content up to the top of the plume at around 6 km height in our measurements. The detailed characterization of aerosol and cloud properties, water vapour, and their uncertainties obtained from OSIRIS and PLASMA2 measurements allows us to study their impacts on the AAC DRE. The high-absorbing load of AAC, combined with high cloud albedo, leads to unprecedented DRE estimates, which are higher than previous satellite-based estimates. The average AAC DRE calculated from the airborne measurements in the visible range is +85 W m−2 (standard deviation of 26 W m−2), with instantaneous values up to +190 W m−2 during intense events. These high DRE values, associated with their uncertainties, have to be considered as new upper cases in order to evaluate the ability of models to reproduce the radiative impact of the aerosols over the southeastern Atlantic region.

This study evaluates the potential of using aerosol optical depth (τa) measurements to characterise the microphysical and optical properties of atmospheric aerosols. With this aim, we used the recently developed GRASP (Generalized Retrieval of Aerosol and Surface Properties) code for numerical testing of six different aerosol models with different aerosol loads. The direct numerical simulations (self-consistency tests) indicate that the GRASP-AOD retrieval provides modal aerosol optical depths (fine and coarse) to within 0.01 of the input values. The retrieval of the fine-mode radius, width and volume concentration are stable and precise if the real part of the refractive index is known. The coarse-mode properties are less accurate, but they are significantly improved when additional a priori information is available. The tests with random simulated errors show that the uncertainty in the bimodal log-normal size distribution parameters increases as the aerosol load decreases. Similarly, the reduction in the spectral range diminishes the stability of the retrieved parameters. In addition to these numerical studies, we used optical depth observations at eight AERONET locations to validate our results with the standard AERONET inversion products. We found that bimodal log-normal size distributions serve as useful input assumptions, especially when the measurements have inadequate spectral coverage and/or limited accuracy, such as moon photometry. Comparisons of the mode median radii between GRASP-AOD and AERONET indicate average differences of 0.013 µm for the fine mode and typical values of 0.2–0.3 µm for the coarse mode. The dominant mode (i.e. fine or coarse) indicates a 10 % difference in mode radii between the GRASP-AOD and AERONET inversions, and the average of the difference in volume concentration is around 17 % for both modes. The retrieved values of the fine-mode τa(500) using GRASP-AOD are generally between those values obtained by the standard AERONET inversion and the values obtained by the AERONET spectral deconvolution algorithm (SDA), with differences typically lower than 0.02 between GRASP-AOD and both algorithms. Finally, we present some examples of application of GRASP-AOD inversion using moon photometry and the airborne PLASMA sun photometer during the ChArMEx summer 2013 campaign in the western Mediterranean.

Abstract. The North China Plain (NCP) has been experiencing serious air quality problems since the rapid urbanization and industrialization and has been the subject of many studies over the years. This work presents mapping at a fine scale of the aerosol spatial and vertical variability obtained during the MOABAI campaign (Mobile Observation of Atmosphere By vehicle-borne Aerosol measurement Instruments) using a van equipped with a micro-pulse LIDAR, a sun photometer and in situ instruments, performing on-road measurements. The campaign was conducted from 5 May to 23 May 2017 and had as a main objective to map the pollutants distribution in Beijing and NCP area. A summary of aerosol properties during all measurement days and a comprehensive case study along the industrial Binhai New Area near Tianjin are presented. The highest AOD at 440 nm (1.34 and 1.9) were recorded during two heavy pollution episodes on 18 May and 19 May 2017, respectively. The lowest PBL (Planetary Boundary Layer) heights (< 1500 m) were recorded during the heavy pollution events, correlated with the highest AOD. Transport of dust from Gobi Desert was captured during the mobile measurements, impacting Beijing in the 9–13 May period. Exploring the NCP outside Beijing provided datasets in regions with lack of aerosol observation sites and allowed mapping higher aerosol concentrations when passing by polluted cities in NCP (Baoding, Tianjin and Tangshan). In this study, we provide the first mass concentration profiles derived from a mobile micro-pulse LIDAR, making use of complementary information on aerosol type from sun photometer and in situ data. The case study of 17 May 2017 revealed mean extinction coefficients of 0.14 ± 0.10 km−1 at 532 nm and total mass concentration of 80 ± 62 μg m−3 in the PBL (< 2000 m) for the mobile transect from Tianjin to Tangshan along the coast of Bohai Sea. The highest extinction (0.56 km−1) and mass concentrations (404 μg m−3) were found in the industrial Binhai New Area. The PM10 and PM2.5 fractions of the total mass concentration profiles were separated using the columnar size distribution derived from sun photometer measurements. A general good agreement was found between the lidar-derived PM concentrations at surface level and the ones recorded at the closest air quality stations along the transect, with the only exception along the industrial region near Tianjin port, where emissions were highly variable.