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This paper presents a collection of lidar-derived aerosol intensive optical properties for several aerosol types, namely the particle linear depolarization ratio, the extinction-to-backscatter ratio (lidar ratio) and the Ångström exponent. The data collection, named DeLiAn, is based on globally distributed, long-term, ground-based, multiwavelength, Raman and polarization lidar measurements, conducted mainly with lidars that have been developed at the Leibniz Institute for Tropospheric Research. The intensive optical properties are presented at two wavelengths, 355 and 532 nm, for 13 aerosol categories. The categories cover the basic aerosol types (i.e., marine, pollution, continental European background, volcanic ash, smoke, mineral dust), as well as the most frequently observed mixtures they form. This extensive collection also incorporates more peculiar aerosol categories, including dried marine aerosol that, compared to marine aerosol, exhibits a significantly enhanced depolarization ratio (up to 15 %). Besides Saharan dust, additional mineral dust types related to their source region were identified due to their lower lidar ratios (Central Asian and Middle Eastern dust). In addition, extreme wildfire events (such as in north America and Australia) emitted smoke into the stratosphere showing significantly different optical properties, i.e., high depolarization values (up to 25 %), compared to tropospheric smoke. The data collection reflects and underlines the variety of aerosol mixtures in the atmosphere and can be used for the development of aerosol-typing schemes. The paper contains the most up-to-date and comprehensive overview of optical properties from aerosol lidar measurements and, therefore, provides a solid basis for future aerosol retrievals in the frame of both spaceborne and ground-based lidars. Furthermore, DeLiAn can assist the efforts for the harmonization of satellite records of aerosol properties performed at different wavelengths.

A dataset of particle optical properties of the highly polluted atmosphere over the Pearl River Delta, Guangzhou, China, is presented in this paper. The data were derived from the measurements of a multi-wavelengths Raman and depolarization lidar PollyXT and a co-located AERONET sun photometer. The measurement campaign was conducted from November 2011 to mid June 2012. These are the first Raman lidar measurements in the PRD that were lasting for several months. A mean value of aerosol optical depth observed by the sun photometer of 0.54 ± 0.33 was observed in the polluted atmosphere over this megacity for the whole measurement period. The lidar profiles frequently show lofted aerosol layers which reach up to altitudes of 2 to 3 km and especially in spring season up to even 5 km. These layers contain between 12 and 56 % of the total AOD, with the highest values in spring. The aerosol in these lofted layers were classified by their optical properties. The observed lidar ratio values are in the range from 30 to 80 sr with a mean value of 48.0 sr ± 10.7 sr. The linear particle depolarization ratio at 532 nm lied mostly below 5 % with a mean value of 3.6 ± 3.7. The majority of the Ångström exponents were observed between 1 and 1.5 indicating a coarse mode in addition to the fine mode in the particle size distribution. These results reveal that mainly urban pollution particles mixed with particles arisen from biomass and industrial burning are present in the atmosphere above the Pearl River Delta. Trajectory analyses show that these pollution mixtures arise mainly from local and regional sources.

Knowledge of the vertical distribution and layering of aerosols and identification of the corresponding aerosol sources are needed to improve our understanding of the spatial and temporal variability of aerosol pollution. To achieve this goal, we combined both passive and active remote-sensing techniques to provide a 3D view of local aerosol levels and regional to long-range pollution transport. We studied aerosol optical depth (AOD) data from the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm at 1-km spatial resolution along with active multiwavelength polarization lidar observations of vertical aerosol profiles in Haifa, Israel, Aerosol Robotic Network (AERONET) sun photometer observations at the lidar site, and local-network observations of aerosol concentrations (PM2.5). This comprehensive dataset enabled analyzing the performance of the MAIAC AOD retrieval in cases of complex aerosol layering and mixing states which are typical of the Eastern Mediterranean. While satellite-derived and ground-based AOD measurements generally showed good agreement, 35 out of 100 measurements showed low correspondence. Analysis of those cases revealed that overestimation of AOD was mostly related to cloud-contaminated pixels and aerosol water-uptake effects in moist, cloud-free air at cloud level. Furthermore, AOD over- and underestimations were related to the presence of complex aerosol mixture and layering conditions, especially when dust was mixed with aged anthropogenic aerosol pollution and marine aerosols with lofted anthropogenic pollution. In these cases 50–70% of measurements were outside of the expected error limit. Perhaps these conditions are not considered in the MAIAC retrieval. Finally, we investigated the link between AOD spatial variability and the MAIAC AOD bias, and performed a cluster analysis corroborating the strong impact of cloud contamination on MAIAC AOD quality. Our observation-based results raise the importance of carefully analyzing the uncertainties in satellite AOD measurements that are used as an important input variable in numerous health-related exposure studies and climate models.

The atmospheric science community demands autonomous and quality-assured vertically resolved measurements of aerosol and cloud properties. For this purpose, a portable lidar called Polly was developed at TROPOS in 2003. The lidar system was continuously improved with gained experience from the EARLINET community, involvement in worldwide field campaigns, and international institute collaborations within the last 10 years. Here we present recent changes of the setup of the portable multiwavelength Raman and polarization lidar PollyXT and discuss the improved capabilities of the system by means of a case study. The latest system developments include an additional near-range receiver unit for Raman measurements of the backscatter and extinction coefficient down to 120 m above ground, a water-vapor channel, and channels for simultaneous measurements of the particle linear depolarization ratio at 355 and 532 nm. Quality improvements were achieved by systematically following the EARLINET guidelines and the international PollyNET quality assurance developments. A modified ship radar ensures measurements in agreement with air-traffic safety regulations and allows for 24∕7 monitoring of the atmospheric state with PollyXT.

MODIS (MOderate-resolution Imaging Spectroradiometer) retrievals of aerosol optical depth (AOD) are biased over urban areas, primarily because the reflectance characteristics of urban surfaces are different than that assumed by the retrieval algorithm. Specifically, the operational "dark-target" retrieval is tuned towards vegetated (dark) surfaces and assumes a spectral relationship to estimate the surface reflectance in blue and red wavelengths. From airborne measurements of surface reflectance over the city of Zhongshan, China, were collected that could replace the assumptions within the MODIS retrieval algorithm. The subsequent impact was tested upon two versions of the operational algorithm, Collections 5 and 6 (C5 and C6). AOD retrieval results of the operational and modified algorithms were compared for a specific case study over Zhongshan to show minor differences between them all. However, the Zhongshan-based spectral surface relationship was applied to a much larger urban sample, specifically to the MODIS data taken over Beijing between 2010 and 2014. These results were compared directly to ground-based AERONET (AErosol RObotic NETwork) measurements of AOD. A significant reduction of the differences between the AOD retrieved by the modified algorithms and AERONET was found, whereby the mean difference decreased from 0.27±0.14 for the operational C5 and 0.19±0.12 for the operational C6 to 0.10±0.15 and -0.02±0.17 by using the modified C5 and C6 retrievals. Since the modified algorithms assume a higher contribution by the surface to the total measured reflectance from MODIS, consequently the overestimation of AOD by the operational methods is reduced. Furthermore, the sensitivity of the MODIS AOD retrieval with respect to different surface types was investigated. Radiative transfer simulations were performed to model reflectances at top of atmosphere for predefined aerosol properties. The reflectance data were used as input for the retrieval methods. It was shown that the operational MODIS AOD retrieval over land reproduces the AOD reference input of 0.85 for dark surface types (retrieved AOD = 0.87 (C5)). An overestimation of AOD = 0.99 is found for urban surfaces, whereas the modified C5 algorithm shows a good performance with a retrieved value of AOD = 0.86.

We present combined Raman and elastic backscatter lidar observations in Zhongshan, PRD (Pearl River Delta), China, during two periods in 2009: one haze period and one moderate pollution period. During the haze period, high Aerosol Optical Depth (AOD) (0.86 and 1.20 at 355 nm) and medium Angstrom exponents (1.23 and 1.35 at 355 nm/532 nm) were observed. In the moderate pollution period, the corresponding parameters were comparatively lower with values of 0.83 and 0.74 at 355 nm for AOD and 1.108 and 0.98 at 355 nm/532 nm for Angstrom exponent. The mean lidar ratios in the two periods were 64 ± 10 sr and 56 ± 9 sr, respectively, at 355 nm. The Angstrom exponent was calculated for the extinction from the wavelength pair 355 nm/532 nm, with high values of around 1.35 for the haze event. The particle size distribution and single-scattering albedo derived from Sun photometer measurements indicate the presence of rather small particles. The 3 day back trajectories from a Hybrid Single-Particle Lagrangian Integrated Trajectory model in the haze period indicate that the air masses in the lower layer were advected from the southeast coast of China, where incomplete combustion of carbonaceous fuels and straw burning are frequently found in Shanghai during the heating period in winter. In the moderate pollution period, the air mass passed through western China, indicating a combination of some pollution from South Asia in case of strong convection, local aerosol aging, and smoke from adjacent fire burning spots in the PRD region.

A global vertically resolved aerosol data set covering more than 10 years of observations at more than 20 measurement sites distributed from 63° N to 52° S and 72° W to 124° E has been achieved within the Raman and polarization lidar network PollyNET. This network consists of portable, remote-controlled multiwavelength-polarization-Raman lidars (Polly) for automated and continuous 24/7 observations of clouds and aerosols. PollyNET is an independent, voluntary, and scientific network. All Polly lidars feature a standardized instrument design with different capabilities ranging from single wavelength to multiwavelength systems, and now apply unified calibration, quality control, and data analysis. The observations are processed in near-real time without manual intervention, and are presented online at http://polly.tropos.de/. The paper gives an overview of the observations on four continents and two research vessels obtained with eight Polly systems. The specific aerosol types at these locations (mineral dust, smoke, dust-smoke and other dusty mixtures, urban haze, and volcanic ash) are identified by their Ångström exponent, lidar ratio, and depolarization ratio. The vertical aerosol distribution at the PollyNET locations is discussed on the basis of more than 55 000 automatically retrieved 30 min particle backscatter coefficient profiles at 532 nm as this operating wavelength is available for all Polly lidar systems. A seasonal analysis of measurements at selected sites revealed typical and extraordinary aerosol conditions as well as seasonal differences. These studies show the potential of PollyNET to support the establishment of a global aerosol climatology that covers the entire troposphere.

The potential of a new generation of ceilometer instruments for aerosol monitoring has been studied in the Ceilometer Lidar Comparison (CLIC) study. The used ceilometer was developed by Jenoptik, Germany, and is designed to find both thin cirrus clouds at tropopause level and aerosol layers at close ranges during day and night-time. The comparison study was performed to determine up to which altitude the ceilometers are capable to deliver particle backscatter coefficient profiles. For this, the derived ceilometer profiles are compared to simultaneously measured lidar profiles at the same wavelength. The lidar used for the comparison was the multi-wavelengths Raman lidar PollyXT. To demonstrate the capabilities and limits of ceilometers for the derivation of particle backscatter coefficient profiles from their measurements two examples of the comparison results are shown. Two cases, a daytime case with high background noise and a less noisy night-time case, are chosen. In both cases the ceilometer profiles compare well with the lidar profiles in atmospheric structures like aerosol layers or the boundary layer top height. However, the determination of the correct magnitude of the particle backscatter coefficient needs a calibration of the ceilometer data with an independent measurement of the aerosol optical depth by a sun photometer. To characterizes the ceilometers signal performance with increasing altitude a comprehensive signal-to-noise ratio study was performed. During daytime the signal-to-noise ratio is higher than 1 up to 4–5 km depending on the aerosol content. In our night-time case the SNR is higher than 1 even up to 8.5 km, so that also aerosol layers in the upper troposphere had been detected by the ceilometer.

Two versions of the portable aerosol Raman lidar system (Polly) are presented. First, the two-channel prototype is depicted. It has been developed for the independent and simultaneous determination of particle backscatter and extinction coefficient profiles at 532 nm. Second, the 3 + 2 Raman lidar PollyXT (3 + 2: three backscatter and two extinction coefficients), the second generation of Polly, is described. The extended capabilities of PollyXT are due to the simultaneous emission of light with three wavelengths, more laser power, a larger main receiver mirror, and seven receiver channels. These systems are completely remotely controlled and all measurements are performed automatically. The collected data are transferred to a home server via the Internet and are displayed on a Web page. This paper describes the details of the optical setup, the housekeeping of the systems, and the used data retrieval routines. A measurement example taken close to Manaus, Brazil, on 15 August 2008 shows the capabilities of PollyXT.

The results of a validation of the European Space Agency’s (ESA) operational Medium-Resolution Imaging Spectrometer (MERIS) cloud-top pressure (CTP) product by airborne lidar measurements are presented. MERIS, mounted on the polar-orbiting ESA Environmental Satellite (ENVISAT), provides radiance measurements within the oxygen A absorption band around 761 nm. The exploitation of these data allows the retrieval of CTP. The validation flights were performed in the northeastern part of Germany between April and June 2004 and were temporally and spatially synchronized with the ENVISAT overpasses. The Cessna 207T of the Freie Universität Berlin was equipped with the portable lidar system (POLIS) of the Ludwig-Maximilians-Universität München and a GPS navigation system. The maximum flying altitude was around 3000 m; therefore, the validation measurements were limited to situations with low-level clouds only. The validation was done by comparing MERIS data and lidar data. The statistical analysis of the observations revealed a high accuracy of the MERIS CTP product for low-level clouds, apart from a slight systematic overestimation of cloud-top heights. The root-mean-square error was 249 m, with a bias of +232 m. In the average top height level of ∼2000 m, these values are commensurate to pressure values of 24 hPa (rmse) and −22 hPa (bias). Furthermore, this validation campaign revealed deficiencies of the MERIS cloud mask to detect small-scale broken clouds.

International audience; In the framework of the Third European Stratospheric Experiment on Ozone project Meridional Transport of Ozone in the Lower Stratosphere (METRO) an airborne ozone lidar has been flown on the French Falcon (Mystere 20) during winter 1998/1999 to investigate polar and subtropical filaments at midlatitudes. The objective of the METRO project is to quantify the proportion of the transport of polar and subtropical air and their mixing into midlatitude air masses. The dynamical evolution of the northern winter hemisphere was simulated using a high‐resolution advection model for potential vorticity (PV): Modele Isentropique de transport Mesoechelle de l'Ozone Stratospherique par Advection (MIMOSA). To validate the model for further studies, it was first utilized to forecast the appearance of filaments inside the range of the airplane so that each flight of the airborne campaign could be planned precisely. The vertical ozone distribution measured along the flight tracks was then compared to the respective PV distribution simulated by the advection model. Correlation coefficients between 0.5 and 0.7 found over the altitude range where the filaments were observed show a good agreement between PV and ozone filaments. An improvement of the correlation up to 0.8 by horizontal shifting of the ozone profiles against the PV evolution showed that small displacements of less than 1° of the modeled PV filaments can occur. These displacements can be explained by the uncertainties of the input wind velocity data and of the lidar data. Thus we can conclude that the PV advection model MIMOSA reproduces the position, size, and structure of polar filaments and subtropical intrusions well in the range of the expected accuracy. The model is a suitable tool for further studies of the quantification of the global, long‐term transport and mixing of polar and subtropical air into mid‐latitudes.

The atmospheric science community demands for autonomous and quality-assured vertically resolved measurements of aerosol and cloud properties. Aiming this goal, TROPOS developed the fully automated multiwavelength polarization Raman lidar Polly since over 10 years [1, 2]. In cooperation with different partner research institutes the system was improved continuously. Our latest lidar developments include aside the "3+2" measurements also a near-range receiver to measure aerosol extinction and backscatter down to 120 m above the lidar, a water-vapor channel, and measurements of the linear depolarization at two wavelengths. The latest system was built in cooperation with the National Observatory of Athens (NOA). Its first campaign however was performed at the Cyprus Institute of Nicosia from March to April 2015, aiming specifically at the observation of ice nuclei with in-situ and lidar remote sensing techniques in the framework of BACCHUS [3, 4].

[1] For the first time, multiwavelength polarization Raman lidar observations of optical and microphysical particle properties over the Amazon Basin are presented. The fully automated advanced Raman lidar was deployed 60 km north of Manaus, Brazil (2.5°S, 60°W) in the Amazon rain forest from January to November 2008. The measurements thus cover both the wet season (Dec–June) and the dry or burning season (July–Nov). Two cases studies of young and aged smoke plumes are discussed in terms of spectrally resolved optical properties (355, 532, and 1064 nm) and further lidar products such as particle effective radius and single-scattering albedo. These measurement examples confirm that biomass burning aerosols show a broad spectrum of optical, microphysical, and chemical properties. The statistical analysis of the entire measurement period revealed strong differences between the pristine wet and the polluted dry season. African smoke and dust advection frequently interrupt the pristine phases during the wet season. Compared to pristine wet season conditions, the particle scattering coefficients in the lowermost 2 km of the atmosphere were found to be enhanced, on average, by a factor of 4 during periods of African aerosol intrusion and by a factor of 6 during the dry (burning) season. Under pristine conditions, the particle extinction coefficients and optical depth for 532 nm wavelength were frequently as low as 10–30 Mm−1 and

Nearly pure Saharan dust was observed with an Aerosol Robotic Network (AERONET) Sun photometer, several Raman and high spectral resolution lidars, and airborne in situ instruments during the Saharan Mineral Dust Experiment (SAMUM) 2006 in Morocco. In the framework of a case study we present particle shapeA¢Â€Âdependent dust properties, i.e., backscatter coefficients, extinctionA¢Â€ÂtoA¢Â€Âbackscatter (lidar) ratios, and linear particle depolarization ratios. These parameters can be inferred from AERONETA¢Â€Â™s latest version of the mineral dust retrieval algorithm. The parameters can be measured with multiwavelength Raman/depolarization lidar without critical assumptions on particle shape. Lidar ratios inferred from the AERONET Sun photometer data tend to be larger than lidar ratios measured directly with lidar. Linear dust depolarization ratios were derived for several measurement wavelengths from the data products of the AERONET Sun photometer. The depolarization ratios tend to be smaller than the depolarization ratios measured with lidar. The largest differences exist in the nearA¢Â€Âultraviolet wavelength range. Particle axis ratios were determined with scanning electron microscopy. The axis ratio distribution differs significantly from the axis ratio distribution that is assumed in the AERONET retrievals. If the axis ratio distributions measured during SAMUM are used, the reproducibility of the lidar data products improves. The differences may in part be caused by an insufficient understanding of the lightA¢Â€Âscattering model that is used in the AERONET algorithm. The results of the present study will be used to develop a dust lightA¢Â€Âscattering model that will serve as the theoretical basis for the inversion of optical data into dust microphysical properties.

In the framework of the Saharan Mineral Dust Experiment (SAMUM) for the first time the spectral dependence of particle linear depolarization ratios was measured by combining four lidar systems. In this paper these measurements are compared with results from scattering theory based on the T-matrix method. For this purpose, in situ measurements—size distribution, shape distribution and refractive index—were used as input parameters; particle shape was approximated by spheroids. A sensitivity study showed that lidar-related parameters—lidar ratio Sp and linear depolarization ratio δp—are very sensitive to changes of all parameters. The simulated values of the δp are in the range of 20% and 31% and thus in the range of the measurements. The spectral dependence is weak, so that it could not be resolved by the measurements. Calculated lidar ratios based on the measured microphysics and considering equivalent radii up to 7.5 μm show a range of possible values between 29 and 50 sr at λ= 532 nm. Larger Sp might be possible if the real part of the refractive index is small and the imaginary part is large. A strict validation was however not possible as too many microphysical parameters influence Sp and δp that could not be measured with the required accuracy.DOI: 10.1111/j.1600-0889.2008.00381.x

Vertical profiles of dust key optical properties are presented from measurements during the Saharan Mineral Dust Experiment (SAMUM) by Raman and depolarization lidar at two ground-based sites and by airborne high spectral resolution lidar. One of the sites, Tinfou, is located close to the border of the Sahara in Southern Morocco and was the main in situ site during SAMUM. The other site was Ouarzazate airport, the main lidar site. From the lidar measurements the spatial distribution of the dust between Tinfou and Ouarzazate was derived for 1 d. The retrieved profiles of backscatter and extinction coefficients and particle depolarization ratios show comparable dust optical properties, a similar vertical structure of the dust layer, and a height of about 4 km asl at both sites. The airborne cross-section of the extinction coefficient at the two sites confirms the low variability in dust properties. Although the general picture of the dust layer was similar, the lidar measurements reveal a higher dust load closer to the dust source. Nevertheless, the observed intensive optical properties were the same. These results indicate that the lidar measurements at two sites close to the dust source are both representative for the SAMUM dust conditions. DOI: 10.1111/j.1600-0889.2008.00404.x

Vertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9°N, –6.9°E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.DOI: 10.1111/j.1600-0889.2008.00396.x

[1] Lidar measurements of the vertical distribution of optical particle properties were performed in January 2006 in the frame of the dry season field experiment of the African Monsoon Multidisciplinary Analyses (AMMA). The measurements were conducted at a remote site in West Africa in the Sahel region just south of the Saharan desert to investigate the influence of particles on the West African monsoon. The portable Raman polarization lidar system POLIS of the University of Munich was operated each forenoon and during selected nights. These measurements are the first Raman and depolarization lidar measurements taken in the Sahel. The particle depolarization ratio and the extinction coefficient profiles allow the characterization of the particles and the distinction between Saharan dust particles and biomass burning aerosol (BBA). The origin of this aerosol was investigated by trajectory analyses. The profiles show a varying dust layer in the lowermost troposphere during the whole period. Above the dust layer a broad layer of BBA reaching up to an altitude of 5 km was frequently observed. The extinction coefficient in the dust layer was between 0.2 and 0.4 km−1 at 355 nm, and the particle depolarization was around 25%. In the BBA layer the extinction coefficient was lower with values between 0.1 and 0.2 km−1, and the particle depolarization ratio was below 10%. Lidar ratio values around 55 sr were found for the dust layer and around 75 sr for the BBA layer. From previous observations, lidar ratios between 40 and 80 sr were expected for BBA.

International audience; This paper presents measurements of the vertical distribution of aerosol extinction coefficient over West Africa during the Dust and Biomass-burning Aerosol Experiment (DABEX)/African Monsoon Multidisciplinary Analysis dry season Special Observing Period Zero (AMMA-SOP0). In situ aircraft measurements from the UK FAAM aircraft have been compared with two ground-based lidars (POLIS and ARM MPL) and an airborne lidar on an ultralight aircraft. In general, mineral dust was observed at low altitudes (up to 2 km), and a mixture of biomass burning aerosol and dust was observed at altitudes of 2-5 km. The study exposes difficulties associated with spatial and temporal variability when intercomparing aircraft and ground measurements. Averaging over many profiles provided a better means of assessing consistent errors and biases associated with in situ sampling instruments and retrievals of lidar ratios. Shortwave radiative transfer calculations and a 3-year simulation with the HadGEM2-A climate model show that the radiative effect of biomass burning aerosol was somewhat sensitive to the vertical distribution of aerosol. In particular, when the observed low-level dust layer was included in the model, the absorption of solar radiation by the biomass burning aerosols increased by 10%. We conclude that this absorption enhancement was caused by the dust reflecting solar radiation up into the biomass burning aerosol layer. This result illustrates that the radiative forcing of anthropogenic absorbing aerosol can be sensitive to the presence of natural aerosol species.

[1] Multiwavelength lidar, Sun photometer, and radiosonde observations were conducted at Ouarzazate (30.9°N, 6.9°W, 1133 m above sea level, asl), Morocco, in the framework of the Saharan Mineral Dust Experiment (SAMUM) in May–June 2006. The field site is close to the Saharan desert. Information on the depolarization ratio, backscatter and extinction coefficients, and lidar ratio of the dust particles, estimates of the available concentration of atmospheric ice nuclei at cloud level, profiles of temperature, humidity, and the horizontal wind vector as well as backward trajectory analysis are used to study cases of cloud formation in the dust with focus on heterogeneous ice formation. Surprisingly, most of the altocumulus clouds that form at the top of the Saharan dust layer, which reaches into heights of 4–7 km asl and has layer top temperatures of −8°C to −18°C, do not show any ice formation. According to the lidar observations the presence of a high number of ice nuclei (1–20 cm−3) does not automatically result in the obvious generation of ice particles, but the observations indicate that cloud top temperatures must typically reach values as low as −20°C before significant ice production starts. Another main finding is that liquid clouds are obviously required before ice crystals form via heterogeneous freezing mechanisms, and, as a consequence, that deposition freezing is not an important ice nucleation process. An interesting case with cloud seeding in the free troposphere above the dust layer is presented in addition. Small water clouds formed at about −30°C and produced ice virga. These virga reached water cloud layers several kilometers below the initiating cloud cells and caused strong ice production in these clouds at temperatures as high as −12°C to −15°C.

The African Monsoon Multidisciplinary Analysis (AMMA) is a major international campaign investigating far-reaching aspects of the African monsoon, climate and the hydrological cycle. A special observing period was established for the dry season (SOP0) with a focus on aerosol and radiation measurements. SOP0 took place during January and February 2006 and involved several ground-based measurement sites across west Africa. These were augmented by aircraft measurements made by the Facility for Airborne Atmospheric Measurements (FAAM) aircraft during the Dust and Biomass-burning Experiment (DABEX), measurements from an ultralight aircraft, and dedicated modeling efforts. We provide an overview of these measurement and modeling studies together with an analysis of the meteorological conditions that determined the aerosol transport and link the results together to provide a balanced synthesis. The biomass burning aerosol was significantly more absorbing than that measured in other areas and, unlike industrial areas, the ratio of excess carbon monoxide to organic carbon was invariant, which may be owing to interaction between the organic carbon and mineral dust aerosol. The mineral dust aerosol in situ filter measurements close to Niamey reveals very little absorption, while other measurements and remote sensing inversions suggest significantly more absorption. The influence of both mineral dust and biomass burning aerosol on the radiation budget is significant throughout the period, implying that meteorological models should include their radiative effects for accurate weather forecasts and climate simulations. Generally, the operational meteorological models that simulate the production and transport of mineral dust show skill at lead times of 5 days or more. Climate models that need to accurately simulate the vertical profiles of both anthropogenic and natural aerosols to accurately represent the direct and indirect effects of aerosols appear to do a reasonable job, although the magnitude of the aerosol scattering is strongly dependent upon the emission data set.

[1] On 4 days in summer and winter the mixing layer height over the municipal area of Munich, Germany, was determined by several remote sensing instruments and in situ probes. The main motivation was to obtain information on aerosols, and therefore we decided to understand the mixing layer as that layer where most of the locally produced aerosols are concentrated. In this paper we wanted to investigate the potential of the quite different methodologies which depend on measurements of aerosol properties and those which do not. The operation of two lidars, a ceilometer, a wind-temperature-radar, a sodar, radiosondes, and aerosol probes onboard of a microlight aircraft allowed such a thorough intercomparison. As the instruments were located at different sites, the horizontal homogeneity of the mixing layer could also be observed. It was found that the agreement between the different methodologies is very good as long as the mixing layer height does not exceed approximately 1 km, which is the common measurement range of all instruments. In summer, however, the mixing layer can reach 2 km and more, so that the lidar turns out to be the most capable remote sensing technique. Another advantage of the lidar is the possibility to clearly derive the internal structure of the mixing layer. The latter is important in cases when simple parameterizations assume vertical homogeneity of aerosol properties within the mixing layer. On the other hand, lidars are quite expensive and require a trained operator. As a conclusion, the development of unattended working lidars including automated data evaluation should be fostered. From the limited data set it was found that the mixing layer height in Munich did not change more than approximately 100 m over a horizontal distance of around 50 km. If this finding can be confirmed by further measurements, the area of Munich is a good test bed for the validation of aerosol retrievals from satellite data with medium spatial resolution and for the validation of the numerical treatment of aerosols in mesoscale chemistry transport models.

International audience; The observed decrease of ozone in the northern midlatitude lower stratosphere is only partially reproduced by chemical models. The transport of ozone‐depleted air from the polar vortex is one of the proposed mechanisms to explain the discrepancy. Here we present a study on the quantification of the air mass transported from the polar vortex to midlatitude during the four winters 1996–1997 to 1999–2000, in relation with vortex filamentation and break up, using the high‐resolution advection model MIMOSA on isentropic surfaces. Sensitivity tests show that the advection model is able to predict the location of polar filaments with accuracy better than 100 km, limited by uncertainties in meteorological advecting wind fields. The effective diffusivity diagnostic is used to evaluate the intensity of the vortex edge barrier and to quantify the transport of air from the polar vortex to midlatitude. The intensity of the polar barrier is increasing with height from 400 to 550 K and is nearly constant above. During periods with a cold and undisturbed vortex, favorable to chlorine activation, the transport is very weak. This suggests that the midwinter vortex filamentation plays a minor role in the midlatitude ozone decline. In the opposite limit, during a stratospheric warming up to 30% of the polar vortex air is transported to midlatitudes. The cumulative transport from early January to end of April across the polar edge varies from almost 5% of the polar vortex air at 675 K in 1997 to 50% at 435 K in 1999.

Coincident observations made over the Moroccan desert during the Sahara mineral dust experiment (SAMUM) 2006 field campaign are used both to validate aerosol amount and type retrieved from multi-angle imaging spectroradiometer (MISR) observations, and to place the suborbital aerosol measurements into the satellite's larger regional context. On three moderately dusty days during which coincident observations were made, MISR mid-visible aerosol optical thickness (AOT) agrees with field measurements point-by-point to within 0.05–0.1. This is about as well as can be expected given spatial sampling differences; the space-based observations capture AOT trends and variability over an extended region. The field data also validate MISR's ability to distinguish and to map aerosol air masses, from the combination of retrieved constraints on particle size, shape and single-scattering albedo. For the three study days, the satellite observations (1) highlight regional gradients in the mix of dust and background spherical particles, (2) identify a dust plume most likely part of a density flow and (3) show an aerosol air mass containing a higher proportion of small, spherical particles than the surroundings, that appears to be aerosol pollution transported from several thousand kilometres away.DOI: 10.1111/j.1600-0889.2008.00398.x

In May–June 2006, airborne and ground-based solar (0.3–2.2 μm) and thermal infrared (4–42 μm) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m-2 per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m-2 were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between −19 and +24 W m-2 for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.DOI: 10.1111/j.1600-0889.2008.00395.x

Ozone measurements from the Total Ozone Mapping Spectrometer on the Nimbus 7 satellite are compared with ground-based measurements from five Dobson stations at northern latitudes to evaluate the accuracy of the TOMS data, particularly in regions north of 50°N. The measurements from the individual stations show mean differences from −2.5% up to +8.3% relative to TOMS measurements and two of the ground stations, Oslo and Longyearbyen, show a significant drift of +1.2% and +3.7% per year, respectively. It can be shown from nearly simultaneous measurements in two different wavelength double pairs at Oslo that at least 2% of the differences result from the use of the CC' wavelength double pair instead of the standard AD wavelength double pair. Since all Norwegian stations used the CC' wavelength double pair exclusively, a similar error can be assumed for Tromso and Longyearbyen. A comparison between the tropospheric ozone content in TOMS data and from ECC ozonesonde measurements at Ny-Alesund and Bear Island shows that the amount of tropospheric ozone in the standard profiles used in the TOMS algorithm is too low, which leads to an error of about 2% in total ozone. Particularly at high solar zenith angles (>80°), Dobson measurements become unreliable. They are up to 20% lower than TOMS measurements averaged over solar zenith angles of 88° to 89°.