ALBERT

Beschreibung: Aerosol particles are important and highly variable components of the terrestrial atmosphere, and they affect both air quality and climate. In order to evaluate their multiple impacts, the most important requirement is to precisely measure their characteristics. Remote sensing technologies such as LIDAR (LIght Detection And Ranging) and sun/sky-photometers are powerful tools for determining aerosol optical and microphysical properties. In our work, we applied several methods to joint or separate LIDAR and sun/sky-photometer data to retrieve aerosol properties. The Raman technique and inversion with regularization use only LIDAR data. The LIRIC (LIdar-Radiometer Inversion Code) and recently developed GARRLiC (Generalized Aerosol Retrieval from Radiometer and LIDAR Combined data) inversion methods use joint LIDAR and sun/sky-photometer data. This paper presents a comparison and discussion of aerosol optical properties (extinction coefficient profiles and LIDAR ratios) and microphysical properties (volume concentrations, complex refractive index values, and effective radius values) retrieved using the above-mentioned methods. The comparison showed inconsistencies in the retrieved LIDAR ratios. However, other aerosol properties were found to be generally in close agreement with the AERONET (AErosol RObotic NETwork) products. It future studies, more cases should be analysed in order to clearly define the peculiarities in our results.

Beschreibung: The paper presents some results of the study on aerosol variability in the period from 2003 to 2011 over the Eastern Europe region, with latitude ranging from 40° N to 60° N and longitude from 20° E to 50° E. The analysis was based on the POLDER/PARASOL and POLDER-2/ADEOS satellites and AERONET (AErosol RObotic NETwork) ground-based sun photometer observations. The aerosol optical thickness (AOT) of the studied area is characterized by values (referenced to 870 nm wavelength) ranging from 0.05 to 0.2, except for in the period of July–August 2010 with strong forest and peat wildfires when the AOT typical values range from 0.3 to 0.5 according to both retrievals. The analysis of seasonal dynamics of aerosol loading has revealed two AOT high value peaks. The spring peak observed in April–May is the result of solitary transportation of Saharan dust in the atmosphere over Eastern Europe, infrequent agricultural fires, transportation of sea salt aerosols by southern winds to Ukraine and Moldova from the Black and Azov seas. The autumn peak in August–September is associated with forest and peat wildfires, considerable transportation of Saharan dust and the presence of soil dust aerosols due to harvesting activity. The maximum values of AOT are observed in May 2006 (0.1–0.15), April 2009 (0.07–0.15) and August 2010 (0.2–0.5). Furthermore, the study has identified a distinct pattern of anthropogenic aerosols over the industrial areas, especially in central Ukraine and eastern Belarus as well as Moscow region in Russia. The comparison of the AOT derived by standard algorithm POLDER/PARASOL with those recomputed from AERONET inversions for fine mode particles with radius 〈 0.3 μm was performed over several AERONET sites. The correlation coefficients for the POLDER/AERONET AOT retrieval comparisons are equal: 0.78 for Moscow site, 0.76 – Minsk, 0.86 – Belsk, 0.81 – Moldova (period 2005–2009), 0.93 – Kyiv and 0.63 for Sevastopol sites (2008–2009). The deviations are explained by the spatial inhomogeneity of the surface polarization that has a stronger effect on aerosol retrieval for clear atmospheric conditions with low aerosol loading when surface impact on satellite observations is more pronounced. In addition, the preliminary analysis of the detailed aerosol properties derived by a new generation PARASOL algorithm was evaluated. The comparison of AOT and single scattering albedo retrieved from the POLDER/PARASOL observations over Kyiv with the closest AERONET retrievals within 30 min of satellite overpass time and with a cloudless day shows acceptable agreement of the aerosol dynamics. The correspondence of those data is observed even for extreme AOT440 value 1.14, which was caused by the forest and peat fires in August 2010.

Beschreibung: This work deals with the applicability of the Linear Estimation technique (LE) to invert spectral measurements of aerosol optical depth (AOD) provided by AERONET CIMEL sun-photometers. The inversion of particle properties using only direct sun AODs allows the evaluation of parameters such as effective radius (reff) and columnar volume aerosol content (V) with significantly better temporal resolution than the operational AERONET algorithm which requires both direct sun and sky radiance measurements. Sensitivity studies performed demonstrate that the constraints on the range of the inversion are very important to minimize the uncertainties, and therefore estimates of reff can be obtained with uncertainties less than 30% and of V with uncertainties below 40%. The LE technique is applied to data acquired at five AERONET sites influenced by different aerosol types and the retrievals are compared with the results of the operational AERONET code. Good agreement between the two techniques is obtained when the fine mode predominates, while for coarse mode cases the LE results systematically underestimate both reff and V. The highest differences are found for cases where no mode predominates. To minimize these biases, correction functions are developed using the multi-year database of observations at selected sites, where the AERONET retrieval is used as the reference. The derived corrections are tested using data from 18 other AERONET stations offering a range of aerosol types. After correction, the LE retrievals provide better agreement with AERONET for all the sites considered. Finally, the LE approach developed here is applied to AERONET and star-photometry measurements at the city of Granada (Spain) to obtain day-to-night time-evolution of columnar aerosol microphysical properties.

Beschreibung: This paper presents a new method for simultaneously retrieving aerosol and surface reflectance properties from combined airborne and ground-based direct and diffuse radiometric measurements. The method is based on the standard Aerosol Robotic Network (AERONET) method for retrieving aerosol size distribution, complex index of refraction, and single scattering albedo, but modified to retrieve aerosol properties in two layers, below and above the aircraft, and parameters on surface optical properties from combined datasets (Cloud Absorption Radiometer (CAR) and AERONET data). A key advantage of this method is the inversion of all available spectral and angular data at the same time, while accounting for the influence of noise in the inversion procedure using statistical optimization. The wide spectral (0.34–2.30 μm) and angular range (180°) of the CAR instrument, combined with observations from an AERONET sunphotometer, provide sufficient measurement constraints for characterizing aerosol and surface properties with minimal assumptions. The robustness of the method was tested on observations made during four different field campaigns: (a) the Southern African Regional Science Initiative 2000 over Mongu, Zambia, (b) the Intercontinental Transport Experiment-Phase B over Mexico City, Mexico (c) Cloud and Land Surface Interaction Campaign over the Atmospheric Radiation Measurement (ARM) Central Facility, Oklahoma, USA, and (d) the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) over Elson Lagoon in Barrow, Alaska, USA. The four areas are dominated by different surface characteristics and aerosol types, and therefore provide good test cases for the new inversion method.

Beschreibung: A contrail from a large-body A380 aircraft at cruise in the humid upper troposphere has been probed with in-situ instruments onboard the DLR research aircraft Falcon. The contrail was sampled during 700 s measurement time at contrail ages of about 1–4 min. The contrail was in the vortex regime during which the primary wake vortices were sinking 270 m below the A380 flight level while the secondary wake remained above. Contrail properties were sampled separately in the primary wake at 90 and 115 s contrail age and nearly continously in the secondary wake at contrail ages from 70 s to 220 s. The scattering phase functions of the contrail particles were measured with a polar nephelometer. The asymmetry parameter derived from these data is used to distinguish between quasi-spherical and aspherical ice particles. In the primary wake, quasi-spherical ice particles were found with concentrations up to 160 cm−3, mean effective diameter Deff of 3.7 μm, maximum extinction of 7.0 km−1, and ice water content (IWC) of 3 mg m−3 at slightly ice-subsaturated conditions. The secondary and primary wakes were separated by an almost particle-free wake vortex gap. The secondary wake contained clearly aspherical contrail ice particles with mean Deff of 4.8 μm, mean (maximum) concentration, extinction, and IWC of 80 (350) cm−3, 1.6 (5.0) km−1, and 2.5 (10) mg m−3, respectively, at conditions apparently above ice-saturation. The asymmetry parameter in the secondary wake decreased with contrail age from 0.87 to 0.80 on average indicating a preferential aspherical ice crystal growth. A retrieval of ice particle habit and size with an inversion code shows that the number fraction of aspherical ice crystals increased from 2% initially to 56% at 4 min contrail age. The observed crystal size and habit differences in the primary and secondary wakes of an up to 4 min old contrail are of interest for understanding ice crystal growth in contrails and their climate impact. Aspherical contrail ice particles cause less radiative forcing than spherical ones.

Beschreibung: The shortwave radiative forcing (ΔF) and the radiative forcing efficiency (ΔFeff) of natural and anthropogenic aerosols have been analyzed using estimates of radiation both at the Top (TOA) and at the Bottom Of Atmosphere (BOA) modeled based on AERONET aerosol retrievals. Six main types of atmospheric aerosols have been compared (desert mineral dust, biomass burning, urban-industrial, continental background, oceanic and free troposphere) in similar observational conditions (i.e., for solar zenith angles between 55° and 65°) in order to compare the nearly same solar geometry. The instantaneous ΔF averages obtained vary from −122 ± 37 Wm−2 (aerosol optical depth, AOD, at 0.55 μm, 0.85 ± 0.45) at the BOA for the mixture of desert mineral dust and biomass burning aerosols in West Africa and −42 ± 22 Wm−2 (AOD = 0.9 ± 0.5) at the TOA for the pure mineral dust also in this region up to −6 ± 3 Wm−2 and −4 ± 2 Wm−2 (AOD = 0.03 ± 0.02) at the BOA and the TOA, respectively, for free troposphere conditions. This last result may be taken as reference on a global scale. Furthermore, we observe that the more absorbing aerosols are overall more efficient at the BOA in contrast to at the TOA, where they backscatter less solar energy into the space. The analysis of the radiative balance at the TOA shows that, together with the amount of aerosols and their absorptive capacity, it is essential to consider the surface albedo of the region on which they are. Thus, we document that in regions with high surface reflectivity (deserts and snow conditions) atmospheric aerosols lead to a warming of the Earth-atmosphere system.

Beschreibung: Recent results from diverse air, ground, and laboratory studies using both radiometric and in situ techniques show that the fractions of black carbon, organic matter, and mineral dust in atmospheric aerosols determine the wavelength dependence of absorption (often expressed as Absorption Angstrom Exponent, or AAE). Taken together, these results hold promise of improving information on aerosol composition from remote measurements. The main purpose of this paper is to show that AAE values for an Aerosol Robotic Network (AERONET) set of retrievals from Sun-sky measurements describing full aerosol vertical columns are also strongly correlated with aerosol composition or type. In particular, we find AAE values near 1 (the theoretical value for black carbon) for AERONET-measured aerosol columns dominated by urban-industrial aerosol, larger AAE values for biomass burning aerosols, and the largest AAE values for Sahara dust aerosols. These AERONET results are consistent with results from other, very different, techniques, including solar flux-aerosol optical depth (AOD) analyses and airborne in situ analyses examined in this paper, as well as many other previous results. Ambiguities in aerosol composition or mixtures thereof, resulting from intermediate AAE values, can be reduced via cluster analyses that supplement AAE with other variables, for example Extinction Angstrom Exponent (EAE), which is an indicator of particle size. Together with previous results, these results strengthen prospects for determining aerosol composition from space, for example using the Glory Aerosol Polarimetry Sensor (APS), which seeks to provide retrievals of multiwavelength single-scattering albedo (SSA) and aerosol optical depth (and therefore aerosol absorption optical depth (AAOD) and AAE), as well as shape and other aerosol properties. Multidimensional cluster analyses promise additional information content, for example by using the Ozone Monitoring Instrument (OMI) to add AAOD in the near ultraviolet and CALIPSO aerosol layer heights to reduce height-absorption ambiguity.

Beschreibung: The global distribution of dust column burden derived from MODIS Deep Blue aerosol products is compared to NH3 column burden retrieved from IASI infrared spectra. We found similarities in their spatial distributions, in particular their hot spots are often collocated over croplands and to a lesser extent pastures. Globally, we found 22% of dust burden collocated with NH3, with only 1% difference between land-use databases. This confirms the importance of anthropogenic dust from agriculture. Regionally, the Indian subcontinent has the highest amount of dust mixed with NH3 (26%), mostly over cropland and during the pre-monsoon season. North Africa represents 50% of total dust burden but accounts for only 4% of mixed dust, which is found over croplands and pastures in Sahel and the coastal region of the Mediterranean. In order to evaluate the radiative effect of this mixing on dust optical properties, we derive the mass extinction efficiency for various mixtures of dust and NH3, using AERONET sunphotometers data. We found that for dusty days the coarse mode mass extinction efficiency decreases from 0.62 to 0.48 m2 g−1 as NH3 burden increases from 0 to 40 mg m−2. The fine mode extinction efficiency, ranging from 4 to 16 m2 g−1, does not appear to depend on NH3 concentration or relative humidity but rather on mineralogical composition and mixing with other aerosols. Our results imply that a significant amount of dust is already mixed with ammonium salt before its long range transport. This in turn will affect dust lifetime, and its interactions with radiation and cloud properties.

Beschreibung: An optimization approach has been developed for simultaneous retrieval of aerosol properties and normalized water-leaving radiance (nLw) from multi-spectral, multi-angular, and polarimetric observations over ocean. The main features of the method are (1) use of a simplified bio-optical model to estimate nLw followed by an empirical refinement within a specified range to improve its accuracy; (2) improved algorithm convergence and stability by applying constraints on the spatial smoothness of aerosol loading and Chlorophyll-a (Chl-a) concentration across neighboring image patches and spectral constraints on aerosol optical properties and on nLw across relevant bands; and (3) enhanced Jacobian calculation by modeling and storing the radiative transfer (RT) in aerosol/Rayleigh mixed layer, pure Rayleigh scattering layers, and ocean medium separately and then coupling them to calculate the field at the sensor. This approach avoids unnecessary and time-consuming recalculations of RT in unperturbed layers in Jacobian evaluations. The Markov chain method is used to model RT in the aerosol/Rayleigh mixed layer and the doubling method is used for the uniform layers of the atmosphere-ocean system. Our optimization approach has been tested using radiance and polarization measurements acquired by the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) over the AERONET USC_SeaPRISM ocean site (6 February 2013) and near the AERONET La Jolla site (14 January 2013), which respectively reported relatively high and low aerosol loadings. Validation of the results is achieved through comparisons to AERONET aerosol and ocean color products and retrievals performed using the Generalized Retrieval of Aerosol and Surface Properties algorithm (Dubovik et al., 2011) on AirMSPI data. Uncertainties of aerosol and nLw retrievals due to random and systematic instrument errors are analyzed by truth-in/truth-out tests with three Chl-a concentrations, five aerosol loadings, three different types of aerosols, and nine combinations of solar incidence and viewing geometries.