ALBERT

Description: 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.

Description: 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.

Description: 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.

Description: Recently, some authors have suggested that the absorption Angstrom exponent (AAE) can be used to deduce the component aerosol absorption optical depths (AAOD) of carbonaceous aerosols in the AERONET database. This "AAE approach" presumes that AAE ≪ 1 for soot carbon, which contrasts the traditional small particle limit of AAE = 1 for soot carbon. Thus, we provide an overview of the AERONET retrieval, and investigate how the microphysics of carbonaceous aerosols can be interpreted in the AERONET AAE product. We find that AAE ≪ 1 in the AERONET database requires large coarse mode fractions and/or imaginary refractive indices that increase with wavelength. Neither of these characteristics are consistent with the current definition of soot carbon, so we explore other possibilities for the cause of AAE ≪ 1. We note that AAE is related to particle size, and that coarse mode particles have a smaller AAE than fine mode particles for a given aerosol mixture of species. We also note that the mineral goethite has an imaginary refractive index that increases with wavelength, is very common in dust regions, and can easily contribute to AAE ≪ 1. We find that AAE ≪ 1 can not be caused by soot carbon, unless soot carbon has an imaginary refractive index that increases with wavelength throughout the visible and near infrared spectrums. Finally, AAE is not a robust parameter for separating carbonaceous absorption from dust aerosol absorption in the AERONET database.

Description: We describe a method of using the aerosol robotic network (AERONET) size distributions and complex refractive indices to retrieve the relative proportion of carbonaceous aerosols and iron oxide minerals. We assume that soot carbon has a spectrally flat refractive index, and that enhanced imaginary indices at the 440 nm wavelength are caused by brown carbon or hematite. Carbonaceous aerosols can be separated from dust in imaginary refractive index space because 95% of biomass burning aerosols have imaginary indices greater than 0.0042 at the 675–1020 nm wavelengths, and 95% of dust has imaginary refractive indices of less than 0.0042 at those wavelengths. However, mixtures of these two types of particles can not be unambiguously partitioned on the basis of optical properties alone, so we also separate these particles by size. Regional and seasonal results are consistent with expectations. Monthly climatologies of fine mode soot carbon are less than 1.0% by volume for West Africa and the Middle East, but the southern Africa and South America biomass burning sites have peak values of 3.0 and 1.7%. Monthly-averaged fine mode brown carbon volume fractions have a peak value of 5.8% for West Africa, 2.1% for the Middle East, 3.7% for southern Africa, and 5.7% for South America. Monthly climatologies of iron oxide volume fractions show little seasonal variability, and range from about 1.1 to 1.7% for coarse mode aerosols in all four study regions. Finally, our sensitivity study indicates that the soot carbon retrieval is not sensitive to the component refractive indices or densities assumed for carbonaceous and iron oxide aerosols, and differs by only 15.4% when these parameters are altered from our chosen baseline values. The associated soot carbon absorption aerosol optical depth (AAOD) does not vary at all when these parameters are altered, however, because the retrieval is constrained by the AERONET optical properties.

Description: The importance of light absorbing organic aerosols, often called brown carbon (BrC), has become evident in recent years. However, there are relatively few measurement-based estimates for the direct radiative effect of BrC so far. In those earlier studies, the AErosol RObotic NETwork (AERONET) measured Aerosol Absorption Optical Depth (AAOD) and Absorption Angstrom Exponent (AAE) have been exploited. However, these two pieces of information are clearly not sufficient to separate properly carbonaceous aerosols from dust, while imaginary indices of refraction would contain more and better justified information for this purpose. This is first time that the direct radiative effect (DRE) of BrC is estimated by exploiting the AERONET-retrieved imaginary indices. We estimated it for four sites in Indo-Gangetic Plain (IGP), Karachi, Lahore, Kanpur and Gandhi College. We found a distinct seasonality, which was generally similar among all the sites, but with slightly different strengths. The monthly warming effect up to 0.5 W m-2 takes place during spring season. On the other hand, BrC results in overall cooling effect in the winter season, which can reach levels close to −1W m-2. We then estimated similarly also DRE of black carbon and total aerosol, in order to assess the relative significance of BrC radiative effect in the radiative effects of other components. Even though BrC impact seems minor in this context, we demonstrated that it is not insignificant and moreover that it is crucial to perform spectrally resolved radiative transfer calculations to obtain good estimates for DRE of BrC.

Description: The evaluation of aerosol radiative effect on broadband hemispherical solar flux is often performed using simplified spectral and directional scattering characteristics of atmospheric aerosol and underlying surface reflectance. In this study we present a rigorous yet fast computational tool that accurately accounts for detailed variability of both spectral and angular scattering properties of aerosol and surface reflectance in calculation of direct aerosol radiative effect. The tool is developed as part of the GRASP (Generalized Retrieval of Aerosol and Surface Properties) project. We use the tool to evaluate instantaneous and daily average radiative efficiencies of several key atmospheric aerosol models over different surface types. We then examine the differences due to neglect of surface reflectance anisotropy, non-sphericity of aerosol particle shape and accounting only for aerosol angular scattering asymmetry instead of using full phase function. For example, it is shown that neglecting aerosol particle nonsphericity causes mainly overestimation of the aerosol cooling effect and that magnitude of this overestimate changes significantly as a function of solar zenith angle (SZA) if only asymmetry parameter is used instead of detailed phase function. It was also found that the nonspherical–spherical differences in the calculated aerosol radiative effect are not modified significantly if detailed BRDF (Bidirectional Reflectance Distribution Function) is used instead of Lambertian approximation of surface reflectance. Additionally, calculations show that usage of only angular scattering asymmetry, even for case of spherical aerosols, modifies dependence of instantaneous aerosol radiative effect on SZA. This effect can be canceled for daily average values, but only if sun reaches the zenith, otherwise a systematic bias remains. Since the daily average radiative effect is obtained by integration over a range of SZAs, the errors vary with latitude and season. In summary, the present analysis showed that use of simplified assumptions causes systematic biases, rather than random uncertainties, in calculation of both instantaneous and daily average aerosol radiative effect. Finally, we illustrate application of the rigorous aerosol radiative effect calculations performed as part of GRASP aerosol retrieval from real POLDER/PARASOL satellite observations.

Description: West Africa and the adjacent oceanic regions are very important locations for studying dust properties and their influence on weather and climate. The SHADOW (Study of SaHAran Dust Over West Africa) campaign is performing a multi-scale and multi-laboratory study of aerosol properties and dynamics using a set of in situ and remote sensing instruments at an observation site located at IRD (Institute for Research and Development) Center, Mbour, Senegal (14° N, 17° W). In this paper, we present the results of lidar measurements performed during the first phase of SHADOW which occurred in March-April, 2015. The multiwavelength Mie-Raman lidar acquired 3β + 2α + 1δ measurements during this period. This set of measurements has permitted particle intensive properties such as extinction and backscattering Ångström exponents (BAE) for 355/532 nm wavelengths corresponding lidar ratios and depolarization ratio at 532 nm to be determined. The mean values of dust lidar ratios during the observation period were about 53 sr at both 532 nm and 355 nm, which agrees with the values observed during the SAMUM 1 and SAMUM 2 campaigns held in Morocco and Cape Verde in 2006, 2008. The mean value of particle depolarization ratio at 532 nm was 30 ± 4.5 %, however during strong dust episodes this ratio increased to 35 ± 5 %, which is also in agreement with the results of the SAMUM campaigns. The backscattering Ångström exponent during the dust episodes decreased to ~ −0.7, while the extinction Ångström exponent though being negative, was greater than −0.2. Low values of BAE can likely be explained by an increase in the imaginary part of the dust refractive index at 355 nm compared to 532 nm. The dust extinction and backscattering coefficients at multiple wavelengths were inverted to the particle microphysics using the regularization algorithm and the model of randomly oriented spheroids. The analysis performed has demonstrated that the spectral dependence of the imaginary part of the dust refractive index may significantly influence the inversion results and should be taken into account.

Description: The analysis of aerosol microphysical properties profiles at different European stations is made in the framework of the ChArMEx/EMEP 2012 field campaign (9–11 July 2012). During and in support to this campaign, five lidar ground-based stations (Athens, Barcelona, Bucharest, Évora and Granada) performed 72 h of continuous lidar and collocated and coincident sun-photometer measurements. Therefore it was possible to retrieve volume concentration profiles with the Lidar Radiometer Inversion Code (LIRIC). Results indicated the presence of a mineral dust plume affecting the Western Mediterranean region (mainly Granada station) whereas a different aerosol plume was observed over the Balkans area. LIRIC profiles showed a predominance of coarse spheroid particles above Granada, as expected for mineral dust, and an aerosol plume composed mainly of fine and coarse spherical particles above Athens and Bucharest. Due to the exceptional characteristics of the ChArMEx database, the analysis of the microphysical properties profiles temporal evolution was also possible. An in depth analysis was performed mainly at Granada station because of the availability of continuous lidar measurements and frequent AERONET inversion retrievals. The analysis at Granada was of special interest since the station was affected by mineral dust during the complete analyzed period. LIRIC was found to be a very useful tool for performing continuous monitoring of mineral dust, allowing for the analysis of the dynamics of the dust event in the vertical and temporal coordinates. Results obtained here illustrate the importance of having collocated and simultaneous advanced lidar and sun-photometer measurements in order to characterize the aerosol microphysical properties both in the vertical and temporal coordinates at a regional scale. In addition, this study revealed that the use of the depolarization information as input in LIRIC in the stations of Bucharest, Évora and Granada was crucial for the characterization of the aerosol types and their distribution in the vertical column, whereas in stations lacking of depolarization lidar channels ancillary information was needed. Results obtained were also used for the validation of different mineral dust models. In general, the models better forecast the vertical distribution of the mineral dust than the column integrated mass concentration, which was underestimated in most of the cases.

Description: Because of its wide coverage over much of the globe, biomass burning has been widely studied in the context of direct radiative forcing. Such study is warranted as smoke particles scatter and at times absorb solar radiation efficiently. Further, as much of what is known about smoke transport and impacts is based on remote sensing measurements, the optical properties of smoke particles have far reaching effects into numerous aspects of biomass burning studies. Global estimates of direct forcing have been widely varying, ranging from near zero to −1 Wm−2. A significant part of this difference can be traced to varying assumptions on the optical properties of smoke. This manuscript is the third part of four examining biomass-burning emissions. Here we review and discuss the literature concerning measurement and modeling of optical properties of biomass-burning particles. These include available data from published sensitivity studies, field campaigns, and inversions from the Aerosol Robotic Network (AERONET) of Sun photometer sites. As a whole, optical properties reported in the literature are varied, reflecting both the dynamic nature of fires, variations in smoke aging processes and differences in measurement technique. We find that forward modeling or "internal closure" studies ultimately are of little help in resolving outstanding measurement issues due to the high degree of degeneracy in solutions when using "reasonable" input parameters. This is particularly notable with respect to index of refraction and the treatment of black carbon. Consequently, previous claims of column closure may in fact be more ambiguous. Differences between in situ and retrieved ωo values have implications for estimates of mass scattering and mass absorption efficiencies. In this manuscript we review and discuss this community dataset. Strengths and lapses are pointed out, future research topics are prioritized, and best estimates and uncertainties of key smoke particle parameters are provided.