Abstract
Abstract. The Sierra Nevada Lidar aerOsol Profiling Experiment I and II (SLOPE I and II) campaigns were intended to determine the vertical structure of aerosols by remote sensing instruments and test the various retrieval schemes for obtaining aerosol microphysical and optical properties with in situ measurements. The SLOPE I and II campaigns were developed during the summers of 2016 and 2017, respectively, combining active and passive remote sensing with in situ measurements at stations belonging to the AGORA observatory (Andalusian Global ObseRvatory of the Atmosphere) in the Granada area (Spain). In this work, we use the in situ measurements of these campaigns to evaluate aerosol properties retrieved by the GRASP code (Generalized Retrieval of Atmosphere and Surface Properties) combining lidar and sun–sky photometer measurements. We show an overview of aerosol properties retrieved by GRASP during the SLOPE I and II campaigns. In addition, we evaluate the GRASP retrievals of total aerosol volume concentration (discerning between fine and coarse modes), extinction and scattering coefficients, and for the first time we present an evaluation of the absorption coefficient. The statistical analysis of aerosol optical and microphysical properties, both column-integrated and vertically resolved, from May to July 2016 and 2017 shows a large variability in aerosol load and types. The results show a strong predominance of desert dust particles due to North African intrusions. The vertically resolved analysis denotes a decay of the atmospheric aerosols with an altitude up to 5 km a.s.l. Finally, desert dust and biomass burning events were chosen to show the high potential of GRASP to retrieve vertical profiles of aerosol properties (e.g. absorption coefficient and single scattering albedo) for different aerosol types. The aerosol properties retrieved by GRASP show good agreement with simultaneous in situ measurements (nephelometer, aethalometer, scanning mobility particle sizer, and aerodynamic particle sizer) performed at the Sierra Nevada Station (SNS) in Granada. In general, GRASP overestimates the in situ data at the SNS with a mean difference lower than 6 µm3 cm−3 for volume concentration, and 11 and 2 Mm−1 for the scattering and absorption coefficients. On the other hand, the comparison of GRASP with airborne measurements also shows an overestimation with mean absolute differences of 14 ± 10 and 1.2 ± 1.2 Mm−1 for the scattering and absorption coefficients, showing a better agreement for the absorption (scattering) coefficient with higher (lower) aerosol optical depth. The potential of GRASP shown in this study will contribute to enhancing the representativeness of the aerosol vertical distribution and provide information for satellite and global model evaluation.
Highlights
The characterization of atmospheric aerosol optical and microphysical properties is difficult due to their high spatial and temporal variability in the atmosphere
During the last few decades, several field campaigns have been carried out for studying atmospheric aerosol properties (e.g. Tanré et al, 2003; Mallet et al, 2016; Veselovskii et al, 2016; Vandenbussche et al, 2020) using observatories with in situ measurements and that are included in global networks based on passive and active remote sensing instruments, such as the AERosol RObotic NETwork (AERONET; Holben et al, 1998) and European Aerosol Research LIdar NETwork (EARLINET; Pappalardo et al, 2014)
For the inter-comparison between the GRASP retrievals and the Sierra Nevada Station (SNS) in situ data, we selected the in situ measurements averaged in ± 15 min around the GRASP retrieval time and the 400 m averaged data of the GRASP retrieval profile at 2500 m a.s.l. (SNS altitude)
Summary
The characterization of atmospheric aerosol optical and microphysical properties is difficult due to their high spatial and temporal variability in the atmosphere. Vertically resolved aerosol observations are needed to discern between the different aerosol layers and to study their radiative properties. In this regard, lidar systems are used for aerosol optical and microphysical properties profiling. Advanced lidar systems provide information on the backscatter elastic and inelastic signals allowing the retrieval of vertical profiles of aerosol backscatter and extinction (α) coefficients using the Raman technique These measurements allow for the retrieval of particle vertical microphysical properties by inversion algorithms using the 3β + 2α configuration (e.g. Müller et al, 1999; Böckmann, 2001; Veselovskii et al, 2002)
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