Overview of the SLOPE I and II campaigns: aerosol properties retrieved with lidar and sun–sky photometer measurements
Authors: Benavent-Oltra Jose Antonio , Casquero-Vera Juan Andrés , Román Roberto , Lyamani Hassan , Pérez-Ramírez Daniel , Granados-Muñoz María José , Herrera Milagros , Cazorla Alberto , Titos Gloria , Ortiz-Amezcua Pablo , Bedoya-Velásquez Andrés Esteban , de Arruda Moreira Gregori , Pérez Noemí , Alastuey Andrés , Dubovik Oleg , Guerrero-Rascado Juan Luis , Olmo-Reyes Francisco José , Alados-Arboledas Lucas 
Published: June 2021

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.