Vertical assessment of the mineral dust optical and microphysical properties as retrieved from the synergy between polarized micro-pulse lidar and sun/sky photometer observations using GRASP code

Highlights

• Synergy of P-MPL and Sun/sky photometer with GRASP to retrieve particle properties.

• Columnar/vertical optical and microphysical properties of Saharan dust are derived.

• Good agreement for columnar properties between GRASP and AERONET products.

• High confidence between GRASP and P-MPL/POLIPHON for deriving vertical properties.

Abstract

The potential of the synergetic use of both polarized Micro-Pulse Lidar (P-MPL) and Sun/sky photometer observations by using the GRASP (Generalized Retrieval of Atmosphere and Surface Properties) code for retrieving dust properties has been analysed in this work. Both the columnar and height-resolved variables have been evaluated in comparison, respectively, with AERONET data and vertical P-MPL-derived profiles of the dust optical and microphysical properties over a mid-latitude site. The linear regression analysis together with the mean fractional bias values and relative differences between datasets have been used as the statistical proxies for assessing the degree of agreement by examining 24 selected dust cases. Regarding columnar variables, the microphysical properties (both the particle volume concentration and effective radius) presented a high confidence level between GRASP and AERONET retrievals by taking into account all the proxies. In addition, the single scattering albedo and the real refractive index presented a good confidence level at all the wavelengths, as well, unlike the imaginary refractive index, which was confident at 440 nm only. By analysing the height-resolved properties of the total particle backscatter coefficient and the total volume concentration, the degree of agreement shows a high confidence between GRASP retrievals and P-MPL-derived variables, which were retrieved by using an alternative, well-validated, polarization-based methodology. Indeed, these results can be essential for application with no-polarized systems (e.g., ceilometers, lidars …). Overall, the synergy between P-MPL and Sun/sky photometer measurements regarding the usual co-location of both MPLNET and AERONET networks in combination with GRASP code represent an improved approach to derive the height-resolved optical and microphysical properties for mineral dust particles with a high degree of confidence.

Introduction

Atmospheric aerosols play an important role in the Earth's radiative budget and climate forcing due to their both direct and indirect radiative effects (Boucher et al., 2013). On one hand, the aerosol optical and microphysical properties can directly modify the Earth-atmosphere radiative forcing sign, based on the scattering and absorption of the incoming solar and outgoing thermal radiation. On the other hand, aerosol particles can act as cloud condensation nuclei (CCN) and ice nucleating particles (INP) (DeMott et al., 2003; Karydis et al., 2011) and, thus, can indirectly modify the cloudy media properties. For both effects, the improvement on the knowledge of the aerosol vertical distribution is crucial. Indeed, an advanced vertical inversion of the dust properties is important when they are used for determining the aerosol radiative impact. Although, in general terms, aerosol particles mainly produce negative radiative forcing (net cooling effect) due to the predominance of the scattering of solar radiation in all directions, the location of certain particle types at given vertical altitudes can cause perturbations. For instance, aerosols are able to heat the atmospheric regions in which they are present when absorbing solar radiation, in spite of the possible net cooling effect for the atmospheric column (Pilewskie, 2007). As demonstrated by Gobbi et al. (2000), dust particles can modify the atmospheric thermal structure by solar radiation absorption. Also, Perrone and Bergamo (2011) and Perrone et al. (2012) evaluated the impact of mineral dust vertical distribution on the aerosol direct radiative effects and heating rates using ground-based lidar, whereas Granados-Muñoz et al. (2019) applied a combination of satellite platforms, including CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation), for that purpose. In addition, the role of dust fine particles in the dust direct radiative effect can be relevant, as revealed from lidar observations, even when dust coarse particles are predominant as usually under dusty conditions (Córdoba-Jabonero et al., 2021a).

During the 90's the worldwide NASA Aerosol Robotic NETwork (AERONET) was established with the aim of achieving comprehension of the aerosol distribution on a global scale (Holben et al., 1998). This network, by using two kinds of photometric measurements, i.e. direct-Sun irradiances and the angular distribution of diffuse sky radiation, and in combination with state-of-the-art retrieval algorithms (Dubovik and King, 2000; Dubovik et al., 2006), provides some column-integrated aerosol optical and microphysical properties that are a key in climate, air quality and health studies, such as Aerosol Optical Depth (AOD), Ångstrom exponent (AE), particle size distribution (PSD), refractive index, single scattering albedo, asymmetry factor and phase function, among others.

Despite its clear advantages regarding spatial coverage, AERONET measurements do not provide vertically-resolved aerosol properties. However, this drawback might be overcome in combination with lidar/ceilometer measurements. Currently, several networks with different levels of developments, spatial/temporal coverage, and scientific objectives are deployed covering most of our planet, such as EARLINET (European Aerosol Research LIdar NETwork) (Pappalardo et al., 2014) and E-PROFILE (EUMETNET Profiling Programme) in Europe, LALINET (Latin-American LIdar NETwork) in South America (Guerrero-Rascado et al., 2016; Antuña-Marrero et al., 2017), ADNET (Asian Dust NETwork) in Asia (Murayama et al., 2001), and NASA MPLNET (Micro-Pulse Lidar NETwork) (Welton et al., 2001) and NDACC (Network for the Detection of Atmospheric Composition Change) (De Mazière et al., 2018) world-widely spread, among others. In this work, aerosol observations at a co-located MPLNET and AERONET site have been used.

Due to the large number of AERONET/MPLNET co-located stations (21 sites at present), a more complete assessment of aerosol properties can be achieved, since both columnar-integrated and vertically-resolved measurements can be combined by using the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) code (Dubovik et al., 2014) in order to provide a significant improvement in retrieving advanced aerosol products. The GRASP code, using the heritage of AERONET inversion scheme (Dubovik and King, 2000; Dubovik et al., 2006), is a versatile and open-source inversion algorithm capable to retrieve aerosol optical and microphysical properties by combining active and passive remote sensing techniques (Lopatin et al., 2013). Hence, this code has been applied, among others, to satellite imagery (e.g., Milinevsky et al., 2014; Kokhanovsky et al., 2015; Chen et al., 2019; Sayer et al., 2018; Remer et al., 2019; Tan et al., 2019; Li et al., 2019, Li et al., 2020; Wei et al., 2020), in situ data (Espinosa et al., 2017, Espinosa et al., 2019; Schuster et al., 2019), different combinations of Sun/sky photometer: only spectral AODs (Torres et al., 2017), spectral AODs, sky radiances and polarized sky radiances (Fedarenka et al., 2016), spectral AODs and sky cameras images (Román et al., 2017), spectral AODs, sky radiances and lidar/ceilometer signals (e.g., Bovchaliuk et al., 2016; Benavent-Oltra et al., 2017; Tsekeri et al., 2017; Román et al., 2018; Herreras et al., 2019; Hu et al., 2019; Titos et al., 2019; Molero et al., 2020), spectral AODs and Raman lidar signals (e.g., Benavent-Oltra et al., 2019; Soupiona et al., 2019) and spectral AODs, sky radiances and airborne backscatter observations (Lopatin et al., 2020). To our knowledge, this is the first time that GRASP code is used together with the combination of Sun/sky photometer and polarized Micro-Pulse Lidar (P-MPL) observations of Saharan dust at mid-latitudes using a particular lidar-based methodology as reference for GRASP assessment.

The aim of this study is to analyse the potential of the GRASP products, as obtained from the synergy between AERONET Sun/sky photometer and elastic P-MPL lidar observations, for deriving columnar and vertically-resolved aerosol properties. For that purpose, particular mineral dust cases as observed at the AERONET/MPLNET El Arenosillo site (Huelva, Spain, 37.1°N 6.7°W) along 2018 have been selected regarding its relatively close proximity to the Saharan dust sources. Hence, a total of 24 cases under dusty conditions with simultaneous AERONET and P-MPL observations have been analysed, fulfilling specific GRASP high-quality criteria.

A description of the measurement site, the instrumentation used, the alternative methodology for comparison purposes, and the GRASP model in addition to the comparative analysis applied for evaluating the performance of GRASP is shown in Section 2. Section 3 shows the results as obtained from the comparison between GRASP-retrieved columnar and height-resolved products and, respectively, AERONET data and P-MPL-derived variables in order to determine the degree of confidence. The main conclusions are introduced in Section 4.