Towards introducing aerosols inhomogeneity into GRASP remote sensing algorithm 

Abstract

Though inhomogeneous particles are common for atmospheric aerosol, inhomogeneity is not taken into account in present-day remote sensing retrieval algorithms. Effects of inhomogeneity on radiation scattering can however have an impact on the quality of aerosol retrievals, as shown in Mishenko et al., 2016. The current study aims to address this gap by an attempt to introduce aerosol inhomogeneity parameterization into the aerosol remote sensing retrieval algorithm – GRASP (Dubovik et al., 2021). First, we focus on AERONET measurements as a global network with an efficient retrieval algorithm with the aim to identify situations where currently employed homogeneous aerosol model does not reproduce correctly the radiation field or gets to the limits of the field of solutions. We examine AERONET retrievals using results of operational AERONET algorithm, but also those obtained using an independent recently developed version of GRASP/Component algorithm (Li et al., 2019) applied to AERONET. A notable part of these retrievals, under atmospheric conditions suspected to cause particles inhomogeneity, present questionable values for retrieved refractive index, i.e. values of its real part reach the algorithmic limit. This situation unveils potential mismatch of employed aerosol microphysical model. At the second step we model the aerosol inhomogeneity by Mie calculations for layered spheres (core/shell) particle structure with an ammonium nitrate/sulfate liquid shell and various composition of core, relying on some field results reported in (Unga et al. 2018). We then examine the response of the obtained optical characteristics to variation in core/shell model. Namely, the phase function and degree of linear polarization are compared for several core radii, refractive indexes and particles size distributions. We present comparative analysis for the effects of structure changes over size changes and study their differences relative to homogeneous particle model. This analysis reveals potential sensitivity of remote sensing to particles inhomogeneity and serves for parameterization of core/shell model in the remote sensing algorithm GRASP. The updated GRASP/Component algorithm will then be applied to previously identified cases of questionable AERONET retrievals. ReferencesDubovik O., Fuertes D., Litvinov P., et al.: A Comprehensive Description of Multi-Term LSM for Applying Multiple a Priori Constraints in Problems of Atmospheric Remote Sensing: GRASP Algorithm, Concept, and Applications. Front. Remote Sens. 2:706851, 2021. doi:10.3389/frsen.2021.706851Unga F., Choël M., Derimian Y., et al. : Microscopic Observationsof Core-Shell Particle Structure and Implications for Atmospheric Aerosol Remote Sensing. Journal of Geophysical research 123:24, 2018. doi:10.1029/2018JD028602Michael I. Mishchenko, Janna M. Dlugach, and Li Liu, "Linear depolarization of lidar returns by aged smoke particles," Appl. Opt. 55, 9968-9973, https://doi.org/10.1364/AO.55.009968, (2016).Li, L., Dubovik, O., Derimian, Y., et al.: Retrieval of aerosol components directly from satellite and ground-based measurements, Atmos. Chem. Phys., 19, 13409–13443, https://doi.org/10.5194/acp-19-13409-2019, 2019.