Abstract
Abstract. This study presents a novel methodology for the remote monitoring of aerosol components over large spatial and temporal domains. The concept is realized within the GRASP (Generalized Retrieval of Aerosol and Surface Properties) algorithm to directly infer aerosol components from the measured radiances. The observed aerosols are assumed to be mixtures of hydrated soluble particles embedded with black carbon, brown carbon, iron oxide, and other (non-absorbing) insoluble inclusions. The complex refractive indices of the dry components are fixed a priori (although the refractive index of the soluble host is allowed to vary with hydration), and the complex refractive indices of the mixture are computed using mixing rules. The volume fractions of these components are derived along with the size distribution and the fraction of spherical particles, as well as the spectral surface reflectance in cases when the satellite data are inverted. The retrieval is implemented as a statistically optimized fit in a continuous space of solutions. This contrasts with most conventional approaches in which the type of aerosol is either associated with a pre-assumed aerosol model that is included in a set of look-up tables, or determined from the analysis of the retrieved aerosol optical parameters (e.g., single scattering albedo, refractive index, among others, provided by the AERONET retrieval algorithm); here, we retrieve the aerosol components explicitly. The approach also bridges directly to the quantities used in global chemical transport models. We first tested the approach with synthetic data to estimate the uncertainty, and then applied it to real ground-based AERONET and spaceborne POLDER/PARASOL observations; thus, the study presents a first attempt to derive aerosol components from satellite observations specifically tied to global chemical transport model quantities. Our results indicate aerosol optical characteristics that are highly consistent with standard products (e.g., R of ∼0.9 for aerosol optical thickness) and demonstrate an ability to separate intrinsic optical properties of fine- and coarse-sized aerosols. We applied our method to POLDER/PARASOL radiances on the global scale and obtained spatial and temporal patterns of the aerosol components that agree well with existing knowledge on aerosol sources and transport features. Finally, we discuss limitations and perspectives of this new technique.
Highlights
Information about atmospheric aerosol chemical composition has a great importance for monitoring and understanding of various aspects of climate and environment
The more abundant presence of particles with the spectral absorption signature of black carbon (BC) is reasonable for the areas near biomass burning emissions, whereas particles with a spectrally dependent absorption signature of brown carbon (BrC) are generally enriched in downwind regions, which can explain the appearance of BrC concentrations in aerosol particles transported over ocean in Northern Hemisphere
We present a new approach for monitoring atmospheric aerosol components with remote sensing observations
Summary
Information about atmospheric aerosol chemical composition has a great importance for monitoring and understanding of various aspects of climate and environment. Schuster et al (2016a) used the AERONET size distributions and complex refractive indices to retrieve the relative proportion of carbonaceous aerosols (BC and BrC) and free iron minerals (hematite and goethite) in fine- and coarse-mode particles All of these methods for retrieving aerosol components rely upon an intermediate retrieval of the refractive index and/or the aerosol absorption optical depth (e.g., one provided by the AERONET operational inversion). The objective of our GRASP/Component approach is to retrieve the aerosol components directly from remote sensing measurements without intermediate retrieval of the complex refractive index, as in previous studies (Arola et al, 2011; Koven and Fung, 2006; Li et al, 2015, 2013; Schuster et al, 2016a, 2009, 2005; Wang et al, 2013). We note that the GRASP/Component approach is only possible (1) if there is significant instrument sensitivity to the parameters that are related to aerosol components (i.e., complex refractive index), and (2) if this sensitivity is maintained while other parameters such as the size distribution are adjusted
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