A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements

Abstract

Abstract. The problem of deriving a complete set of aerosol optical properties from Sun and sky radiance measurements is discussed. Algorithm development is focused on improving aerosol retrievals by means of including a detailed statistical optimization of the influence of noise in the inversion procedure. The methodological aspects of such an optimization are discussed in detail and revised according to both modern findings in inversion theory and practical experience in remote sensing. Accordingly, the proposed inversion algorithm is built on the principles of statistical estimation: the spectral radiances and various a priori constraints on aerosol characteristics are considered as multisource data that are known with predetermined accuracy. The inversion is designed as a search for the best fit of all input data by a theoretical model that takes into account the different levels of accuracy of the fitted data. The algorithm allows a choice of normal or lognormal noise assumptions. The multivariable fitting is implemented by a stable numerical procedure combining matrix inversion and univariant relaxation. The theoretical inversion scheme has been realized in the advanced algorithm retrieving aerosol size distribution together with complex refractive index from the spectral measurements of direct and diffuse radiation. The aerosol particles are modeled as homogeneous spheres. The atmospheric radiative transfer modeling is implemented with well-established publicly available radiative transfer codes. The retrieved refractive indices can be wavelength dependent; however, the extended smoothness constraints are applied to its spectral dependence (and indirectly through smoothness constraints on retrieved size distributions). The positive effects of statistical optimization on the retrieval results as well as the importance of applying a priori constraints are discussed in detail for the retrieval of both aerosol size distribution and complex refractive index. The developed algorithm is adapted for the retrieval of aerosol properties from measurements made by ground-based Sun-sky scanning radiometers used in the Aerosol Robotic Network (AERONET). The results of numerical tests together with examples of experimental data inversions are presented.

Aerosol trends as a potential driver of regional climate in the central United States: evidence from observations

“…For single scattering albedo and asymmetry parameters of the aerosol, we rely on measurements from two long-term AERONET sites located close to the SURFRAD sites: Bondville, IL, and Huntsville, AL (Dubovik and King, 2000). Gan et al (2014) found close agreement between AOD measurements at the Bondville SURFRAD site and nearby AERONET sites, so we assume that AERONET aerosol properties represent those at SURFRAD.…”

Impact of dust aerosols on the radiative budget, surface heat fluxes, heating rate profiles and convective activity over West Africa during March 2006

“…AERONET database provides spectral optical thickness directly measured from the surface and the detailed microphysical aerosol properties including size distribution, refractive index, SSA derived from both direct sun and diffuse sky-radiances measurements using inversion algorithm by Dubovik and King (2000). Recently, this algorithm was updated with several improvements (generating Version 2 retrievals used here).…”

“…Therefore, in order to take into account the most realistic dust SSA in our numerical simulations, we have used dust size distributions as simulated by MesoNH (Tulet et al, 2005;Grini et al, 2006) together with dust refractive indexes retrieved from AERONET/PHOTONS radiometers measurements (Dubovik and King, 2000) during the dust outbreak. AERONET database provides spectral optical thickness directly measured from the surface and the detailed microphysical aerosol properties including size distribution, refractive index, SSA derived from both direct sun and diffuse sky-radiances measurements using inversion algorithm by Dubovik and King (2000).…”

“…This kind of "on-line" model enables to investigate how the aerosols direct radiative forcing impact the surface-atmosphere system. Due to the significant impact of dust SSA on the radiative budget (both at TOA, surface SRF, and into the dust layer) and heating rate profiles, we used dust refractive indexes retrieved by AERONET (Dubovik and King, 2000) together with the size distribution for calculating the shortwave dust SSA used in our MesoNH simulations. In parallel, experimental observations obtained at Djougou (Northern Benin) allowed to complement modelling simulations.…”

Satellite Retrievals of Aerosol Optical Depth over a Subtropical Urban Area: The Role of Stratification and Surface Reflectance

“…However, emission patterns in Santiago have changed significantly since this period (e.g., Gallardo et al, 2012b) and, lacking recent data and instead of using results from Horvath and Trier (1993), we use AOD derived from the AERONET record collected in Santiago from August 2001 to October 2002 to compare with our simple model AOD simulations (This model is described later in the paper). The AERONET data set contains ~7800 AOD measurements of Level 2 data and 53 inversions (Dubovik and King, 2000) for aerosol properties (e.g., size distribution, phase function, single scattering albedo) over Santiago. These inversions have an average asymmetry parameter of 0.7 for a 441-nm wavelength and 0.63 for a 675-nm wavelength, with standard deviations of 0.03 and 0.04, respectively.…”