Night-Time Ground Hyperspectral Imaging for Urban-Scale Remote Sensing of Ambient PM—Modal Concentrations Retrieval

Abstract

Retrieval of aerosol loading in vertical atmospheric columns is a common product of satellite and ground instruments that measure spectral extinction of solar radiation throughout the entire atmosphere. Here we study ground hyperspectral imaging of artificial light sources as a complementary method for retrieving fine aerosol concentrations along quazi-horizontal ambient open paths. Previously, we reported hyperspectral measurements of the aerosol optical thickness in the 500-900 nm range over urban-scale distances (180 m to 4 km), measuring the extinction of radiation emitted from a halogen source. Here we confirm in a laboratory-setup the basic premise that different accumulation-size aerosols generate distinct hyperspectral signatures in this spectral range. Measured hyperspectral attenuation signatures of fine aerosols were comparable to calculated Mie scattering signatures, suggesting that modal aerosol concentrations can be retrieved. A genetic algorithm was adapted to estimate the aerosol modal concentrations from its hyperspectral extinction signature. Retrievals of aerosol concentrations from measured and synthetic hyperspectral signatures indicated a robust algorithm, with an expected retrieval error of 0.2-22% for typical ambient concentrations along an urban-scale open path. The retrieval accuracy was found to depend on the relative aerosol modal concentrations, especially when there is a substantial overlap between the modal spectral signatures.

Night-Time Ground Hyperspectral Imaging for Urban-Scale Remote Sensing of Ambient PM. I. Aerosol Optical Thickness Acquisition

“…It has been noted (Alakian et al 2009) that the spectral response of ambient aerosols at the visible-NIR wavebands (0.4-2.5 µm) shows only moderate variation and, therefore, many features of the hyperspectra in this range (i.e., possible spectral variables) are expected to be redundant. However, different ambient PM concentrations and composition may generate partially overlapping signatures (Etzion et al 2010), which call for a strategy of nonlinear curve fitting and favors multivariable analysis. Moreover, optimally, to obtain the best retrieval the spectral range for analysis should be dynamic and change according to the characteristics of the true environmental (i.e., aerosol) conditions.…”