Ground-based and airborne hyperspectral imaging sensor measurements of NO2 total column amounts

Abstract

The hyperspectral imaging sensor (HIS) captures 3-dimensional cubes of spatial images and spectral radiance. The HISs, used in this study, each have a spectral range of 250-500 nm (UV-VIS) and 400-1000 nm (VNIR) and measures scattered solar radiance. Due to their mobility, we utilized them for both ground-based (downward radiance) and airborne (upward radiance) measurements in South Korea. From the HIS measurements, total column amounts of nitrogen dioxide (TCN) were retrieved to investigate the spatial distribution of NO2. For the TCN retrieval, the method of radiance fitting between radiative transfer model simulation and HIS measurements was used, and a parameter defined as radiance ratio of wavelength pairs, composed of wavelengths with strong and weak NO2 absorption, was utilized for the fitting. To estimate the uncertainty of the TCN retrieval, we conducted sensitivity analyses to the observation geometries (solar zenith angle, viewing zenith angle and relative azimuth angle) and the vertical profile of NO2. The arbitral biases of the observation geometries caused insignificant errors to the TCN retrieval. However, we found that NO2 vertical distribution contributed to uncertainty in retrieved TCN due to the effective optical path length. Additional sensitivity analysis to surface reflectance was done for airborne measurement retrievals. For validation, we compared TCN retrieved from HIS and that from Pandora spectrophotometer at Yonsei University in Seoul, South Korea. In the case of ground-based measurement retrievals, the temporal variation of TCN showed good agreement with a slight positive bias.