FOCAL-AIR: Adaption of the Fast atmOspheric traCe gAs retrievaL to airborne high spectral resolution remote sensing measurements

Abstract:

Methane (CH₄) and carbon dioxide (CO₂) are the two most important anthropogenic greenhouse gases. Their sources have been studied by a variety of different instruments, including satellite, airborne and ground based remote sensing and airborne and ground based in situ instruments. However, large uncertainties regarding emissions from different sectors still remain for both gases.The new MAMAP-2D instrument, a grating imaging spectrometer with spectral resolution <0.5 nm at 1650 nm and <0.2 nm at 760 nm being currently built at the University of Bremen brings the opportunity to map emissions over large areas with sufficient spatial resolution to resolve point sources at facility level, and sufficient spectral resolution to observe small concentration changes and therefore emission plumes of emitting regions over longer distances. This can be used for estimating emissions and validating satellite based measurements with high spatial resolution.

To fully exploit the capabilities of MAMAP-2D, a retrieval algorithm is needed, which can simultaneously retrieve CH4 and CO2 column mixing ratios and at the same time accounts for the light path changes due to scattering and other atmospheric effects with a precision of a few 0.1%. The FOCAL algorithm developed for OCO-2 satellite measurements implements a forward model, in which the multiple scattering problem is approximated by a thin, isotropic scattering layer. In combination with a lambertian surface, this can be solved analytically and results in a simple forward model formula, that makes FOCAL comparably computationally efficient as simple absorption models. For an airborne system, which observes the backscattered solar radiation from within the atmosphere, especially the scattering parts of the problem need to be developed further.

Here, we present the adaption of the FOCAL approach to airborne MAMAP-2D measurements (FOCAL-AIR). Additionally, we demonstrate the retrieval performance using simulated MAMAP-2D spectra, as well as slightly lower spectral resolution MAMAP measurements from past airborne campaigns.