A Fast Atmospheric Trace Gas Retrieval for Hyperspectral Instruments Approximating Multiple ScatteringPart 1: Radiative Transfer and a Potential OCO-2 XCO2 Retrieval Setup

Satellite retrievals of the atmospheric dry-air column-average mole fraction of CO2 (XCO2) based on hyperspectral measurements in appropriate near (NIR) and short wave infrared (SWIR) O2 and CO2 absorption bands can help to answer important questions about the carbon cycle but the precision and accuracy requirements for XCO2 data products are demanding. Multiple scattering of light at aerosols and clouds can be a significant error source for XCO2 retrievals. Therefore, so called full physics retrieval algorithms were developed aiming to minimize scattering related errors by explicitly fitting scattering related properties such as cloud water/ ice content, aerosol optical thickness, cloud height, etc. However, the computational costs for multiple scattering radiative transfer (RT) calculations can be immense. Processing all data of the Orbiting Carbon Observatory-2 (OCO-2) can require up to thousands of CPU cores and the next generation of CO2 monitoring satellites will produce at least an order of magnitude more data. Here we introduce the Fast atmOspheric traCe gAs retrievaL FOCAL including a scalar RT model which approximates multiple scattering effects with an analytic solution of the RT problem of an isotropic scattering layer and a Lambertian surface. The computational performance is similar to an absorption only model and currently determined by the convolution of the simulated spectra with the instrumental line shape function (ILS). We assess FOCAL's quality by confronting it with accurate multiple scattering vector RT simulations using SCIATRAN. The simulated scenarios do not cover all possible geophysical conditions but represent, among others, some typical cloud and aerosol scattering scenarios with optical thicknesses of up to 0.7 which have the potential to survive the pre-processing of a XCO2 algorithm for real OCO-2 measurements. Systematic errors of XCO2 range from 2.5 ppm (6.3%) to 3.0ppm (7.6%) and are usually smaller than 0.3ppm (0.8%). The stochastic uncertainty of XCO2 is typically about 1.0ppm (2.5%). FOCAL simultaneously retrieves the dry-air column-average mole fraction ofH2O (XH2O) and the solar induced chlorophyll fluorescence at 760nm (SIF). Systematic and stochastic errors of XH2O are most times smaller than 6ppm and 9 ppm, respectively. The systematic SIF errors are always below 0.02mW/ m2/ sr/ nm, i. e., it can be expected that instrumental or forward model effects causing an in-filling of the used Fraunhofer lines will dominate the systematic errors when analyzing actually measured data. The stochastic uncertainty of SIF is usually below 0.3mW/ m2/ sr/ nm. Without understating the importance of analyzing synthetic measurements as presented here, the actual retrieval performance can only be assessed by analyzing measured data which is subject to part 2 of this publication.