A geostationary thermal infrared sensor to monitor the lowermost troposphere: O3 and CO retrieval studies

This paper describes the capabilities of a nadir thermal infrared (TIR) sensor proposed for deployment on-board a geostationary platform to monitor ozone (O-3) and carbon monoxide (CO) for air quality (AQ) purposes. To assess the capabilities of this sensor we perform idealized retrieval studies considering typical atmospheric profiles of O-3 and CO over Europe with different instrument configuration (signal to noise ratio, SNR, and spectral sampling interval, SSI) using the KOPRA forward model and the KOPRA-fit retrieval scheme. We then select a configuration, referred to as GEO-TIR, optimized for providing information in the lowermost troposphere (LmT; 0-3 km in height). For the GEO-TIR configuration we obtain similar to 1.5 degrees of freedom for O-3 and similar to 2 for CO at altitudes between 0 and 15 km. The error budget of GEO-TIR, calculated using the principal contributions to the error (namely, temperature, measurement error, smoothing error) shows that information in the LmT can be achieved by GEO-TIR. We also retrieve analogous profiles from another geostationary infrared instrument with SNR and SSI similar to the Meteosat Third Generation Infrared Sounder (MTG-IRS) which is dedicated to numerical weather prediction, referred to as GEO-TIR2. We quantify the added value of GEO-TIR over GEO-TIR2 for a realistic atmosphere, simulated using the chemistry transport model MOCAGE (MOdele de Chimie Atmospherique a Grande Echelle). Results show that GEO-TIR is able to capture well the spatial and temporal variability in the LmT for both O-3 and CO. These results also provide evidence of the significant added value in the LmT of GEO-TIR compared to GEO-TIR2 by showing GEO-TIR is closer to MOCAGE than GEO-TIR2 for various statistical parameters (correlation, bias, standard deviation).