Influence of the a priori profile on CO2 total columns at Paris during pollution episodes

Abstract

International audienceGround-based Fourier transform spectrometry (FTIR) using solar absorption spectroscopy is a powerful tool for monitoring atmospheric trace gases and validate satellite measurements. The Total Carbon Column Observing Network (TCCON) is an international FTIR network operating in the near-infrared spectral region and dedicated to the retrieval of greenhouse gases. Accurate and precise column-averaged abundances of atmospheric constituents like CO2, CH4, CO, N2O, HF, H2O and HDO are provided on a routinely basis. These data are linked to the WMO reference scale and used to validate satellite data worldwide. Only two TCCON instruments are operated in megacities that globally are important sources of anthropogenic CO2: one instrument is located in Pasadena, California (United States) and the other one in Paris, France. Paris is the third European megacity. We address the question in as much a priori CO2 profiles provided by GFIT (TCCON retrieval algorithm) are suited to retrieve CO2 abundances during pollution episodes in Paris. CO2 a priori profiles from GFIT, as provided by the standard TCCON retrieval algorithm, yield typical surface volume mixing ratios (VMR) of CO2 between 385 and 415 ppm, while in situ measurements at the ground can easily values as high as 450 ppm. From complementary in-situ CO2 and LIDAR measurements at the TCCON-Paris station, we construct improved a priori profiles that comply with the local conditions at the ground. These new a priori profiles are then incorporated into the GFIT code to study their influence on the retrieved total column of CO2. The presentation will give an overview of the method and discuss the sensitivity of the retrieved XCO2 to the improved a priori profile