Retrieval of vertical profiles and tropospheric CO₂ columns based on high-resolution FTIR over Hefei, China

Abstract

High-resolution solar absorption spectra, observed by ground-based Fourier Transform Infrared spectroscopy (FTIR), are used to retrieve vertical profiles and partial or total column concentrations of many trace gases. In this study, we present the tropospheric CO₂ columns retrieved by mid-infrared solar spectra over Hefei, China. To reduce the influence of stratospheric CO₂ cross-dependencies on tropospheric CO₂, an a posteriori optimization method based on a simple matrix multiplication is used to correct the tropospheric CO₂ profiles and columns. The corrected tropospheric CO₂ time series show an obvious annual increase and seasonal variation. The tropospheric CO₂ annual increase rate is 2.71 ± 0.36 ppm yr^-1, with the annual peak value in January, and CO₂ decreases to a minimum in August. Further, the corrected tropospheric CO₂ from GEOS-Chem simulations are in good agreement with the coincident FTIR data, with a correlation coefficient between GEOS-chem model and FTS of 0.89. The annual increase rate of XCO₂ observed from near-infrared solar absorption spectra is in good agreement with the tropospheric CO₂ but the annual seasonal amplitude of XCO₂ is only about 1/3 of dry-air averaged mole fractions (DMF) of tropospheric CO₂. This is mostly attributed to the seasonal variation of CO₂ being mainly dominated by sources near the surface.