Background CO2 levels and error analysis from ground-based solar absorption IR measurements in central Mexico

In this investigation we analyze two common optical configurations to retrieve CO2 total column amounts from solar absorption infrared spectra. The noise errors using either a KBr or a CaF2 beam splitter, a main component of a Fourier transform infrared spectrometer (FTIR), are quantified in order to assess the relative precisions of the measurements. The configuration using a CaF2 beam splitter, as deployed by the instruments which contribute to the Total Carbon Column Observing Network (TCCON), shows a slightly better precision. However, we show that the precisions in X CO2 (=0.2095.Total Column CO2/Total Column O-2) retrieved from > 96% of the spectra measured with a KBr beam splitter fall well below 0.2 %. A bias in X CO2 (KBr-CaF2) of +0.56 +/- 0.25 ppm was found when using an independent data set as reference. This value, which corresponds to +0.14 +/- 0.064 %, is slightly larger than the mean precisions obtained. A 3year X CO2 time series from FTIR measurements at the highaltitude site of Altzomoni in central Mexico presents clear annual and diurnal cycles, and a trend of +2.2 ppm yr(-1) could be determined.