Retrieval of tropospheric column-averaged CH 4 mole fraction by solar absorption FTIR-spectrometry using N 2O as a proxy
Tropospheric column-averaged CH 4 mole fractions were derived from ground-based column absorption measurements. The method uses stratospheric N 2O columns to correct for the stratospheric contribution to the CH 4 total column. The method was applied to four Total Carbon Column Observing Network (TCCON) sites covering locations from the Northern Arctic to the tropics. It performs well for all sites. The derived tropospheric CH 4 concentrations were compared with profiles measured by aircraft at three sites. The results indicate an inter-site consistency within 6 ppb (~0.3%). With aircraft profiles up to 3 km, the seasonal behavior of the derived tropospheric CH 4 concentration was also checked, revealing a difference of around 20 ppb. The mean relative uncertainty of the four sites, as estimated from the daily standard deviations, is 0.23%.