ECMWF_atmosphere_file fileNo spatial interpolation of the values is done. The atmospheric data nearest to the specified time will be selected (or linearly interpolated if time_interpolate is switched on). Atmospheric profiles, which are not provided by the ECMWF file (O2, CO2, NO2) are taken from the atmosphere_file. Per default, also the atmosphere above the ECMWF data is taken from the atmosphere_file. In order to avoid this, please have a look at the option: ECMWF_levels_only.