Original source:
  https://ads.atmosphere.copernicus.eu/cdsapp#!/dataset/cams-global-greenhouse-gas-inversion?tab=form
 
for CH4, select Methane,concentration, surface air sample and satellite,
    daily mean, latest version, 2019
for CO2, select Carbon dioxide, concentration, surface air sample,
    instantaneous, latest version, 2020
for N2O, select Nitrous oxide, concentration, surface air sample,
    instantaneous, latest version, 2020
 
You will get 12 monthly files of the gas data on a hybrid sigma vertical
coordinate system with 39 levels, n2o 96 in lat, 97 in lon, 8 time / day
co2 same, ch4 34 levels,90 in lat, 120 in lon, 1 time / day
 
When distributing the data, include the following
'Generated using Copernicus Atmosphere Monitoring Service information [Year]’
with [Year] 2019 for CH$ and N2O and 2020 for CO2
 
Local storage for data:
The ECMWF files and final average file are under area:
~/data/ancillary/ecmwf/cams/base_ecmwf_data
in the following sub-directories:  CH4_2019  CO2_2020  N2O_2019
 
Program:
 
The program to make the average files is in
~/src/idl_lib/cams_avg/lcl_code
To run:
- edit the file monthly_mean_trace_gas_concentrations_on_merra_grid.pro
and insert the gas to process: 'co2', 'n2o', or 'ch4'
- invoke IDL
- compile the routines:
   .compile monthly_mean_trace_gas_concentrations_on_merra_grid
   .compile ncdf_read_all
   .compile cams_monthly_average
   .compile run
- Make the average:
  run
 
final file will be in the CH4_2019  CO2_2020  N2O_2019 directories
respectively, names:
  cams73_latest_ch4_conc_surface_satellite_dm_2019.nc
  cams73_latest_co2_conc_surface_inst_2020.nc
  cams73_latest_n2o_conc_surface_inst_2019.nc
 
They are averaged for each month and on the 42 level pressure coordinates
used by MERRA2