Data use policy for the TROPOMI level 2 science products CO, CH4, H2O/HDO from 
SRON Netherlands Institute for Space Research (beta version of the operational S5P product):

The data can be freely downloaded via the ftp site  
ftp://ftp.sron.nl/ open-access-data-2/TROPOMI/tropomi/ 
and is accessible within SRON under /nfs/Tropomi/lv2/SCIENCE. 

The science products are free to use but any use of the data in scientific publications 
and/or its preparation require co-authorship to be granted to the members of the SRON level 2 team.

Here is a list of CH4 versions available on the ftp:

CH4 Version 19_446 : Updated fit of the surface reflectance spectral dependency to a third-order polynomial fit. Lorente et al. (2022), AMT.                      Accounting for surface reflectance spectral features in TROPOMI methane retrievals.

CH4 Version 18_17  : Updated version which includes retrievals for ocean observations under sun-glint geometry. Lorente et al. (2022), AMT. E                     valuation of the methane full-physics retrieval applied to TROPOMI ocean sun glint measurements.

CH4 Version 14_14  : Lorente et al. (2021), AMT. Independent bias correction, updated spectroscopy, DEM and regularisation.  
                     https://amt.copernicus.org/articles/14/665/2021/

CH4 Version 10_9   : ONE band retrieval (Hu et al. 2018) 

CH4 Version 10_101 : TWO band retrieval (Hu et a. 2016), same as operational version, processed only over TCCON stations.

In version 10_101 the bias correction has not been applied yet. To calculate the bias corrected CH4, apply the following to the original XCH4 product (see ATBD for details):

XCHcorr4 = XCH4* (c1 + c2 * A+ c3 * A2)

Where A is the surface albedo in the SWIR, and the correction parameters c1 = 1.0173; c2 =-0.1538 and c3 = 0.2036. 

See Hasekamp et al. 2019 (S5P-XCH4 ATBD) for more details on this bias correction.