apply_tccon_airmass_correction

Purpose

apply_tccon_airmass_correction does two things:

1. Converts gas column densities (in molecules per area) to column averages by dividing by the O2 column and multiplying by the mean O2 atmospheric dry mole fraction.
2. Applies a solar zenith angle-dependent correction to those column averages that require it, as defined by a configuration file.

This can operate either on individual windows' column densities or on column densities calculated by averaging together all the windows for a given gas. The former is considered to be more accurate, as it allows for different airmass dependence per window (which depends on the spectroscopy in that window), but the latter is preserved for backwards compatibility.

Examples

This program requires two arguments: a path to a file defining the airmass corrections and a path to either a .vsw file (created by collate_tccon_results) or .vav file (created by average_results):

$GGGPATH/bin/apply_tccon_airmass_correction CORRECTION_FILE VSW_OR_VAV_FILE

The CORRECTION_FILE will usually be one of those supplied with GGG, in the $GGGPATH/tccon subdirectory. See the configuration section for the details of this file's format if you need to modify one or create your own.

Use in TCCON standard processing

For TCCON standard processing, the CORRECTION_FILE must be $GGGPATH/tccon/corrections_airmass_preavg.dat, as these are the airmass correction factors derived for the required TCCON data version. It must be run on the vsw file output by collate_tccon_results, as the TCCON standard processing uses per-window airmass corrections. This is automatically configured in the post_processing.sh file, therefore standard users should rely on the post_processing.sh script to run the required post processing steps in the correct order.

Use in EM27/SUN standard processing

As of GGG2020, EM27/SUNs still use per-gas airmass corrections, rather than per-window. Therefore, this must be run on the .vav file output by average_results using the EM27/SUN-specific airmass corrections included with EGI-RS. If using EGI-RS correctly, it will automatically create a post_processing.sh file with the correct post processing order for an EM27/SUN, so normal users should rely on that.