How to alter a variable with a shell script using sed

I am attempting to alter the values in a .dat file for a model I am running. The issue is that the code cannot simply be changed with the sed -i 's/old_text/new_text/'

The .dat file I am attempting to change looks like

0.000E-00          !Argon
7.956E-03          !Methane
0.000E-00          !Ethane
1.945E-03          !Carbon Dioxide
9.901E-01          !Nitrogen
1.000E-40          !Oxygen
0.000E-00          !Hydrogen
0.000E-00          !Nitrogen Dioxide
22                 !Tropopause layer

I am attempting to alter the values for methane, nitrogen, and carbon dioxide, however I cannot simply tell it to replace '7.965E-03 !Methane' because after the first time I run the shell script, that value will change.

I have attempted to replace !Methane with $'variable name' !Methane, but that does not solve the issue of removing the number, nor will it work after the first time I compile the script.

The current script is

#file=atmos-master-copy1/CLIMA/IO/mixing_ratios.dat                                                                                        
#sed -i 's/Methane.*/'$muCH4_1'    !Methane/' $file

But I dont know how to make it start by altering the contents at the beginning of the file.

If you want to replace the value for !Methane with the value held by the variable $muCH4_1, then:

sed -Ei 's/^\S+\s+(!Methane)$/'"$muCH4_1"' /' "$file"

• ^ - Start of line anchor
• \S+ - One or more non-whitespace characters
• \s+ - One or more whitespace characters
• (!Methane) - Capture !Methane into capture group 1
• $ - End of line anchor

Substitute with

• '"$muCH4_1"' - The value held by the variable muCH4_1
• - A literal space
• \1 - !Methane from capture group 1

Note: For the second column to line up just like in the original file, muCH4_1 must be padded with spaces to the right. If that's what you want, you can use printf to do the padding:

sed -Ei 's/^\S+\s+(!Methane)$/'"$(printf "%-18s" "$muCH4_1")"' /' "$file"