How to modify ozone dry deposition velocity?

I attempted to modify the following code :

depvel_gas( n,c,r ) = 1.0 / ( rsurf + rac )

but this code seems will change many other gases.

There is not enough information in your question to provide a specific answer. Which version of CMAQ are you using, and which dry deposition module (M3Dry or STAGE) have you enabled when building the executable? Which specific components influencing ozone dry deposition would you like to modify - e.g., stomatal, cuticular, or soil resistances, deposition to vegetated or non-vegetated portions of the grid cell, wet and/or dry conditions, etc.? Depending on your plans, modifications would have to be made in multiple portions of the code. On the other hand, if you’re only trying to conduct a brute-force sensitivity simulation with a bulk scaling of ozone dry deposition velocities, you could probably accomplish this by modifying the GC namelist file to use a scaling factor other than 1 to map O3 to its dry deposition surrogate VD_O3. For example, in CMAQv5.4, the following modification to the FAC column following the DRYDEP SURR column should implement a 50% reduction in ozone dry deposition by scaling the ozone dry deposition velocity with 0.5 when calculating the dry deposition fluxes, though I don’t think I have tested this out myself:

'O3' , 48.0 ,'' ,-1 ,'' ,-1 ,'VD_O3' , 0.5 ,'O3' , 1 ,'' ,'O3' ,'Yes' ,'Yes' ,'Yes' ,'Yes',