It's there the same reason I don't allow certain breeds of dogs, smoking, or charcoal grills on the porch.

Make sure you don't have a dead mouse in the unit somewhere....could take it outside and power wash it and bleach it....could try a couple bags of charcoal....use the coal for a barbeque if it doesnt work.

If you are repainting anyway, I have always like charcoal gray with red brick and your brick looks to have some gray.

Does it depends on whether it's charcoal or gas? 

I repaired a deck one time that some renter had put the charcoal grill on the composite porch.

You might be able to install a charcoal filter hood.  

They lit a charcoal fire in a gas grill once. 

Last door I did was a charcoal colour and combined well with that.

I've used some super duper charcoal stuff on carpet smells before to no avail.

If it's charcoal and the roofs gone definitely know thats a much bigger more expensive job going into.