Step 1:
Remove any loose particles with a vacuum cleaner attachment.
Step 2:
Rub the area with a dry sponge or a soot sponge, a chemically treated sponge that absorbs soot and dirt into its pores.
Step 3:
Wipe down the wall with a solution of trisodium phosphate (TSP); use about 1 tbsp. TSP to 1 gallon warm water. Rinse thoroughly.
same method worked for me
