I Have Been Coughing Up Black Mucous Or Grey In

I have been coughing up black mucous or grey in the morning . I live in LA where there is a lot of pollution. I am not a smoker but I am a musician and I play at venues where people smoke. I have no other symptoms at all. Went to the doctor and he said irvwas probably from all the pollution in my area. Said my lungs were completely clear and very healthy. I have had this on and off for several weeks. How long does this take yo clear up?