Model calculations show that hydroxyl is a rather conservative specie with respect to abundance and diurnal variation. Calculated maximum concentrations range from about 6¿10-5 to 4¿10-4 ppl. for winter and summer, clear and clouded sky, temperature ranging from 0 to 25 ¿C and pollutant emissions ranging from average values for Western industrialized countries to urban areas. Peroxy radiacals show a much stronger variability in concentration partly due to their strong dependence on HO. Gas phase sulfate conversion rates up to 3% hr-1 are calculated. Sulfate and ozone are efficiently generated in moderately polluted air.