Bromoform as a source of stratospheric bromine

We have measured bromoform (CHBr 3 ) in the troposphere and lower stratosphere (sea level to 17 km) from balloons, aircraft and ground stations. The concentrations ranged from 2 to 20 ppt at sea level, and 0.1 to 1 ppt in the upper troposphere. Above the tropopause the concentrations declined sharply to less than 0.01 ppt above 14 km. CHBr 3 accounted for 3% of total measured organic bromine (Br o ) at the extratropical tropopause, and appeared to potentially contribute at least 9% of reactive bromine (Br y ) in stratospheric air of mean age less than 1 year. Inclusion of dibromomethane and the mixed bromochloromethanes increased these figures to about 10% of Br o and 20% of Br y respectively. Observations of CHBr 3 in both Arctic and mid‐latitudinal stratospheric air, and in air masses with mean ages of more than 1.5 years, suggests that the CHBr 3 in these locations originates predominantly from extratropical stratosphere‐troposphere exchange.