Simulation of bromoform's contribution to stratospheric bromine

The chlorine and bromine that are contained in gases emitted at the Earth's surface enter the stratosphere in two ways. First, those gases that are in ert in the troposphere may be transported to the stratosphere before they a re oxidized or photolyzed. In this case these compounds act as an in situ s ource of stratospheric reactive chlorine and bromine. Second, the chlorine and bromine that are released into the troposphere from those gases that ar e reactive in the troposphere may also be subsequently transported to the s tratosphere. We evaluate the relative influence of these processes on strat ospheric bromine in a three-dimensional chemistry and transport model that simulates the distribution of bromoform (CHBr3). Many of the observed featu res of CHBr3 are simulated well, and comparisons with observations show tha t the model represents aspects of transport in the upper troposphere and lo wer stratosphere that are critical to the evaluation. In particular, the mo del maintains the observed troposphere-stratosphere distinctness in transpo rt pathways and reproduces the observed seasonal dependence of the mixture of air in the middle- and high-latitude lowermost stratosphere. CHBr3 is sh ort-lived and is destroyed by photolysis and by reaction with hydroxyl (OH) : which yields inorganic bromine (Br-y) In our simulations, stratospheric d estruction of CHBr3 produces Br-y in amounts that are comparable to that tr ansported into the stratosphere after photolysis and oxidation of CHBr3 in the troposphere. The mass of Br-y produced from the destruction of CHBr3 do es not exceed the mass of Br-y produced from the destruction of the long-li ved brominated compounds (halons and methyl bromide) globally at any level in the stratosphere. However. Bry from the loss of CHBr3 accounts for appro ximately one third of the total Br, in the lowest kilometer of the stratosp here. We estimate that adding CHBr3 to models that already include the long -lived organic brominated compounds will increase the simulated stratospher ic mass of Br-y by, at most, 15%.