A simulation of bromoform's contribution to stratospheric bromine

Abstract

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 inert in the troposphere may be transported to the stratosphere before they are oxidized or photolyzed. In this case these compounds act as an in situ source of stratospheric reactive chlorine and bromine. Second, the chlorine and bromine that are released into the troposphere from those gases that are reactive in the troposphere may also be subsequently transported to the stratosphere. We evaluate the relative influence of these processes on stratospheric bromine in a three-dimensional chemistry and transport model that simulates the distribution of bromoform (CHBr3). Many of the observed features of CHBr3 are simulated well, and comparisons with observations show that the model represents aspects of transport in the upper troposphere and lower stratosphere that are critical to the evaluation. In particular, the model maintains the observed troposphere-stratosphere distinctness in transport pathways and reproduces the observed seasonal dependence of the mixture of air in the middle- and high-latitude lowermost stratosphere. CHBr3 is short-lived and is destroyed by photolysis and by reaction with hydroxyl (OH), which yields inorganic bromine (Bry). In our simulations, stratospheric destruction of CHBr3 produces Bry in amounts that are comparable to that transported into the stratosphere after photolysis and oxidation of CHBr3 in the troposphere. The mass of Bry produced from the destruction of CHBr3 does not exceed the mass of Bry produced from the destruction of the long-lived brominated compounds (halons and methyl bromide) globally at any level in the stratosphere. However, Bry from the loss of CHBr3 accounts for approximately one third of the total Bry in the lowest kilometer of the stratosphere. We estimate that adding CHBr3 to models that already include the long-lived organic brominated compounds will increase the simulated stratospheric mass of Bry by, at most, 15%. Copyright 2001 by the American Geophysical Union.