The trend in stratospheric bromine from 1987 to 2000 deduced from in situ measurements of BrO in the Winter Polar Vortices

Measurements of BrO obtained in situ from the NASA ER-2 aircraft and balloons (HALOZ balloon-borne ClO/BrO instrument) between 1987 and 2000 are used to estimate the trend in total inorganic bromine (Bry). Despite the significance of ozone destruction by BrO, due to the analytical difficulty in quantifying very low abundances of bromine radicals present in the stratosphere these were the only direct observations of the dominant species of inorganic bromine that were made often enough over this period to allow for this analysis. In particular, this work focuses on observations in the perturbed polar vortices when BrO chemistry is dominated by reaction with ClO, and thus is less sensitive to systematic errors due to uncertainties in reactions with compounds such as HOx and NOx that dominate at lower latitudes. We use a detailed one-dimensional photochemical model with recommended rate parameters for key chemical processes, initialized with simultaneous observations of ClO and O₃, to determine the atmospheric bromine partitioning under the specific photochemical conditions of each observation. We find that total inorganic bromine increased by ~43% over this time and reaches a value in 2000 that agrees well with the other observations that have since shown a leveling off, and small decrease, of inorganic bromine in the stratosphere since 2000. Within the accuracies of the BrO measurements and modeled partitioning, our results are consistent with a relatively constant value of 4 ppt find for very short-lived bromine substances (VSLSs) over the period of these observations. These results extend the Bry record deduced in recent WMO ozone assessments by 13 years and add further confidence to our understanding of ozone depletion due to bromine compounds over this critical period when inorganic bromine was increasing rapidly due to emissions of anthropogenic compounds, such as Halons and methyl bromide used in fire extinguishers and for fumigating crops.