Evaluating the Stratospheric Loading of Halogens: Brominated Very Short-Lived Substances and Lower Stratospheric Inorganic Chlorine

Abstract

Bromine and chlorine compounds catalyze the removal of stratospheric ozone. The Montreal Protocol and subsequent regulations have decreased the atmospheric abundance of anthropogenic, long-lived halogen sources. This dissertation quantifies the stratospheric supply of halogens from very short-lived substances (VSLS), which are important drivers of ozone loss in the lower stratosphere. First, the contribution of natural, biogenic VSLS to stratospheric bromine (BryVSLS) is calculated using the first comprehensive sampling of organic and inorganic bromine gases in the Tropical Western Pacific (TWP), obtained by two aircraft campaigns in winter 2014. Measurements of BrO and organic bromine compounds, including VSLS, were used to determine that the value of BryVSLS is 5.0 ± 2.1 ppt, about 25% of total stratospheric bromine. This analysis indicates that 60% of BryVSLS enters the stratosphere as organic compounds, higher than previous estimates. Second, the representation of brominated VSLS within 14 global models used to project the recovery of the ozone layer is examined. Bromine within global models that explicitly simulate VSLS compare better with TWP observations than bromine within models that utilize longer-lived chemicals as a surrogate for VSLS. Both methods for incorporating VSLS are significant improvements over previous simulations that only included long-lived bromine sources. Third, ground and satellite-based measurements of column BrO collected over Fairbanks, Alaska during spring 2011 are examined. Ground-based retrievals support the central, 5 ppt value of our TWP estimate of BryVSLS. Satellite measurements of BrO are consistently higher than reported by the ground-based instrument. Satellite observations indicate either the upper, 7 ppt limit of BryVSLS or, if we consider 5 ppt of BryVSLS, the presence of 1.2 ppt of tropospheric BrO uniformly mixed between 2 km and the tropopause. Finally, inorganic chlorine in the midlatitude lower stratosphere is evaluated. Ground and satellite-based measurements of HCl and ClONO2 support a stratospheric contribution of anthropogenic chlorinated VSLS as well as recently proposed updates to parameterizations of the stratospheric decomposition of long-lived halocarbons. Simulations including chlorinated VSLS and the updated parameterization project a slower recovery of the ozone layer than previously expected.