Clive Rodgers

Abstract:

Spectroscopic measurements of BrO using direct sun and zenith sky viewing geometries are combined in an optimal estimation retrieval algorithm to obtain tropospheric and stratospheric columns of BrO. Twenty-two twilight periods are investigated over Arrival Heights, Antarctica (77.8°S, 166.7°E) during the polar spring period of 2002. This paper presents the first tropospheric and stratospheric BrO column retrievals from UV-visible ground-based measurements for a polar location. A direct comparison is made between stratospheric columns retrieved at 80°, 84°, and 88° solar zenith angles (SZA) from the spectroscopic measurements and those calculated by the SLIMCAT three-dimensional chemical transport model. The ground-based column BrO observations are consistent with a SLIMCAT stratospheric Bry loading of 21.2 parts per trillion at 20 km. SLIMCAT reproduces the observed sunrise column BrO increase but does not match the sunset observations, which display less variation. The significant warming of the Antarctic polar stratosphere in 2002 led to highly variable stratospheric columns being observed. The observed column BrO decreased with the transition from vortex to extravortex air on 21 September but did not change much following the return of the vortex on 12 October. For the tropospheric column, an almost normal distribution consistent with a "background" of 0.3 ± 0.3 × 10¹³ molecules cm^-2 is observed from the ground (80°, 84°, and 88° for both sunrise and sunset). A statistically significant "bromine explosion" event (at the 2σ level) was detected at the end of October with a tropospheric column of 1.8 ± 0.1 × 10¹³ molecules cm^-2. The measured tropospheric columns are compared with the tropospheric Model of Atmospheric Transport and Chemistry-Max Planck Institute for Chemistry version model. The tropospheric BrO sunrise column observations can only be explained with an additional bromine source other than decomposition of CH3Br and downward transport of long-lived bromine from the stratosphere. A comparison with the spaceborne Global Ozone Monitoring Experiment (GOME) found the total columns observed from the ground to be 16-25% smaller than the total columns observed by GOME for SZAs between 80° and 88°. Copyright 2006 by the American Geophysical Union.