Bromoform and dibromomethane above the Mauritanian upwelling: Atmospheric distributions and oceanic emissions

Abstract

Natural sources of bromoform (CHBr 3) and dibromomethane (CH 2 Br 2), including oceanic emissions, contribute to stratospheric and tropospheric O 3 depletion. Convective transport over tropical oceans could deliver large amounts of these short-lived organic bromine species to the upper atmosphere. High mixing ratios of atmospheric CHBr 3 in air masses from the northwest African coast have been hypothesized to originate from the biologically active Mauritanian upwelling. During a cruise into the upwelling source region in spring 2005 the atmospheric mixing ratios of the brominated compounds CHBr 3 and CH 2 Br 2 were found to be elevated above the marine background and comparable to measurements in other coastal regions. The shelf waters were identified as a source of both compounds for the atmosphere. The calculated sea-to-air emissions support the hypothesis of a strong upwelling source for reactive organic bromine. However, calculated emissions were not sufficient to explain the elevated concentrations observed in the coastal atmosphere. Other strong sources that could contribute to the large atmospheric mixing ratios previously observed over the Atlantic Ocean must exist within or near West Africa. Copyright 2007 by the American Geophysical Union.