Seasonal cycle of seawater bromoform and dibromomethane concentrations in a coastal bay on the western Antarctic Peninsula

摘要

Sea-to-air emissions of bromocarbon gases are known to play an important role in atmospheric ozone depletion. In this study, seawater concentrations of bromoform (CHBr_3) and dibromomethane (CH_2Br_2) were measured regularly between February 2005 and March 2007 at the Rothera Oceanographic and Biological Time Series (RaTS) site located in Marguerite Bay on the Antarctic Peninsula. Strong seasonality in CHBr_3 and CH_2Br_2 concentrations was observed. The highest bromocarbon concentrations (up to 276.4 ± 13.0 pmol CHBr_3 L~(-1) and 30.0 ± 0.4 pmol CH_2Br_2 L~(-1)) were found to coincide with the annual microalgal bloom during the austral summer, with lower concentrations (up to 39.5 pmol CHBr_3 L~(-1) and 9.6 ± 0.6 pmol CH_2Br_2 L~(-1)) measured under the winter fast ice. The timing of the initial increase in bromocarbon concentrations was related to the sea-ice retreat and onset of the microalgal bloom. Observed seasonal variability in CH_2Br_2/CHBr_3 suggests that this relationship may be of use in resolving bromocarbon source regions. Mainly positive saturation anomalies calculated for both the 2005/2006 and 2006/2007 summers suggest that the bay was a source of CHBr_3 and CH_2Br_2 to the atmosphere. Estimates of bromocarbon sea-to-air flux rates from Marguerite Bay during ice-free periods are 84 (-13 to 275) CHBr_3 nmol m~(-2) d~(-1) and 21 (2 to 70) nmol CH_2Br_2 m~(-2) d~(-1). If these flux rates are representative of the seasonal ice edge zone bloom which occurs each year over large areas of the Southern Ocean during the austral summer, sea-to-air bromocarbon emissions could have an important impact on the chemistry of the Antarctic atmosphere.