Bridging the gap between bromocarbon oceanic emissions and upper air concentrations
1IFM-GEOMAR, Kiel, Germany
2Research Institute for Global Change, JAMSTEC, Yokohama, Japan
3Norwegian Institute for Air Research (NILU), Kjeller, Norway
4National Centre for Atmospheric Science (NCAS), Cambridge, UK
5University of Cambridge, Department of Chemistry, Cambridge, UK
Abstract. Oceanic emissions of halogenated very short-lived substances (VSLS) are expected to contribute significantly to the stratospheric halogen loading and therefore to ozone depletion. Estimates of the amount of VSLS transported into the stratosphere are highly uncertain and based on sporadic observations around the tropical tropopause layer (TTL) and on modeling studies which use prescribed emission scenarios to reproduce observed atmospheric concentrations. Actual measurements of VSLS emissions at the ocean surface have not been linked to the stratospheric halogen loading until now. Here we use observations of oceanic VSLS emissions in the western Pacific and an atmospheric Lagrangian transport model to estimate the direct contribution of bromoform (CHBr3), and dibromomethane (CH2Br2) to the stratospheric bromine loading. Our emission-based estimates of VSLS profiles provide the first link between observed oceanic emissions and in situ TTL measurements. The emission-based and observed profiles of CHBr3 show good agreement, confirming the importance of the western Pacific as a source region. However, CH2Br2 emission-based estimates are considerable smaller than current upper air observations as a result of relatively low western Pacific emissions. We estimate the relative importance of the highly variable emission rates and the surface to stratosphere transport for the contribution of the two bromocarbons to the stratospheric bromine budget. Our results show that stratospheric entrainment of bromine in form of VSLS or their degradation products is highly variable and that this variability is primarily linked to the variability of the observed sea-to-air flux. Together, both bromocarbons contribute to the stratospheric bromine budget with 0.4 pptv on average and 2.3 pptv for cases of maximum emissions.