Quantifying the vertical transport of CHBr₃ and CH₂Br₂ over the western Pacific

Abstract. We use the GEOS-Chem global 3-D atmospheric chemistry transport model to interpret atmospheric observations of bromoform (CHBr₃) and dibromomethane (CH₂Br₂) collected during the CAST and CONTRAST aircraft measurement campaigns over the western Pacific, January–February 2014. We use a new linearized, tagged version of CHBr₃ and CH₂Br₂, allowing us to study the influence of emissions from specific geographical regions on observed atmospheric variations. The model describes 32 %–37 % of CHBr₃ and 15 %–45 % of CH₂Br₂ observed variability during CAST and CONTRAST, reflecting model errors in vertical transport. The model has a mean positive bias of 30 % that is larger near the surface, reflecting errors in the poorly constrained prior emission estimates. We find using the model that observed variability of CHBr₃ and CH₂Br₂ is driven by open ocean emissions where there is deep convection. Atmospheric variability above 6 km includes a significant contribution from coastal oceans, but it is still dominated by emissions from the open ocean and by older air masses that originate upwind. In the absence of reliable ocean emission estimates, we use a new physical age-of-air simulation to determine the relative abundance of halogens delivered by CHBr₃ and CH₂Br₂ to the tropical transition layer (TTL). We find that 76 % (92 %) of air masses that originate from the ocean reach the TTL within two (three) atmospheric e-folding lifetimes of CHBr₃ and almost all of them reach the TTL within one e-folding lifetime of CH₂Br₂. Over the duration of CAST and CONTRAST, and over our study region, oceans delivered a mean (range) CHBr₃ and CH₂Br₂ mole fraction of 0.46 (0.13–0.72) and 0.88 (0.71–1.01) pptv, respectively, to the TTL, and a mean (range) Br_y mole fraction of 3.14 (1.81–4.18) pptv from source gases to the upper troposphere.