We use the GEOS-Chem global 3-D atmospheric chemistry transport model to interpret atmospheric observations of bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ) collected during the CAST and CONTRAST aircraft measurement campaigns over the western Pacific, January–February 2014. We use a new linearized, tagged version of CHBr 3 and CH 2 Br 2 , allowing us to study the influence of emissions from specific geographical regions on observed atmospheric variations. The model describes 32 %–37 % of CHBr 3 and 15 %–45 % of CH 2 Br 2 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 CHBr 3 and CH 2 Br 2 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 CHBr 3 and CH 2 Br 2 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 CHBr 3 and almost all of them reach the TTL within one e -folding lifetime of CH 2 Br 2 . Over the duration of CAST and CONTRAST, and over our study region, oceans delivered a mean (range) CHBr 3 and CH 2 Br 2 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.