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. Oceanic very short-lived substances (VSLSs), such as bromoform( CHBr3 ), contribute to stratospheric halogen loading and, thus, toozone depletion. However, the amount, timing, and region of bromine deliveryto the stratosphere through one of the main entrance gates, the Indian summermonsoon circulation, are still uncertain. In this study, we created twobromoform emission inventories with monthly resolution for the tropicalIndian Ocean and west Pacific based on new in situ bromoform measurements andnovel ocean biogeochemistry modeling. The mass transport and atmosphericmixing ratios of bromoform were modeled for the year 2014 with the particledispersion model FLEXPART driven by ERA-Interim reanalysis. We compareresults between two emission scenarios: (1) monthly averaged and (2) annually averagedemissions. Both simulations reproduce the atmospheric distribution ofbromoform from ship- and aircraft-based observations in the boundary layerand upper troposphere above the Indian Ocean reasonably well. Using monthly resolved emissions, the main oceanic source regions for thestratosphere include the Arabian Sea and Bay of Bengal in boreal summer andthe tropical west Pacific Ocean in boreal winter. The main stratosphericinjection in boreal summer occurs over the southern tip of India associatedwith the high local oceanic sources and strong convection of the summermonsoon. In boreal winter more bromoform is entrained over the west Pacificthan over the Indian Ocean. The annually averaged stratospheric injection ofbromoform is in the same range whether using monthly averaged or annually averagedemissions in our Lagrangian calculations. However, monthly averaged emissionsresult in the highest mixing ratios within the Asian monsoon anticyclone inboreal summer and above the central Indian Ocean in boreal winter, whileannually averaged emissions display a maximum above the west Indian Ocean inboreal spring. In the Asian summer monsoon anticyclone bromoform atmosphericmixing ratios vary by up to 50 % between using monthly averaged and annuallyaveraged oceanic emissions. Our results underline that the seasonal andregional stratospheric bromine injection from the tropical Indian Ocean andwest Pacific critically depend on the seasonality and spatial distributionof the VSLS emissions.

Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere