Abstract. Oceanic very short-lived substances (VSLSs), such as bromoform
(CHBr3), contribute to stratospheric halogen loading and, thus, to
ozone depletion. However, the amount, timing, and region of bromine delivery
to the stratosphere through one of the main entrance gates, the Indian summer
monsoon circulation, are still uncertain. In this study, we created two
bromoform emission inventories with monthly resolution for the tropical
Indian Ocean and west Pacific based on new in situ bromoform measurements and
novel ocean biogeochemistry modeling. The mass transport and atmospheric
mixing ratios of bromoform were modeled for the year 2014 with the particle
dispersion model FLEXPART driven by ERA-Interim reanalysis. We compare
results between two emission scenarios: (1) monthly averaged and (2) annually averaged
emissions. Both simulations reproduce the atmospheric distribution of
bromoform from ship- and aircraft-based observations in the boundary layer
and upper troposphere above the Indian Ocean reasonably well. Using monthly resolved emissions, the main oceanic source regions for the
stratosphere include the Arabian Sea and Bay of Bengal in boreal summer and
the tropical west Pacific Ocean in boreal winter. The main stratospheric
injection in boreal summer occurs over the southern tip of India associated
with the high local oceanic sources and strong convection of the summer
monsoon. In boreal winter more bromoform is entrained over the west Pacific
than over the Indian Ocean. The annually averaged stratospheric injection of
bromoform is in the same range whether using monthly averaged or annually averaged
emissions in our Lagrangian calculations. However, monthly averaged emissions
result in the highest mixing ratios within the Asian monsoon anticyclone in
boreal summer and above the central Indian Ocean in boreal winter, while
annually averaged emissions display a maximum above the west Indian Ocean in
boreal spring. In the Asian summer monsoon anticyclone bromoform atmospheric
mixing ratios vary by up to 50 % between using monthly averaged and annually
averaged oceanic emissions. Our results underline that the seasonal and
regional stratospheric bromine injection from the tropical Indian Ocean and
west Pacific critically depend on the seasonality and spatial distribution
of the VSLS emissions.
Supplementary material to "Importance of seasonally resolved oceanic emissions for bromoform delivery from the tropical Indian Ocean and west Pacific to the stratosphere"