Sammendrag
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.
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