Delivery of halogenated very short-lived substances from the west Indian Ocean to the stratosphere during the Asian summer monsoon
Halogenated very short-lived substances (VSLSs) are naturally produced in the ocean and emitted to the atmosphere. When transported to the stratosphere, these compounds can have a significant influence on the ozone layer and climate. During a research cruise on RV <i>Sonne</i> in the...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-06-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/6723/2017/acp-17-6723-2017.pdf |
| Summary: | Halogenated very short-lived substances (VSLSs) are naturally
produced in the ocean and emitted to the atmosphere. When transported to the
stratosphere, these compounds can have a significant influence on the ozone
layer and climate. During a research cruise on RV <i>Sonne</i> in the
subtropical and tropical west Indian Ocean in July and August 2014, we
measured the VSLSs, methyl iodide (CH<sub>3</sub>I) and for the first time bromoform
(CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>), in surface seawater and
the marine atmosphere to derive their emission strengths. Using the
Lagrangian particle dispersion model
FLEXPART with ERA-Interim meteorological fields,
we calculated the direct contribution of observed VSLS emissions to the
stratospheric halogen burden during the Asian summer monsoon. Furthermore, we
compare the in situ calculations with the interannual variability of
transport from a larger area of the west Indian Ocean surface to the
stratosphere for July 2000–2015. We found that the west Indian Ocean is a
strong source for CHBr<sub>3</sub> (910 pmol m<sup>−2</sup> h<sup>−1</sup>), very
strong source for CH<sub>2</sub>Br<sub>2</sub> (930 pmol m<sup>−2</sup> h<sup>−1</sup>), and an average source for
CH<sub>3</sub>I (460 pmol m<sup>−2</sup> h<sup>−1</sup>). The atmospheric transport from the
tropical west Indian Ocean surface to the stratosphere experiences two main
pathways. On very short timescales, especially relevant for the
shortest-lived compound CH<sub>3</sub>I (3.5 days lifetime), convection above the
Indian Ocean lifts oceanic air masses and VSLSs towards the tropopause. On a
longer timescale, the Asian summer monsoon circulation transports oceanic
VSLSs towards India and the Bay of Bengal, where they are lifted with the monsoon
convection and reach stratospheric levels in the southeastern part of the
Asian monsoon anticyclone. This transport pathway is more important for the
longer-lived brominated compounds (17 and 150 days lifetime for CHBr<sub>3</sub>
and CH<sub>2</sub>Br<sub>2</sub>). The entrainment of CHBr<sub>3</sub> and CH<sub>3</sub>I from the
west Indian Ocean to the stratosphere during the Asian summer monsoon is
lower
than from previous cruises in the tropical west Pacific Ocean during boreal
autumn and early winter
but higher than from the tropical Atlantic during boreal
summer. In contrast, the projected CH<sub>2</sub>Br<sub>2</sub> entrainment was very high
because of the high emissions during the west Indian Ocean cruise. The
16-year July time series shows highest interannual variability for the
shortest-lived CH<sub>3</sub>I and lowest for the longest-lived CH<sub>2</sub>Br<sub>2</sub>. During
this time period, a small increase in VSLS entrainment from the west Indian
Ocean through the Asian monsoon to the stratosphere is found. Overall, this
study confirms that the subtropical and tropical west Indian Ocean is an
important source region of halogenated VSLSs, especially CH<sub>2</sub>Br<sub>2</sub>, to
the troposphere and stratosphere during the Asian summer monsoon. |
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| ISSN: | 1680-7316 1680-7324 |