Quantifying the vertical transport of CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub> over the western Pacific
<p>We use the GEOS-Chem global 3-D atmospheric chemistry transport model to interpret atmospheric observations of bromoform (CHBr<sub>3</sub>) and dibromomethane (CH<sub>2</sub>Br<sub>2</sub>) collected during the CAST and CONTRAST aircraft measurement ca...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/13135/2018/acp-18-13135-2018.pdf |
| Summary: | <p>We use the GEOS-Chem global 3-D atmospheric chemistry transport model to
interpret atmospheric observations of bromoform (CHBr<sub>3</sub>) and
dibromomethane (CH<sub>2</sub>Br<sub>2</sub>) 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<sub>3</sub> and
CH<sub>2</sub>Br<sub>2</sub>, allowing us to study the influence of emissions from
specific geographical regions on observed atmospheric variations. The model
describes 32 %–37 % of CHBr<sub>3</sub> and 15 %–45 % of
CH<sub>2</sub>Br<sub>2</sub> 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<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub> 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<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub> 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
<i>e</i>-folding lifetimes of CHBr<sub>3</sub> and almost all of them reach the TTL
within one <i>e</i>-folding lifetime of CH<sub>2</sub>Br<sub>2</sub>. Over the duration of
CAST and CONTRAST, and over our study region, oceans delivered a mean (range)
CHBr<sub>3</sub> and CH<sub>2</sub>Br<sub>2</sub> 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<sub><i>y</i></sub>
mole fraction of 3.14 (1.81–4.18) pptv from source gases to the upper
troposphere.</p> |
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| ISSN: | 1680-7316 1680-7324 |