Quantifying the vertical transport of CHBr3 and CH2Br2 over the Western Pacific

Other literature type English OPEN
Butler, Robyn ; Palmer, Paul I. ; Feng, Liang ; Andrews, Stephen J. ; Atlas, Elliot L. ; Carpenter, Lucy J. ; Donets, Valeria ; Harris, Neil R. P. ; Montzka, Stephen A. ; Pan, Laura L. ; Salawitch, Ross J. ; Schauffler, Sue M. (2016)

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&ndash;February, 2014. We use a new linearised, 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&thinsp;%&ndash;37&thinsp;% of CHBr<sub>3</sub> observed variability and 15&thinsp;%&ndash;45&thinsp;% of CH<sub>2</sub>Br<sub>2</sub> observed variability during CAST and CONTRAST, reflecting errors in vertical model transport. The model has a mean positive bias of 30&thinsp;% 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 ocean emissions, particularly by the open ocean above which there is deep convection. We find that contributions from coastal oceans and terrestrial sources over the Western Pacific are significant above altitudes >&thinsp;6&thinsp;km, but is still dominated by the open ocean emissions and by air masses transported over longer time lines than the campaign period. 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 6&thinsp;% (47&thinsp;%) of air masses with halogen released by the ocean reach the TTL within two (three) atmospheric e-folding lifetimes of CHBr<sub>3</sub> and almost all of them reach the TTL within one e-folding lifetime of CH<sub>2</sub>Br<sub>2</sub>. We find these gases are delivered to the TTL by a small number of rapid convection events during the study period. 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&ndash;0.72) and 0.88 (0.71&ndash;1.01) pptv, respectively, to the TTL, and a mean (range) Bry mole fraction of 3.14 (1.81&ndash;4.18)&thinsp;pptv to the upper troposphere. Open ocean emissions are responsible for 75&thinsp;% of these values, with only 8&thinsp;% from coastal oceans.
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