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

Atmospheric variability above 6km 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 CHBr3 and CH2Br2 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 e-folding lifetimes of CHBr3 and almost all of them reach the TTL within one e-folding lifetime of CH2Br2.

Over the duration of CAST and CONTRAST, and over our study region, oceans delivered a mean (range) CHBr3 and CH2Br2 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) Bry mole fraction of 3.14 (1.81-4.18)pptv from source gases to the upper troposphere.