J. Geophys. Res. doi:10.1029/2008JD009940 (2008)

A narrow atmospheric boundary in the Western Pacific keeps apart the more polluted air of the Northern Hemisphere from the cleaner air of the south. This newfound divide is markedly farther north than the Intertropical Convergence Zone (ITCZ), a tropical low-pressure belt that is thought to separate air masses elsewhere according to their hemispheric origin.

Jacqueline Hamilton of the University of York, UK, and her team found that carbon monoxide pollution from biomass burning in Thailand and Indonesia dropped steeply across the 50-kilometre-wide boundary. They conclude that storms may lift air from the Northern Hemisphere into the upper troposphere — where pollutants remain longer — preventing it from mixing with southern air masses.