Meteorologists have long known about a boundary of convective storms in equatorial regions that moves north and south with the seasons.
It is the zone where trade winds from the northern and southern hemispheres converge.
The Intertropical Convergence Zone was once thought to be what kept northern air pollution from infiltrating the cleaner environs south of the equator.
But writing in the Journal of Geophysical Research - Atmospheres, a team from the University of York says they found a separate barrier about 30 miles wide in the cloud-free western Pacific, where carbon monoxide levels were four times higher on the northern side.
The team predicts the discovery will provide clues to help scientists model simulations of the movement of pollutants in the atmosphere more accurately, and to assess the impact of pollution on climate.