plume, they set utheir mobile lab in a unique, elevated location between t0 and t1, called pico de tres padres. we're about a thousand meters above each of these two sites. so we have an opportunity at this location to actually look at the lofted plume that's coming to us. narrator: in the morning this location has relatively clean air since it is above the boundary layer a layer near the ground that does not mix well with the atmosphere above. this layer traps the pollution below in the basin of mexico city. but as the sun heats the earth the boundary layer rises. herndon: but what we're observing right now -- we're above the mixing height. all of the pollution and emissions that are taking place are not able to mix up and come up to this location. what happens is that the sun comes up and begins to heat the surface of the earth. and just like putting a pan of boiling water onto the stove, it begins to mix and boil, moving the air upward, upward. and so it mixes up and up and up. and we're located up here at this location and suddenly we begin to see much of the city pollution and emissions c