research teams measure trace gases, aerosol concentrations and solar-radiation levels as well as meteorological data. molina: we have to measure the pressure, we measure the temperature we measure the relative humidity, and the wind speed -- the wind direction. these all affect the transport of the pollutants. narrator: the aerodyne team traveled to mexico city as part of the milagro campaign. to help monitor the 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 takin