NO and NO₂ in the troposphere: Technique and measurements in regions of a folded tropopause

During the 1984 spring program of the NASA Global Tropospheric Experiment, measurements of NO, NO 2 , O 3 , and CO were made during two aircraft flights that encountered tropopause fold events. The technique used to measure NO x (NO + NO 2 ) is described. In the neighborhood of both tropopause fold events, CO and O 3 were strongly anticorrelated [ Danielsen et al ., 1987; Hipskind et al. , 1987]. In one fold, encountered over the eastern Pacific Ocean at night, NO x was positively correlated with O 3 , negatively correlated with the dew point, and negatively correlated with CO, consistent with the stratospheric origin of the air mass. In this fold at altitudes between 5.5 and 6.0 km, NO x reached mixing ratios near 150 parts per trillion by volume (pptv), considerably larger than the levels of 10–50 pptv in the air external to the fold. In contrast, in a daytime encounter with a fold over the southwestern United States at altitudes of 6.7–8.8 km, NO x did not correlate well with CO, O 3 , or dew point, but remained roughly constant at levels of 100–200 pptv. Outside of the fold, in a region of strong convective activity, NO x peaked to just over 500 pptv. It appears that mixing of tropospheric air containing elevated NO x weakened the expected trends for the fold over the continent compared to the fold over the ocean, and therefore NO x may not always be a good tracer of air of recent stratospheric origin. However, the observations emphasize that the stratospheric source of NO x acts to introduce NO x over a short period and through a large vertical region of the troposphere, not just near the tropopause, as is assumed in many tropospheric models.