SOURCES AND CHEMISTRY OF NOX IN THE UPPER TROPOSPHERE OVER THE UNITED-STATES

The origin of NOx in the upper troposphere over the central United Sta tes is examined using aircraft observations obtained during the SUCCES S campaign in April-May of 1996. Correlations between NOy (sum of NOx and its oxidation products) and CO at 8-12 km altitude indicate that N Ox originates primarily from convective transport of polluted boundary layer air. Lightning and aircraft emissions appear to be only minor s ources of NOx. Chemical steady state model calculations constrained by local observations of NO underestimate the measured NOx/NOy concentra tion ratio at 8-12 km altitude by a factor of two on average. The magn itude of the underestimate is correlated with concentrations of conden sation nuclei, which we take as a proxy for the age of air in the uppe r troposphere. We conclude that the NOx/O-y ratio is maintained above chemical steady state by frequent convective injections of fresh NOx f rom the polluted boundary layer and by the long lifetime of NOx in the upper troposphere (5-10 days). In contrast to previous studies, we fi nd no evidence for fast heterogeneous recycling from HNO3 to NOx in th e upper troposphere.