Chemical NOx budget in the upper troposphere over the tropical South Pacific

The chemical NOx budget in the upper troposphere over the tropical South Pa cific is analyzed using aircraft measurements made at 6-12 km altitude in S eptember 1995 during the Global Tropospheric Experiment (GTE) Pacific Explo ratory Mission (PEM) Tropics A campaign. Chemical loss and production rates of NOx along the aircraft flight tracks are calculated with a photochemica l model constrained by observations. Calculations using a standard chemical mechanism show a large missing source for NOx; chemical loss exceeds chemi cal production by a factor of 2.4 on average. Similar or greater NOx budget imbalances have been reported in analyses of data from previous field stud ies. Ammonium aerosol concentrations in PEM-Tropics A generally exceeded su lfate on a charge equivalent basis, and relative humidities were low (media n 25% relative to ice). This implies that the aerosol could be dry in which case N2O5 hydrolysis would be suppressed as a sink for NOx. Suppression of N2O5 hydrolysis and adoption of new measurements of the reaction rate cons tants for NO2 + OH + M and HNO3 + OH reduces the median chemical imbalance in the N-x budget for PEM-Tropics A from 2.4 to 1.9. The remaining imbalanc e cannot be easily explained from known chemistry or long-range transport o f primary NOx and may imply a major gap in our understanding of the chemica l cycling of NOx in the free troposphere.