Chemical budget in the upper troposphere over the tropical South Pacific

The chemical budget in the upper troposphere over the tropical South Pacific is analyzed using aircraft measurements made at 6-12 km altitude in September 1996 during the Global Tropospheric Experiment (GTE) Pacific Exploratory Mission (PEM) Tropics A campaign. Chemical loss and production rates of along the aircraft flight tracks are calculated with a photochemical model constrained by observations. Calculations using a standard chemical mechanism show a large missing source for ; chemical loss exceeds chemical production by a factor of 2.4 on average. Similar or greater budget imbalances have been reported in analyses of data from previous field studies. Ammonium aerosol concentrations in PEM-Tropics A generally exceeded sulfate on a charge equivalent basis, and relative humidities were low (median 25% relative to ice). This implies that the aerosol could be dry in which case hydrolysis would be suppressed as a sink for . Suppression of hydrolysis and adoption of new measurements of the reaction rate constants for and reduces the median chemical imbalance in the budget for PEM-Tropics A from 2.4 to 1.9. The remaining imbalance cannot be easily explained from known chemistry or long-range transport of primary and may imply a major gap in our understanding of the chemical cycling of in the free troposphere.