Signatures of the N2O5 hydrolysis by sulfate aerosols have been previously documented, primarily from balloon and remote-sensing platforms, by measurements of nitrogen species aboard the NASA ER-2 flying at an altitude of approximately 20 km and some ER-2 and DC-8 measurements near the tropopause during stratospheric campaigns. This study documents such signatures in the NOx/NOy ratios derived from DC-8 measurements during Pacific Exploratory Measurements in the Western Pacific Ocean (PEM-West B) in stratospherically influenced air masses sampled during a level leg at an altitude of 10.7 km in flight 17 out of Japan. Despite the very low abundance of total bromine, we also show that heterogeneous hydrolysis of BrNO3 on sulphate aerosols can catalytically convert NOx and liquid H2O into HNO3 and OH and thereby lower the calculated equilibrium NOx/NOy by about 20 to 35% in these air masses, bringing closer agreement with the nitrogen partitioning deduced from measurements. However, the NOx/NOy ratios calculated from a model including heterogeneous chemistry were a factor of 3 smaller than ratios derived from data for a segment of this flight leg when DC-8 measurements indicated a stronger tropospheric influence. We also modeled the equilibrium partitioning of nitrogen species for all upper tropospheric air masses encountered by the DC-8; since NOy in the troposphere may contain nonnegligible contributions from long-lived nitrates (such as peroxyacetylnitrate), we have compared instead modeled and measured NOx/HNO3. The calculated equilibrium NOx/HNO3 ratios using only gas-phase chemistry are on the average smaller than those deduced from measurements in upper tropospheric air masses; inclusion of N2O5 hydrolysis reduces these ratios by an additional 20%, thus worsening the discrepancy. These results suggest a rapid transition from denoxified conditions in the lower stratosphere to renoxified conditions in the upper troposphere. This transition could be due to intrinsically different chemistry in the troposphere. Alternatively, rapid transport in the troposphere could keep the NOx and HNO3 away from chemical equilibrium. Detailed analysis of current and future tropospheric data could shed light on this issue. ¿ 1997 American Geophysical Union