EVIDENCE OF HETEROGENEOUS CHEMISTRY ON SULFATE AEROSOLS IN STRATOSPHERICALLY INFLUENCED AIR MASSES SAMPLED DURING PEM-WEST-B

Signatures of the N2O5 hydrolysis by sulfate aerosols have been previo usly 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 ne ar 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. Despi te the very low abundance of total bromine, we also show that heteroge neous hydrolysis of BrNO3 on sulphate aerosols can catalytically conve rt NOX and liquid H2O into HNO3 and OH and thereby lower the calculate d equilibrium NOx/NOy by about 20 to 35% in these air masses, bringing closer agreement with the nitrogen partitioning deduced from measurem ents. However, the NOx/NO ratios calculated from a model including het erogeneous chemistry were a factor of 3 smaller than ratios derived fr om data for a segment of this flight leg when DC-8 measurements indica ted a stronger tropospheric influence. We also modeled the equilibrium partitioning of nitrogen species for all upper tropospheric air masse s encountered by the DC-8; since NOy in the troposphere may contain no nnegligible contributions from long-lived nitrates (such as peroxyacet ylnitrate), we have compared instead modeled and measured NOx/HNO3. Th e calculated equilibrium NOx/HNO3 ratios using only gas-phase chemistr y are on the average smaller than those deduced from measurements in u pper tropospheric air masses; inclusion of N2O5 hydrolysis reduces the se ratios by an additional 20%, thus worsening the discrepancy. These results suggest a rapid transition from ''denoxified'' conditions in t he lower stratosphere to ''renoxified'' conditions in the upper tropos phere. This transition could be due to intrinsically different chemist ry in the troposphere. Alternatively, rapid transport in the troposphe re could keep the NOx and HNO3 away from chemical equilibrium. Detaile d analysis of current and future tropospheric data could shed light on this issue.