Number concentration and size distribution of aerosol particles in the middle troposphere over the Western Pacific Ocean

Number concentration and size distribution of aerosol particles were measured on board aircraft during the PACE (Pacific Atmospheric Chemistry Experiment) campaign from Australia to Japan in January 1994. The spatial distribution of condensation nuclei (CN) (r 4 nm) at 5–6 km altitude showed large variabilities in concentrations from 102 to 103 mg−1 that is, the concentrations were low (70–500 mg−1) in the intertropical convergence zone, high (400–1500 mg−1) in the subtropical highpressure area, and low again in the higher latitudes. An apparent opposite tendency was present between CN and large particle (r 0.15 μm) concentrations. The size distributions in the subtropical region exhibited high number concentrations of very fine particles (r < 0.02 μm). Together with the horizontal observation, vertical observations of aerosols were carried out over some areas. In the subtropical area (Saipan), CN concentration increased with altitude in contrast to the large particle concentration. Also most of the particles collected at 6 km altitude over Saipan contained sulfuric acid. These results are consistent with the results of Clarke (1993, J. geophys. Res. 98, 20,633–20,647) that new particle formation is favored in the upper troposphere.