Abstract. During November and December 1995 the First Aerosol Characterization Experiment (ACE 1) was undertaken as part of the International Global Atmospheric Chemistry (IGAC)Program. A key objective of the aircraft component of this experiment included the identification of source regions for new particles in the remote marine atmosphere. No evidence was found for particle production in the marine boundary layer (MBL), but extensive observations of enhanced layers of "new" particles were found in the free troposphere (FT). These layers were generally found at altitudes that corresponded to nearby cloud top heights and exhibited concentrations that exceeded MBL air by about 1000 to 10,000 cm -3. Many layers were also associated with enhanced concentrations of water vapor and sulfuric acid. Focused cloud experiments demonstrated that these particles were recently formed and originated in the outflow region of clouds preferentially after late morning when photochemical processes had become sufficiently active. The production and growth of these particles were rapid, and they appeared to evolve and merge with a background nuclei spectra on the scale of hours to a day. These measurements in midlatitude postfrontal air undergoing shallow convection indicated that particles were produced in the trailing cloud outflow region as low as 2 km altitude and that the base of this layer extended down to the inversion in the region of postfrontal subsidence. Other ACE 1 measurements made in transit near equatorial convection also revealed small nuclei aloft at altitudes up to 6 km and a trend in decreasing concentrations, in conjunction with steadily increasing size, during descent toward the surface. The concentration and size distributions in this these regions indicate that significant numbers of new nuclei are formed aloft in various cloud outflow regions and that they can provide a source for the MBL aerosol via subsidence. This nucleation appears to be favored when existing surface areas approach or drop below about 5-10 pm 2 cm -3. Other observations have tended to indicate that gas to particle (GPC) conversion in the MBL for regions with high DMS was nature of the conversion process prompted speculation that new more likely to result in growth of existing aerosol to larger sizes particle formation might be initiated via gas to particle conversion than

New particle formation in a tropical marine boundary layer setting was characterized during NASA's Pacific Exploratory Mission-Tropics A program. It represents the clearest demonstration to date of aerosol nucleation and growth being linked to the natural marine sulfur cycle. This conclusion was based on real-time observations of dimethylsulfide, sulfur dioxide, sulfuric acid (gas), hydroxide, ozone, temperature, relative humidity, aerosol size and number distribution, and total aerosol surface area. Classic binary nucleation theory predicts no nucleation under the observed marine boundary layer conditions.