The characteristics and sources of what are believed to be newly formed 3 to 4 nm particles in anthropogenic plumes advecting from Asian are reported. Airborne measurements were made from March to April 2001 as part of the NASA TRACE-P experiment at latitudes ranging from North of the Philippines to Northern Japan (20 to 45°N). In the more polluted plumes, high concentrations of 3 to 4 nm diameter particles (>100cm−3) were observed both within and along the upper outer edges of plumes that were identified by enhanced carbon monoxide and fine particulate sulfate concentrations. The results from two research flights are investigated in detail. Three to four-nm particle concentrations are generally correlated with gas phase sulfuric acid and found in regions of low surface areas relative to the immediate surroundings or where there are steep transitions to lower surface areas. Sulfuric acid and surface area concentrations in the most polluted plume reached 6 × 107 cm−3 and 750 μm2 cm−3, respectively, in regions of particle formation. In contrast to these anthropogenic plumes, few 3 to 4 nm particles were observed in the clean background and few were detected within a volcanic plume where the studies highest H2SO4 concentrations (>108 cm−3) were recorded. Enhanced SO2 concentrations in the range of approximately 2 to 7 ppb, in conjunction with other unidentified, possibly coemitted species, appear to be the driving factor for nucleation.