Nanoparticles of aluminum metal were generated by passing an aerosol of rnicrometer-scale (mean 50 mum) particles in argon through an atmospheric pressure plasma torch operated at less than 1000 W. A designed experiment was conducted to investigate the effects of plasma gas flow rate, aerosol gas flow rate, and applied power on the shape, size, and size distribution of the final particles. The size and shape of the metal particles were dramatically impacted by the operating parameters employed. At relatively low powers or at high powers and short residence times, virtually all the particles are spherical. Under other conditions, the particles had spherical heads, and virtually all had tails, some quite long. The particle size distributions also were influenced by the operating conditions. Under most conditions the size distributions were log-normal, consistent with growth by agglomeration. However, under some conditions, the population of particles above or below the mode was far too great to be consistent with a log-normal distribution. For example, the particle distributions tend to show an unusual concentration of very small particles at relatively short residence times and low aluminum feed rates. The distributions tend to show an unusual concentration of large particles at relatively long residence times and high aluminum feed rates. On the basis of the data collected, some simple models of the mechanism of nanoparticle formation were postulated which should be of value in future studies of the process.