Particle growth in high-speed particle beam inlets

Physical and chemical characterization of airborne particles is essential for determining their role in air pollution. Characterization instruments typically employ the use of sonic nozzles that transmit a wide range of particle sizes to a low-pressure region. The carrier gas generally undergoes expansion resulting in wide temperature and pressure variations, and as a result Volatile species may condense or evaporate. This is undesirable since it may change the size and composition of the particles and lead to uncertainities in measurements, especially during surface chemical characterization. In this work we investigate the effect of heterogeneous condensation of volatile species on particles in the size range 0.1-10 mu m, in such nozzles. From available experimental and theoretical data, we first conclude that the clusters formed due to homogeneous condensation are too small to influence characterization. From theoretical estimates, we then show that for atmospheric sampling, water vapor is the main cause of particle growth, and using numerical simulations, we illustrate particle growth in some commonly used nozzles and its dependence on the vapor concentration of volatile species in the carrier gas. Fire further investigate growth dependence on the surface accommodation coefficient of the particle. Finally, we illustrate the particle exit velocity characteristics and draw some useful conclusions about inlet design. Copyright (C) 1997 Elsevier Science Ltd