1. Aerosol Optical Properties via Cavity Ring-Down Technology Virtual Impactor for Sub-micron Aerosol Particles A. A. Boľshakov, A. W. Strawa, A. G. Hallar NASA Ames Research Center, Moffett Field, CA 94035 abolshakov@arc.nasa.gov, astrawa@arc.nasa.gov, ahallar@arc.nasa.gov Aerosol Optical Properties via Cavity Ring-Down Technology 0893 Particle size profiles at different flow rates through the virtual impactor Particle size profiles with and without use of virtual impactor at 0.12 l/min Particle size profiles obtained without use of virtual impactor Particle size profiles with and without use of virtual impactor at 0.22 l/min 12 3 4 5 6 Excess Pump out Make-up air 1 – TSI 3076 nebulizer at air pressure 35 psig; 2 – buffer volume/container (box or bottle) with or without silica gel; 3 – virtual impactor; 4 – TSI 3081 electrostatic classifier - differential mobility analyzer; 5 – air filter; 6 – TSI 3025A particle counter. We have developed a virtual impactor to concentrate aerosol particles of known sizes for the purpose of calibrating the optical cavity ring-down instrument designed to measure extinction and scattering of atmospheric aerosols. No such low-flow virtual impactors for sub-micron particles are commercially available. Our virtual impactor features two separable concentric parts: the nozzle and the probe. An internal diameter ratio of the probe to the nozzle was selected as 0.742:0.55 mm. The particles’ 50%- cut-off diameters, characteristic for our virtual impactor were between 500 and 900 nm depending on the flow rates used. Performance of our virtual impactor was evaluated in an experimental set-up using a scanning mobility particle size spectrometer. An aqueous suspension of solid polystyrene latex spheres was aspirated through the nebulizer to generate aerosol standards. Aerosol sample flow rates after passing the virtual impactor were in a range of 0.1–0.3 l/min. The particle size profiles exhibited a broad maximum of moisture particles at ~100 nm and a narrow peak of monodisperse polystyrene spheres of one of the specified standard sizes at 500, 700, or 900 nm. Under experimental conditions optimized for the best performance of the virtual impactor, we were able to concentrate the 700-nm polystyrene particles no less than 15-fold. However, under experimental conditions optimized for calibrating our cavity ring-down instrument, a concentration factor attained was 4 to 5. During calibration experiments, maximum realized particle number densities were 190, 300 and 1600 cm -3 for the 900-nm, 700-nm and 500-nm spheres, respectively. Calibration Facility Instrument in development for Unmanned Aerial Vehicles  Nebulize aerosols with an extensive size range  Create selective narrow size particle distribution  Includes: Virtual Impactor, Desiccator, DMA, CPC ~25 lb package will measure forward & backward scattering with extinction simultaneously at two wavelengths Cadenza measures atmospheric aerosol scattering and extinction simultaneously at same temperature, RH, pressure CADENZA II Nozzle Probe flow in flow out Example: