Atmospheric aerosols, generally considered to be dispersions of particles that range in diameter from a few nanometers to perhaps ten micrometers, play an important role in regulating the Earth's climate and may have a detrimental impact on air quality and human health, even though the concentration is usually quite low (from 50 μg/m3 in rural regions to 150 μg/m3 in polluted urban areas). Other aerosols of interest include those emitted by combustion sources, those that are administered therapeutically for medical reasons, and those that may be employed by terrorists or renegade nations. In all of these examples, rapid acquisition of information concerning the size, concentration and chemical composition of these airborne particles is essential to the development of appropriate strategies.
For this purpose, we have developed instruments to conduct real-time sampling, concentrating and measuring of the atmospheric aerosols in various size ranges. Particles of 70-600nm are detected by an aerosol mass spectrometer (AMS), in which a “standard” aerodynamic lens focuses the particles into a narrow beam and efficiently transports them into vacuum where aerodynamic particle size is determined via a particle time-of-flight measurement. Flash vaporization of the particle on a resistively heated surface then permits mass spectrometric analysis. Particles of 100-2000nm can similarly be measured by an AMS system in a “high-pressure” lens working at about 20Torr instead of 1.3Torr, as employed in the standard lens. For even larger particles (for example, bacilli of 1-10μm), an “atmospheric-pressure” lens has been developed to focus and concentrate the particles in order to permit laser-induced breakdown spectroscopy (LIBS). In all the above cases, computational fluid mechanics (CFD) and experimental verification are interactively used to predict the lens performance and optimize the lens design. The results provide a rational framework for the design of such devices and a physical interpretation of the observed behavior.