The objective of this research is to develop improved techniques and instruments for measuring the physics and chemistry of small airborne particles, especially toxic aerosols. The problem of characterizaton of the toxicity of airborne particulates is difficult. Even though substantial effort has been spent by many groups working in instrument development, resulting in significnt improvements, adequate instrumentation is still not available. For example, no single instrument can presently measure the size distribution of an aerosol over the full respirable size range (between .01 and 3 micron particle diameter). We are attempting to use aerosol beam techniques to obtain improved instruments. We have already demonstrated that time of flight (TOF) measurement provides the size of the particle in the aerosol beam. The present limitations of the method are the lack of a technique for detecting the very small particles so that their TOF can be measured. To solve this problem we are investigating the use of electron beam techniques for detecting the small particles. If these prove successful, as we hope, we will have developed an instrument capable of measuring the particle size distribution over the full respirable size range. It should also be possible to couple the size measurement with a measurement of the particle composition to fully determine the toxic nature of the particle. We are investigating this and related problems.