Measuring and tracking velocity of individual aerosol particles in a bio-threat detection system are accomplished using a single beam laser source in combination with a birefringent crystal that splits the laser beam into two beams having orthogonal polarization. Scattered light is collected with an elliptical reflector and directed into two detection channels, sampling total elastic scatter in the first channel and sampling polarized elastic scatter in the second channel. The difference in intensity of the scattered light in the polarized channel is used to identify the position of the particles. By taking the ratio of signal output from the polarized detector to the total scatter detector, a threshold level can be established to determine the presence of particles traversing the two beams. The particles are time stamped as they traverse the two beams and the time difference between the pulses can be used to measure the velocity of the particles.