TEOM stands for tapered element oscillating microbalance.  This instrument measures particle concentrations in air continuously.  Sampled air is drawn through a filter which collects particles.  The filter sits on top of a hollow tapered element which vibrates at a known frequency (oscillates).  The frequency of oscillation changes by an amount proportional to the mass of particles deposited on the filter.  Because the sample air is heated to a high temperature the semi-volatile fraction of the particle mass can be destroyed.  For this reason TEOMs can underestimate PM levels.  Fitting a TEOM with a FDMS (filter dynamics measuring system) can overcome this problem.