The resonant frequency of vibration of the rod changes with the changing filter mass due to the accumulation of particles, this is assessed through the feedback response to the applied field, allowing a near real time measure of the increasing filter mass, and hence the mass concentration of aerosol. In the commercial systems, the inlet airstream is preheated to approximately 50’c to drive off water, this also leads to the loss of some volatile components of the aerosols and so TEOMs trend to underestimate the actual aerosol mass; a correction factor (commonly a value of 1.3 is used for urban and suburban boundary layer environments) is applied in an attempt to allow for the effect.