Stands for beta attenuation mass monitor.  This instrument measures particle concentrations continuously in air.  Sampled air is drawn through a filter which collects particles.  A beam of beta radiation is passed through the filter and measured on the other side of the filter by a detector.  As the mass of particles on the filter increases the beta count is reduced.  The relationship between the decrease in count and particle mass is calculated according to a known equation (Beer-Lambert law).