Passive Versus Active Aerosol Monitoring

Dynamic aerosol monitors are frequently used today in a passive mode in which the ambient convective air currents transport the aerosol to the sensing chamber, thereby alleviating the use of a pump. The response of an aerosol monitor operated at different flow rates was compared to its response in the passive mode. As expected, the light-scattering sensor always responds to the aerosol concentration it sees. However, the transport of aerosols to the sensing chamber may modify the concentration and size distribution of the monitored aerosol concentration. The aerosol concentration recorded by the tested aerosol monitor was found to vary significantly as a function of flow rate which indicates an effect of flow rate on the sampling efficiency of the inlet to the sampling chamber and on the particle losses inside the chamber. The magnitude of these effects depends on the external aerosol dynamics, the inlet geometry, and the aerosol size distribution. Therefore, the instrument needs to be calibrated against the gravimetric sample for the specific dust and flow configuration to be used during subsequent routine testing. The presented data show that passive sampling of large-size dust should be avoided so that particle deposition to the surfaces of the sensing chamber and, therefore, increases in stray light are kept to a minimum.