Determination of submicron aerosols effective density using two-stage low-pressure impactor and aerosol electrometer...

A novel methodology was presented for determining the representative effective density of aerosols of a given size distribution, using a lab-made two-stage low-pressure impactor and an aerosol electrometer. Electrical currents upstream (I_{measured, up}) and downstream (I_{measured, down}) of the 2^nd stage of the impactor were measured using a corona charger and the aerosol electrometer. In addition, the electrical currents upstream (I_{calculated, up}) and downstream (I_{calculated, down}) of the 2^nd stage of the impactor were calculated using the aerosol charging theory. Then, the difference between the ratio of I_{measured,down} to I_measured,up and the ratio of I_{calculated,down} to I_{calculated,up} was iterated with varying the presumed effective density until the difference was smaller than 0.001. The methodology was validated using poly-disperse sodium chloride (NaCl) particles. The effective densities of ambient aerosols were then obtained from indoor and outdoor environments and compared with those calculated from a relation between mobility (scanning mobility particle sizer (SMPS) measurement) and aerodynamic (electrical low-pressure impactor (ELPI) measurement) diameters. Compared to the effective densities obtained with SMPS and ELPI measurements, the effective densities obtained using the methodology introduced in this paper differed within 10 % deviation, depending on measurement location. After an averaged effective density for given size distribution is obtained at a measurement location, the number-based size distribution can be easily converted to mass-based size distribution using the representative effective density.