Efficiency of portable counters in measuring particle number and lung deposited surface area concentrations

Aerosol exposure is of great concern for medical and air-quality experts due to the particles’ ability in crossing human respiratory system, depositing in its deepest and defenceless regions and carrying condensed toxic compounds. In particular, epidemiological, toxicological and environmental researchers are trying to understand what is the particle property mainly related to negative effect on human health. About the size the interest of such experts is moving from particle mass concentrations to particle number and surface area concentrations mainly characterized in terms of sub-micrometer and ultrafine particles (UFPs). Particle number concentrations are usually measured through condensation particle counters (CPCs) which are able to count particles down to few nanometers by optical methods after their growth by condensation of a working fluid onto the particles. Several CPC models were designed having different minimum detectable particle diameter (cut-off size) as function of the working fluid, the aerosol losses in the inlet of the CPC, the efficiency of the optical system, and the particle activation efficiency (Petaja et al. 2006). Due to their operation methods, such CPCs were mainly used for laboratory purposes. Anyway, in order to perform UFP exposure assessments on personal scale, hand-held, battery powered devices are needed. To this purpose a particle counter (NanoTracer, Philips) based on diffusion charging technique was developed. It does not require working fluid to growth the particles, and it is able to provide number concentration (in the range 10-300 nm), average particle diameter, and lung deposited surface area concentration measurements. Shortly, aerosol accurately charged is captured by fibrous filter in a Faraday cage producing a current signal proportional to the amount of charged particles, and their average diameter. Through a semi-empiric algorithm the device is also able to evaluate the different fractions of the lung deposited surface area (Marra et al. 2010). In the present work a calibration method of two NanoTracers in terms of total number concentration, average diameter and total alveolar deposited surface area is proposed and applied. In order to perform such calibration an aerosol generation system (3940 TSI Inc.) was used. It is able to generate particles with a specific mode and total concentration. The first step was the calibration of a CPC 3775 (TSI Inc.) making a comparison with an Aerosol Electrometer 3068 (TSI Inc.) according the procedure reported in Hameri et al. (2002). In order to calibrate the NanoTracers a buffer volume (about 20 L) was used. Monodisperse and polidisperse aerosols accurately produced was flown in the volume where two NanoTracers were placed. Simultaneously, particles were sampled from the volume through a CPC3775, a Nanoparticle Surface Area Monitor (NSAM 3550, TSI Inc.) and a Scanning Mobility Particle Sizer spectrometer (SMPS 3936, TSI Inc.) in order to measure total particle number concentration, total alveolar deposited surface area concentration and particle number distribution, respectively. The following ranges are investigated: 104-105 part. cm-3 in terms of particle number concentrations and 30-200 nm for the mean diameter. Preliminary results obtained at a diameter of 50 nm and low concentrations (8.4×103 ± 51 part. cm-3) are encouraging: maximum differences of 10% were found in terms of particle number and alveolar deposited surface area concentrations, as well as of 4% in terms of mean diameter