Comparison of instruments for particle number size distribution measurements in air quality monitoring

• We compared number size distributions from ELPI, SMPS, FMPS and APS.
• Results from four lab generated aerosols were compared in a wind tunnel.
• Good correlation was found between instruments in their middle size ranges.
• At the lower and upper particle diameters there were divergences.
• Particle type (size and shape) affected the correlation between instruments.

Number size distributions of airborne particles are relevant to fields including ambient monitoring, pharmaceutical and automotive measurements. A number of commercially available instruments can be used to determine particle number size distributions including the Electrical Low Pressure Impactor (ELPI), Scanning Mobility Particle Sizer (SMPS), Fast Mobility Particle Sizer (FMPS) and the Aerodynamic Particle Sizer (APS). The comparability of the data provided by these instruments has not been fully tested for different kinds of aerosols. This study compared number size distributions of laboratory generated aerosols (TiO2, NaCl, fumed silica and soot) in a wind tunnel. Reasonable agreement was noted between the different instruments, though there were divergences. For example the ELPI was inconsistent at the upper and lower limits of its working size (at low concentrations). Instruments responded variably to different particle types, which has important implications for sampling heterogeneous particle mixtures such as those found in urban air. This study highlights the need for caution when comparing data obtained from different particle instruments, and demonstrates the requirement for further comparison studies in controlled settings using an assortment of particle types with the aim to standardise and harmonise particle sampling protocols.