Comparability of three spectrometers for monitoring urban aerosol

The comparability was tested of three aerosol “spectrometers”, used in a program for monitoring the spectra of fine and ultrafine particles in three European cities. Droplets of sebacate, solid ammonium sulfate and agglomerates of elemental carbon were used in the tests, representing the major chemical and structural types of particles encountered in urban aerosol. Particles in the ultrafine range (10–100 nm) are sized by electrical mobility (SMPS, DMPS and EAS) and the “spectrometers” gave very similar size distributions for these aerosols. The integrated number concentrations were on average within 20% of the directly measured total number concentrations. Particles with a size between 0.1 and 2.5 μm, in which most of the volume/mass is concentrated, are being differently classified in the three “spectrometers”, respectively, with a low- and a high-flow LAS-X, and field charging in the EAS. The agreement between the three instruments in this size range was less good, which was partly caused by signal overload in the high-flow optical sizer, which was solved using a larger threshold. A complication occurred with the elemental carbon, which was composed of highly agglomerated entities. Particles, sized by the mobility instrumentation as being in the range of 100–400 nm, were not detected by the optical sizers. Volume (spectra) for ammonium sulfate deduced from the number spectra were compared with the mass (spectra) obtained with cascade impactors. The comparison was good for the LAS-Xs; the EAS overestimated volume/mass.