An instrument for monitoring number and mass of ambient particles in coarse, fine and ultrafine size ranges

Abstract

Airborne particulate matter is today recognized as an important category of air pollutants. Current air quality standards, and hence most particle instruments, are based on particle mass concentration-the European standard, for example, on the PM10 fraction. For other metrics, such as number concentration, there is currently a lack of suitable instrumentation for monitoring purposes. A novel realtime particle-counting instrument has been developed in an attempt to fill this gap. The new instrument is capable of detecting and counting particles from about 10 nm to 10 pm particle diameter. This instrument, aimed at monitoring ambient air quality, uses a parallel combination of optical particle counting for larger particles plus condensation particle counting for the smallest particles. The particles are classified into several size fractions, which allows discrimination between ultrafine, fine, and coarse particles. The instrument also enables gravimetric measurement of PM10 or PM2.5 providing the possibility of comparing the measured number concentrations with the mass concentration standards. Furthermore, a conversion of the number concentration data into PMIO.P. M2.5,a nd also PMl mass concentration is possible. The new instrument is unique in offering number and mass information over the complete size range of interest in urban air quality monitoring. The design of the instrument and the development and construction of first prototypes are described as well as calibration and performance results. The performance tests included side-by-side comparisons of two identical prototypes and comparison studies with traditional instrumentation in an urban field environment at a monitoring station in Birmingham (UK). Here, very good correlation was observed between the ultrafine particle concentration indicated by the new instrument and the total number concentration measured by a CPC (TSI model 3022A). Number-to-mass conversion results correlated well with mass concentration measured by TEOM. Possibilities for a further size fractionation in the ultrafine particle size range were investigated and selected techniques tested. Size separation using diffusion devices was identified as a suitable technique to be implemented in the new instrument for ultrafine fractionation. Other future possibilities for further developments are also discussed.