Development of a near-continuous monitor for measurement of the sub-150 nm PM mass concentration

Population exposures to ambient particulate matter (PM) have recently received considerable attention due to the association between ambient particle concentrations and mortality. Recent toxicological studies suggest that ultrafine PM (diameter <100 nm) may be responsible for the observed health effects. However, even though ultrafine mass concentrations vary drastically over short time scales in the atmosphere, no monitor currently measures ultrafine PM mass continuously. The need for monitors that can perform ultrafine particle concentration measurement in shorter time intervals is of paramount importance to environmental health, as such a monitor can lead to substantial improvements in population exposure assessment to ambient ultrafine PM. In this study, a modified BAM (Beta Attenuation Monitor) is employed to measure near-ultrafine (i.e., <0.15 mum or PM0.15) particulate mass concentration. The BAM is preceded by a 0.15 mum cutpoint impactor, which is designed to have very low pressure drop. The BAM is operated in a 2 h cycle at a downwind receptor site in the Los Angeles Basin in Claremont. Among the other instruments colocated with the BAM are scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), and a Micro-Orifice Uniform Deposit Impactor (MOUDI). Our results indicate that the PM0.15 mass concentrations obtained by means of the modified BAM and MOUDI are in excellent agreement. The PM0.15 SMPS-to-BAM concentration ratio is generally smaller than 1 and follows a rather distinct diurnal profile, with a maximum towards the middle of the day and minima during the early morning and nighttime periods, presumably due to the classification of fractal-structured ultrafine particles in the accumulation mode by the SNIPS. The lack of correlation between PM2.5 and PM0.15 mass concentrations further corroborates the need for developing monitors such as the modified BAM for the documentation of the short-term variation of ultrafine mass measurements.