| Abstract: | A new automated version of the piezoelectric microbalance measures the mass concentration of airborne particles smaller than a preselected aerodynamic cutoff diameter. It is designed for near-real-time, unattended, round-the-clock measurements of nearly any aerosol environment inhabited by humans. The instrument uses an electrostatic precipitator to deposit particles with greater than 95% efficiency onto a piezoelectric quartz crystal sensor which is able to detect a deposit of 0.005 μg. The precipitator and sensor are nearly identical to those in the portable instrument reported previously. Measurements comparing within ± 15% with gravimetrically measured filter samples are documented for a wide variety of aerosols in the 50 μg/m3 to 5.5 mg/m3 range. The instrument measures particles from 10 μm down to 0.01 μm in diameter, including submicron combustion smokes and metallic fumes. The piezoelectric microbalance technique senses the mass concentration of the aerosol, rather than light scattering properties as is done by photometers and nephelometers. The piezobalance, with 1 L/min sample flow, is more sensitive than any other mass-sensing instrument, making it especially suited for low concentration indoor measurements, even below 50 μg/m3. An automatic piezobalance recently measured respirable aerosol mass concentrations in several offices. Each measurement was the average concentration during a 30-min period. The 24-h/day measurements continued for several days. Especially interesting is the diurnal pattern, both for offices with and without smokers. The effect of a single nearby smoker was clearly illustrated when the smoker was absent one day in the middle of a week. Normal daytime peak concentrations in that office reached 80–110 μg/m3 with a smoker present, but only 50–60 μg/m3 when the smoker was absent. Curious smokers who briefly stopped byt o see the instrument caused single half-hour averages to triple, to values as high as 294 μg/m3 in one office. |
