PM_0.1 non-bouncing impactor (NBI) for ultrafine particle mass and number measurements

This study designed and tested a 30 L/min PM_0.1 non-bouncing impactor (PM_0.1 NBI) for ultrafine particle (UFP) mass concentration measurement. The PM_0.1 NBI uses a wetted glass fiber filter with continuous water injection to maintain a clean surface, preventing particle loading and bounce effects. Lab tests showed the 30 L/min PM_0.1 NBI outperformed the PM_0.1 stage of NCTU micro-orifice cascade impactor (NMCI, non-rotating) with a silicon oil-coated aluminum foil substrate in collecting solid particles, as the particle collection efficiency of the latter decreased obviously with increasing particle aerodynamic diameter for particles larger than ca. 300 nm, an apparent particle bounce phenomenon. The ambient field tests showed very close UFP mass concentrations between the PM_0.1 NBI sampler (the PM_0.1 NBI preceded by 3 stages with 18, 10, and 2.5 μm cutoff sizes) and the reference rotating 10-stages NMCI with the difference of ±0.5 μg/m³ only. In addition, the 3 L/min PM_0.1 NBI was combined with a condensation particle counter to measure the UFP number concentrations which were shown to be very close to those of the reference with the difference of ±5% only in the laboratory tests for loaded particle mass up to 300 μg. Good agreement was also found in the field tests. Therefore, the current 3 L/min PM_0.1 NBI and the future PM NBI with smaller flow rates (0.2–2.1 L/min) and larger cut-sizes (such as 0.4–0.9 μm) can be used as viable UFP classifiers for improving UFP measurement accuracy both in mass concentrations and number concentrations and reducing the maintenance needs for the UFP classifiers which are important to long-term sampling and monitoring.