The performance of the PM_2.5 VSCC and oil-wetted M-WINS in long-term field sampling studies

Most of PM_2.5 FRM samplers and FEM monitors use a well impactor ninety-six (WINS) or very sharp cut cyclone (VSCC) to classify particles smaller than 2.5 μm in diameter. However, the WINS has the particle loading effect and needs to be cleaned after 3 to 5 sampling days. The VSCC was claimed not to be affected by the particle loading effect in a laboratory study but without the support of field studies. In this field comparison study using daily cleaned VSCC as the reference, the uncleaned VSCC showed overall good performance in three long-term testing periods: period #1 (21 days), period #2 (124 days), and period #3 (30 days). However the fluctuation in PM_2.5 concentrations of the uncleaned VSCC occurred in day 14 (D14) and D16 of period #1 due to particle bounce and re-entrainment effects; and also in D19, D31 and D44 of period #2, and in D28 and D29 of period #3 due to particle loading effect. After the sampling periods longer than 21 days in period #2 and #3, the particle loading effect resulted in persistently lower PM_2.5 concentrations due to smaller cut-sizes and smoother penetration curves. In comparison, the oil-wetted M-WINS designed by our group showed a better performance with less than ±5% bias and ±2 μg m⁻³ concentration difference during two long-term tests: periods #2 and #3. This study proved once again that the M-WINS is a viable PM_2.5 sampling inlet that meets the long-term PM_2.5 sampling or monitoring need.