Performance evaluation of four cascade impactors for airborne UFP collection: Influence of particle type, concentration, mass and chemical nature

Abstract. Ultrafine particles (UFP) have aerodynamic diameters of 100 nm or less. As UFP potentially impact human and environmental health, their chemical composition is of interest. However, their small mass presents challenges to techniques originally developed for larger particles. Therefore, we conducted a comprehensive characterization and comparison of four cascade impactors suitable to separate and collect UFP, namely 120R MOUDI (Micro-Orifice Uniform Deposit Impactor), ultraMOUDI, ELPI (Electrical Low-Pressure Impactor), and PENS (Personal Nanoparticle Sampler), under controlled laboratory conditions and in a field application.

In the laboratory, we evaluated pressure drop, cut-off diameters, steepness of the cut-off curve, losses, particle bounce, and transmitted particle mass. We observed performance differences among the impactors due to design and test aerosol mixture variations, including salt particles (NaCl), simulated secondary organic aerosol (SimSOA), and soot (with cut-off diameters of 59–68 nm, 70–74 nm, and 102–116 nm, respectively, as determined by electromobility diameter). All impactors successfully separated UFP, with the best agreement in cut-off diameters for SimSOA, showing maximum deviations of about 4 nm. The cut-off curve was steeper for soot compared to SimSOA and NaCl. Pressure drops were measured at 260±1 hPa (PENS), 420±2 hPa (ultraMOUDI), 600±3 hPa (120R MOUDI), and 690±3 hPa (ELPI). Losses were assessed as maximum transmission in the ultrafine fraction at 30 nm, resulting in 83±8 % for PENS, 77±8 % for ultraMOUDI, 75±8 % for 120R MOUDI, and 69±7 % for ELPI. We identified two additional factors crucial for mass-based analysis of organic marker compounds: evaporation of semi-volatile compounds due to high pressure drop across the impactor and material addition from larger particles bouncing off upper stages. Bounce-off was influenced by particle number concentration in the sampled air and could be mitigated by applying a coating to the upper impaction plates.

In the field application, we deployed the four cascade impactors side-by-side under environmental conditions to sample urban and semi-industrial air. We analyzed six markers representing typical UFP sources and various molecular properties using HPLC-MS/FLD (high-performance liquid chromatography with mass spectrometry and fluorescence detection). The markers included polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]pyrene (BaP) and benzo[b]fluoranthene (BbF), levoglucosan (Levo), pinic acid (PA), terpenylic acid (TA), and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD). The impactors showed the best agreement for the two PAHs. BaP had an average mass concentration of 175±25 pg/m³ across all impactors and sampling days, but concentrations varied by about 29 % higher or 30 % lower when analyzed with the PENS and the 120R MOUDI, respectively, indicating a maximum disagreement of nearly 60 %. The PENS consistently reported higher mass concentrations for all marker compounds compared to the other impactors. Potential reasons for this include the effects of pressure drop on gas-particle partitioning of semi-volatile compounds and material addition from particle bounce, despite the applied coating. Semi-volatile markers PA, TA, and Levo exhibited decreasing absolute deviations from the average mass concentration with increasing pressure drop, suggesting comparably higher evaporation losses during sampling with the ELPI and lower losses with the PENS. Marker mass concentrations increased with higher air concentrations, correlating with increased absolute deviations, likely due to bounce-off adding mass from larger particles. This effect was strongest for the PENS, followed by ultraMOUDI, ELPI, and 120R MOUDI.

Overall, this study demonstrates the impact of impactor design, operational conditions, and aerosol mixture on observed mass concentrations of organic markers in airborne UFP. Our findings highlight the complexities of accurately separating, collecting, and analyzing UFP mass. While all four impactors can sample UFP, they each have distinct strengths and limitations that must be considered when comparing atmospheric UFP study results.