Performance evaluation of CEN-SMPS with a novel fast scan mode for better identification of UFPs in MI-TRAP

Introduction: Precise measurement and characterization of ultrafine particles (UFPs) is one of the key objectives of the MI-TRAP (Mitigating Transport Related Air Pollution in Europe) project. The scanning mobility particle sizers (SMPS) have been conventionally used worldwide for measuring the particle number size distributions (PNSD) from transport emissions as well as in ambient air. Consequently, they are an integral part of the air quality monitoring network under the MI-TRAP consortia and the measurements are harmonized with the standard protocol (CEN/TS 17434:2020) by the European Committee for Standardization (CEN). This standard specifies the diameter range for a SMPS scan from 10 nm up to 800 nm, consequently leading to large scan time over 5 minutes. However, the size distribution of UFPs monitored by SMPS at several European sites have been found to be predominately centered around geometric mean diameter (GMD) < 100 nm ^[1]. Moreover, the introduction of new European emission standards like Euro 6 have led to decrease the GMDs of transport emissions as well (GMD = 50-60 nm observed for Euro 6 gasoline and port fuel injected engines) ^[2]. Therefore, this wide scan range is less efficient and time consuming for monitoring UFPs from transport emissions. All these factors necessitate the introduction of a fast-scanning mode with a narrower scan range without compromising the accuracy and tailored for the specific needs of UFP measurements.

Methodology: Our study presents the performance evaluation of the novel fast-scanning mode (1-minute) of a CEN-SMPS and investigates its suitability/applicability to measure UFPs in its scan range (10-237 nm). The reference SMPS (TSI-3938, CPC-TSI-3755) system was compared against a reference CPC (TSI-3750) measuring the total particle number (TPN). Another reference CPC (TSI-3750) was used in combination with catalytic stripper (CS) to simulate the solid particle number (SPN). Therefore, soot particles with varying parameters like particle number (PN), PNSD, fuels to gas ratio in soot generator (λ), and organic load (OL), were generated using a Mini-CAST (5303C) and aerosol conditioning facility of PTB, Germany. Furthermore, the impact of varying λ and PNSD on organic fractions of soot particles was evaluated through SPN / TPN, and finally a shrinking ratio was estimated based on ratio of GMD (Solid particles) to GMD (Total Particles) i.e., SGMD/TGMD.

Results & Discussion: The figure below presents our first findings for described metric, only λ = 0.9 (lean conditions) is shown here. Similar analyses were performed for λ = 1.3 (moderate), and 1.8 (fat), respectively. The TPN, SPN measured by reference CPCs and simultaneous cumulative particle number measured by reference SMPS (CS-, CS+) were in good agreement (within ±10%). Additionally, the SPN/TPN and SGMD/TGMD ratios decreased with decrease in the GMD, suggesting either a higher organic fraction or higher diffusion losses in CS at lower GMD.

References: 

[1]  https://doi.org/10.1016/j.envint.2023.107744 

[2]  https://dx.doi.org/10.3390/catal9070586