Tompkins, J T; Allerton, J J; Williams, K R; Butterfield, D M; Brown, A S (2025) Particle size comparison between TSI 3936 scanning mobility particle sizer and TSI 3938W50-CEN scanning mobility particle sizer conducted at NPL and London Marylebone Road monitoring sites. NPL Report. ENV 62

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Abstract

A new TSI 3938W50-CEN-7 scanning mobility particle sizer (SMPS) system was purchased and trialled in 2022 for use across the Particle Concentrations and Numbers Network in 2023. This new model of SMPS had a larger size range (nominally 10 - 800 nm) than the older model TSI 3936 SMPS (nominally 16 - 600 nm). In July and August 2022, a validation campaign took place at NPL (Teddington) and London Marylebone Road for the new TSI 3938W50-CEN-7 SMPS, consisting of two parts.

Firstly, laboratory tests were performed at NPL using ambient air to: (a) confirm that the new SMPS instruments could operate continuously when installed at site and, (b) co-locate the old TSI 3936 SMPS and new TSI 3938W50-CEN-7 SMPS models to challenge the instruments with “urban background” ambient air to check for any differences in the particle size distributions between the two instruments. Secondly, tests at London Marylebone Road, an “urban traffic” site, to: (a) confirm that the new SMPS instruments could operate continuously when installed at a roadside site and (b) co-locate the old and new SMPS models to challenge the instruments with roadside air and check for any differences in the particle size distributions between the two instruments.

The TSI 3938W50-CEN-7 SMPS was shown to operate successfully over a month at both NPL and London Marylebone Road sites. The size distribution comparison at NPL showed a good correlation within the between the two SMPS systems meeting the ± 10 % bounds above a particle diameter of 50 nm and the ± 20 % bounds at particle diameters between 20 nm and 50 nm. The size distribution comparison at the London Marylebone Road site indicated a reasonable correlation (± 20 %) between the two SMPS systems for particle diameters of 20 nm and 50 nm. At both sites the two SMPS size distributions deviated by more than 20 % below a particle diameter of 20 nm, but this was to be expected due to the different D50 and condensation particle counter size cut offs of the two SMPS instruments.

Above a particle diameter of 50 nm at London Marylebone Road, a higher particle number concentration was measured by the TSI 3938W50-CEN-7 SMPS compared to the TSI 3936 SMPS. There is not a simple explanation for this difference, indeed the measurement of different particle number concentrations by different models of SMPS is being discussed by CEN TC264 WG32 . As this short study demonstrated that the two SMPS systems behaved differently when challenged with roadside aerosol composition, this shows that additional care must be taken when comparing results from different SMPS systems at roadside sites such as London Marylebone Road.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	ENV 62
Subjects:	Environmental Measurement > Air Quality and Airborne Particulates
Divisions:	Atmospheric Environmental Sciences
Identification number/DOI:	10.47120/npl.ENV62
Last Modified:	29 Aug 2025 13:04
URI:	https://eprintspublications.npl.co.uk/id/eprint/10221