| 29 Sep 2025
Results and insights from the first ACTRIS intercomparison workshop on sub-10 nm aerosol sizing instrumentation
Abstract. Sub-10 nm aerosol particles play a critical role in the atmosphere due to their involvement in new particle formation - a key process influencing cloud condensation nuclei availability. In recognition of their significance, international observation frameworks such as the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS) are in the process of integrating their observation into standardised protocols. ACTRIS aims to enhance the understanding of atmospheric processes including those related to particle dynamics and their interactions with clouds and climate through harmonized long-term observations. To evaluate the performance of current instrumentation for sub-10 nm particle measurements, as well as the procedures for doing so, the ACTRIS Cluster Calibration Center in Helsinki, Finland, held its first workshop on the intercomparison of instrumentation for sub-10 nm aerosol particle number size distribution measurements in November 2023.
In this workshop, three mobility-based systems (GRIMM PSMPS, TSI 1 nm SMPS and TSI 3938N56) and five activation-based systems (three Airmodus A11 and two Airmodus A12) were evaluated. The focus lay on assessing their number concentration response and size dependent detection efficiency – including determination of the diameter with 50 % detection efficiency d50 – and sizing accuracy. In addition to these parameters, the instruments were compared with each other while measuring aerosol particle number size distributions side-by-side from an aerosol chamber. Beyond instrument performance evaluation, the workshop aimed to test and assess the calibration and comparison methods to identify where further refinement is needed to support ACTRIS compliance.
While this work highlights key strengths of the different measurement techniques and instruments, several challenges remain. Mobility-based systems showed high sizing accuracy especially for particles larger than 2 nm, while encountering challenges in measuring particles in the lower atmospheric concentration range. Activation-based systems proved more sensitive at lower particle number concentrations and particle sizes with the drawback of slight unit-to-unit variability. Additionally, a systematic size shift was identified in aerosols generated by 4-way-cross glowing wire generator setups, indicating a need for further investigation of this effect and development of the calibration equipment in the sub-10 nm size range.
Competing interests: Herbert Hartl, Joonas Vanhanden and Joonas Purén are employed by Airmodus Oy. Gerhard Steiner is employed by Grimm Aerosol Technik GmbH. Amine Koched and Sebastian Schmitt are employed by TSI GmbH. The latter is also on the editorial board of the Aerosol Research journal.
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Hartl et al. report results from a workshop for measuring the number size distribution of sub-10 nm particles. The study intercompares instruments with different measurement principles. A chamber experiment was used to evaluate the performance. Good agreement was obtained for measurements at high number concentration. Some instruments performed less well at low number concentration.
This is an excellent and timely manuscript. Measuring sub-10 nm particles are receiving increasing interest due their importance for understanding new particle formation and health effects. The study is well designed and I recommend the manuscript to be published. I have a few comments below for the author’s consideration.
Major comments:
Although the manuscript touches on various inverse methods, uncertainties from inversions are not fully discussed. For mobility spectrometers, there are significant questions regarding the charging efficiency of sub-10 nm particles and the treatment of measurement noise. It is not clear if all MPSS used the same inversion or custom inversions, which may result in additional uncertainty just from the inversion method (Wiedensohler et al., 2012). It would be good to investigate if the high noise flow of M1, M2, and perhaps M3 is due to amplification of rogue counts or due to some treatment in the inversion. A section on “inversion uncertainties” would be much appreciated.
There is limited focus on the dependence of chemical composition. Particles other than WOx are needed to gauge the variance of performance under field conditions. Also charge is mentioned to have a potential effect on activation/particle detection (“and it is also assumed that negatively charged particles of the same composition achieve higher activation efficiencies at small sizes, regardless of the CPCs use”). A discussion section of “composition and charging uncertainties” would also be appreciated, if only to outline what types of experiments are recommended for the next workshop.
A workshop in ice nucleation instrumentation (DeMott et al., 2018) used a blind experiment design where instrument owners were not told the composition, size, or concentration of particles sampled. Instrument operators would setup/calibrate the instrument to the best of their ability, perform the measurement, and submit their answer. Although this is sometimes challenging to do in a collaborative workshop setting, this activity tests uncertainties that might be present under field conditions. The authors might want to comment on the extent to which the experiments were blind and/or make recommendations for future workshops about the feasibility to conduct such a blind intercomparison.
Other comments
Measurable coincidence starts to play a role at concentrations of the order of 103 cm−3 -> This depends strongly on the sample flow rate through the optical. It is my understanding that some nanoCPCs, e.g. TSI3776 and newer models reduced this flow substantially, thus pushing this value much higher. Also, later it is stated “at particle number concentrations approaching or exceeding 107 cm−3, these systems become decreasingly precise due to limitations in single-particle counting and potential coincidence errors”, which contradicts the previous statement.
“For the calibration of the Half-Mini DMA, Tetraheptyl Ammonium Bromide (THAB) ions were generated using electrospray. In this context, term calibration refers to establishing the DMA’s voltage-to-particle diameter relationship by using well-characterized ions of known electrical mobility as references. THAB ions serve as mobility standards that allow comparison between detected ion peaks and theoretical mobility values, as described by Ude and Fernández de la Mora (2005)” -> It might be helpful to state why such a calibration is needed. The design of the traditional DMA is that of a primary calibration standard. They are not calibrated, but size is fully predicted by the sheath flow rate, DMA dimensions, and applied voltage. The sizing is typically tested and if it doesn’t agree, then the causes can be investigated. Why is this not the same for the half-mini DMA?
nano meter -> nanometer
References
DeMott, P. J., et al.: The Fifth International Workshop on Ice Nucleation phase 2 (FIN-02): laboratory intercomparison of ice nucleation measurements, Atmos. Meas. Tech., 11, 6231–6257, https://doi.org/10.5194/amt-11-6231-2018, 2018.
Wiedensohler et al..: Mobility particle size spectrometers: harmonization of technical standards and data structure to facilitate high quality long-term observations of atmospheric particle number size distributions, Atmos. Meas. Tech., 5, 657–685, https://doi.org/10.5194/amt-5-657-2012, 2012.