Comment on ar-2025-27

<strong class="journal-contentHeaderColor">Abstract.</strong> 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 &ndash; including determination of the diameter with 50 % detection efficiency d₅₀ &ndash; 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.