Establishment of specific calibration procedures for MPSS for ambient measurements and the provision of calibration facilities for CPCs to the CEN standard FprCEN/TS 16976 The overall goal of this work package is to substantially improve the quality of number-based aerosol measurements in order to meet regulatory needs and to make environmental aerosol data more reliable and comparable. Therefore the aims of this work package are twofold: a) to develop calibration procedures for MPSS in the size range up to 1000 nm, in support of the standardisation requirements provided by ISO TC 24 WG 12 and CEN TC 264 WG 32 and to provide SI-traceable calibration facilities (Task 3.1 and Task 3.3) b) to provide SI-traceable calibration facilities for CPC a) for the measurement of particle number concentration in ambient air (according to the standard FprCEN/TS 16976) (Task 3.2 and Task 3.3). The standardisation for MPSS is currently under development within CEN TC 264 WG 32. In this context, ISO 15900 and ISO 27891 will be applied in combination. Currently standards for MPSS and CPC quality assurance and calibration within Europe are not harmonised among the atmospheric research community and NMIs. Only MPSS employed at atmospheric observatories within the European Research Infrastructure ACTRIS and the WMO-GAW programmes are quality-assured at the World Calibration Centre Aerosol Physics (WCCAP) at TROPOS. This done with regular based instrumental calibration intercomparison workshops (ca. 5 calibration workshops per year) and following specific ACTRIS- and WMO recommendations (see also WMO-GAW report 227; 2016). However, there aren’t currently any official calibration facilities that can be used to make these quality assurance measures SI-traceable as (e.g. EUR6 automotive exhaust). Task 3.1: Calibration procedures of MPSS and traceability of particle number size distributions The aim of the task is to collect, evaluate, and summarise the state of art of MPSS calibration procedures. The current calibration procedures used for the quality assurance in ACTRIS and GAW will be the starting point to define certificated particle size standards and reference instruments for MPSS. Existing conventions on constants and formulas needed for the inversion routines (ISO 15900) will be considered as well as additional parameterisation such as new bipolar diffusion charger based on soft x-ray. Regular feedbacks to standardisation committees (CEN & ISO) and observational networks (ACTRIS & GAW) will enable a continuous improvement and the output of the task will be harmonised calibration procedures. Task 3.2: Provision and implementation of calibration facilities for CPCs The aim of this task is to provide and implement calibration facilities to certify the performance of CPCs following the CEN standard FprCEN/TS 16976. The calibration procedure for CPCs following the standard FprCEN/TS 16976 needs clarification/improvement with respect to losses of small particles below 10 nm in aerosol electrometers used as references. A traceable correction procedure for these losses will be developed. The lower total particle number concentration limit of the primary reference method (FCAE) is typically above 1000 particles/cm³. Further reduction of this limit is desirable in order to increase the concentration range of the CPC linearity test (i.e. deviation of CPC concentration reading from the reference value). This task will finally provide and implement calibration facilities to certify the performance of CPCs following the CEN standard FprCEN/TS 16976. Task 3.3: Reference instruments intercomparisons workshop The aim of this task is to conduct a workshop for reference aerosol electrometers and reference CPCs The results of the instrumental intercomparison workshop will form the base for: a) Provision of calibration facilities for CPCs following the standard FprCEN/TS 16976 b) Development of a correction for aerosol electrometer measurements for very small particles with low fraction of electrically charged particles c) Definition of realistic uncertainty ranges for MPSS and CPCs used for atmospheric measurements.