The Particulate Matter (PM) Measurement work package (WP4), was undertaken with three aims namely: 1) to understand uncertainty in current CAEP10/11 nvPM regulatory practices (D4.1) 2) to investigate the requirement of potential corrections to be considered towards reduced uncertainty during CAEP/12 (D4.2) and 3) to assess likely benefits future technologies and further regulation will offer in terms of reducing the impact of nvPM beyond CAEP/12 (D4.3). To achieve the aims of D4.1 & 4.2, aero-engine relevant RQL combustor rig tests were performed at Cardiff University’s Gas Turbine Research Centre (GTRC) in Dec 2020, Feb 2021 and Dec 2021 in addition a laboratory scale fuel assessment was performed by ONERA using a liquid-fuel CAST. Towards informing the relevant technical committees (SAE E31 & ICAO WG3) an assessment of currently unquantified uncertainties, namely system variability and drift, limits of detection/quantification (LOD/LOQ) of mass analysers and calibration methodologies was undertaken to understand relative differences in two ICAO Annex 16 compliant systems over a calibration cycle. This assessment was performed by the European (EUR) and Swiss (CH) reference nvPM sampling and measurement systems, which made regulatory compliant measurements in parallel, initially following joint calibration and then after a further 12-month period. During the campaigns > 250 discrete test-points were performed across various power conditions, using seven fuels of varying hydrogen content (aromatic content), which afforded a wide range of number and mass concentrations and particle size distributions representative of large civil aviation gas turbines. It was seen that on average the two systems agreed within expected uncertainty levels, with mass comparisons exhibiting an 11% offset and number agreeing within 2% on average immediately following calibration. After a 12-month period, without recalibration, a better agreement of 3% for mass and 0.2% for number were observed. It is proposed that these observed reductions likely stem from improved protocols in rig operation, sampling and cleanliness checks for the final campaign, which were adopted as an outcome of lessons learned during the initial tests which highlighted discrepancies significantly increased at low mass concentrations indicating shedding of particles from the cyclone. In both comparison campaigns intra-comparisons within individual systems highlighted good agreement between different instrument types for both mass and number. VPR penetration data collected on aero-engine representative soot was seen to closely match that reported on the ICAO compliant calibration certification, offering confidence that current methodologies employing a diffusion flame calibration source are robust. In terms of CAEP/12 improvements and corrections, intensive size measurement were performed towards understanding system loss correction. Size instruments observed good agreement between the three analyser types compared (SMPS, DMS-500 & EEPS) with particle size and shape agreeing within 6% and 4% of the mean for GMD and GSD respectively. It was shown that size measurement potentially offers a more rigorous opportunity for loss correction compared to the current ICAO method (also described in ARP 6481), particularly when the measured nvPM mass is near LOD/LOQ and for distributions which are neither lognormal or monomodal. It was demonstrated that with loss correction methodology, correlation could be found between regulatory and size derived number concentrations. In terms of fuel correction, it was observed that changes in sulphur and aromatic content, within the ranges permitted in ASTM, resulted in differences in reported nvPM mass and number, with data generated on the RQL rig supporting the requirement for a CAEP/12 fuel correction methodology based on fuel hydrogen content. Additionally, humidity studies highlighted competing influence dependant on humidity injection location. Reductions in nvPM were observed at elevated primary flame zone humidity but conversely increased nvPM correlated with increased humidity in the secondary zone leading to inconclusive results of the net impact humidity plays in a full Rich burn engine. Desk based review highlighted that currently unregulated engines (<26.7kN) emit nvPM which is similar in nature to those regulated, with an assessment of future technologies and fuels indicating that future aircraft will likely emit significantly lower nvPM concentrations than those of today. As a result it is proposed that future regulation will likely need to have greater sensitivity and be inclusive of volatile PM emanating from both the exhaust and oil lubrication system in order to ensure protection of LAQ surrounding future airports.