Preparation of standard VOC mixtures for climate monitoring

European legislation regulates ambient concentrations of VOC key compounds relevant for indoor air monitoring and for air quality and climate monitoring programmes, e.g., the Global Atmosphere Watch (GAW) programme of the WMO. Data Quality Objectives refer to an expanded uncertainty lower than 10% for VOC measurement at concentration lower than 100 nmol/mol. Traceable and accurate standards and standardised dilution methods for VOC concentration are thus essential for the reliability and harmonisation of the measurement data provided. International research projects are ongoing to develop dynamic dilution systems from several sources (static cylinders and dynamic generation) to prepare standards at 1 - 100 nmol/mol VOC concentration with an uncertainty < 2-7 %. The uncertainty sources for VOC mixtures prepared by a single stage dynamic dilutor are here analyzed. Dilution flow rate, purity of the dilution gas, VOC losses and concentration variability due to imperfect gas mixing, wall-adsorption effects and leaks were considered as influence quantities. The dynamic dilutor performance was described by material balances on which a uncertainty budget was redacted. Mixing performances, wall interactions and leak tests were considered to design the dilutor. Design criteria for the dynamic dilutor and design specification for the air purification system were developed. This work contribute to develop standardized dilution methods for a better calibration of monitoring GAW instruments, it will helps to reduce uncertainty in climate change forecasting models.