Preparation of standard VOC mixtures for climate monitoring

The growing need for global volatile organic compounds (VOC) data at trace level for industrial applications, domestic and working health, air quality and climate change is driving research towards the preparation of traceable and accurate standards for the VOC amount of substance fraction in gas phase.The World Meteorological Organization (WMO) has implemented a worldwide VOC monitoring programme, named Global Atmosphere Watch (GAW), which data quality objectives refer to an expanded uncertainty lower than 10% for VOC measurement at amount of substance fractions lower than 100 nmol∙mol−1.Currently, gaseous standard atmospheres are prepared in high pressure gas cylinders, but they do not fulfil GAW requirements of accuracy and stability at ppb level, especially for oxygenated VOCs. Dynamic methods are proposed as an alternative to provide a stable amount of substance fraction of the mixture over time. Diffusion rate, dilution and carrier air flow rates, purity of air, VOC losses due to wall-interactions and leakages were considered as influence quantities of the VOC amount of substance fraction dynamically prepared in gas phase by the diffusion method. The contribution of the VOC losses to standard mixtures uncertainty and their sampling procedure are under study and a solution has not yet been proposed.In this paper, a conservative quantification of VOC losses is proposed, discussed and applied to a case study: a single stage dynamic dilutor with an acetone generation. An uncertainty budget was prepared to calculate target uncertainty for VOC losses due to leakages and VOC–wall interactions on Teflon®, Pyrex® and stainless steel tubing. Teflon® showed negligible interaction effects when the pipe was clean and long term effects were not considered. Uncertainty was not negligible for glass and stainless steel for which more accurate analyses are necessary. A leakage test was developed to assure negligible leakage contribution to uncertainty.