Long-term, accurate measurements of volatile organic compounds (VOCs) are crucial to understanding the chemistry of the atmosphere and to addressing the effects of climate change. For the monitoring of background levels of oxygen-containing volatile organic compounds in the atmosphere (e.g., alcohols, aldehydes, ketones) , gas reference materials are required that enable the calibration of measurement equipment at amount fractions typically at nmol mol-1 levels or below. Measurement standards and reference materials containing ethanol, methanol and acetone have been developed at 5 µmol mol-1 by the national metrology institutes of the United Kingdom and the Netherlands. Equivalence has been demonstrated in a bilateral comparison. In this contribution, we show how this milestone was achieved and how the effects of, e.g., cylinder wall adsorption have been considered. The assignment of composition values and uncertainties to the amount fractions of methanol, ethanol and acetone requires a good understanding of the adsorption behaviour and the use of a second, primary method. This work has provided the foundation for a new key comparison coordinated by VSL within the CCQM-GAWG (Gas analysis working group of the Consultative Committee for Amount of Substance: Metrology in Chemistry and Biology) targeting considerably lower amount fractions (0.1 to 1 µmol mol-1) for ethanol, methanol and acetone. As a reference, a dynamic gravimetric method will be used based on diffusion according to ISO 6145-8. There is an established track record with this system at VSL. In a first step to fully characterise the uncertainty associated with the amount fractions of methanol, ethanol and acetone, the diffusion rate of the said components has been reassessed, and it has been checked whether this rate depends on, e.g., the level of filling of the cells. The design of this key comparison is based on a longstanding tradition of key comparisons run against a dynamic primary reference method, where using static gravimetry meets its limitations due to, e.g., reactivity or adsorption effects. The ultimate goal to serve the WMO community is to create a reference gas standard and gaseous reference materials at the nmol mol-1 level, which is at least two orders of magnitude lower than contemplated for the ongoing key comparison. Based on the current state-of-the-art, the key challenges are to improve sampling systems for instrumental analysis and gas cylinder passivation techniques to reduce the response time and the adsorption losses. These improvements will enable supplying reference materials with the required uncertainty and, moreover, stability of composition in time.
Topic : Theme 1: Atmospheric Chemistry and Physics.
Reference : T1-A18
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