A practical uncertainty budget for ambient mercury vapour measurement

Measurements of mercury in ambient air are assuming greater importance, because of increasing health concerns and European legislation. In order to have confidence in measured values and to assess compliance with target values, measurements must be made with validated methodologies which have traceable uncertainty statements associated with them. This paper presents a practical uncertainty budget for the measurement of vapour-phase mercury in ambient air, sampling onto a gold-coated silica adsorption tube and measuring with atomic fluorescence spectrometry. Moreover, this budget may be generalised for other related measurement methods for mercury vapour and other ambient air pollutants. All significant sources of uncertainty are discussed and estimated. Expanded relative uncertainties at the 95% confidence interval of approximately 17% are estimated for exemplar measurements made by the National Physical Laboratory (NPL) as part of the UK Heavy Metals Monitoring Network.