State of UK emissions monitoring of stacks and flues: an evaluation of proficiency testing data for SO2, NO and particulates

We report an examination of the UK stack testing industry’s proficiency for monitoring industrial emissions of SO2, NO and particulates from 2000 to 2011. Data were taken from three proficiency testing schemes run by the National Physical Laboratory (NPL), UK; Calibration Gas Scheme (gas bottle certified reference materials), Gas Measurement Scheme (using a Stack Simulator Facility to test the entire measurement system) and Particulate Scheme (foil shims and salt solutions—i.e., filter and probe washing simulants). In each round of each scheme, participants’ deviations from assigned value were normalised to an allowable deviation based on the required uncertainty for stack emission measurements stipulated in the European Union’s Industrial Emissions Directive. This normalisation produced a z-score and limits were set to define satisfactory, warning and unsatisfactory participant performance. As a function of time, it was found that across all schemes, the number of unsatisfactory/outlier scores decreased, evidencing an overall improvement in industry proficiency. With regard to the gas schemes, it was found that the industry had a poorer proficiency for SO2 than NO and that there was a distribution bias toward negative scores in the Gas Measurement scheme consistent with SO2 sample losses in drying units. It was evident that this industry bias was insufficient to force the vast majority of the industry outside of the satisfactory z-score limits; however, it was noted that this issue should be carefully monitored in the future.