Stack simulators for sampling uncertainties The overall aim of this work package is to initially review existing data to provide some broader context before moving onto the larger part of the work package which is to acquire data on Stack Simulator Facilities at INERIS and NPL, as well as the flow rig at CMI and to go forward with uncertainty analysis in Task 2.2. Task 1.1 will review historical and recent studies of HCl acidification and summarise how the understanding of the impact of this important pollutant has changed. In addition, it will determine what can be learnt from existing data in the application of the HCl SRM (EN 1911), both in terms of any biases that have emerged through real field experience of using this method and from audits carried out witnessing implementation practices of stack testing organisations. Task 1.2 will extend the capability of Stack Simulators based at INERIS and NPL down to at least 10 mg.m-3 HCl, mirroring emissions at industries regulated via BAT Conclusions documents, and adopted under the Industrial Emissions Directive (IED 2010/75/EU). Furthermore, this extended capability will be used to carry out an ILC of 10 stack testing organisations implementing EN 1911 and a comparison of the 10 (5 pairs) of portable optical technologies. It will be geared towards generating data under emission conditions found at industries regulated via BAT Conclusions documents. Task 1.3 will complement the above with work on measurement uncertainty caused by flow, namely, in terms of both flow calibration and swirling flow in stacks affecting droplet distribution and the impact this has on isokinetic sampling.