## 1 Introduction  
Hydrogen chloride (HCI) is a harmful gas which is released to the atmosphere, e.g., from fossil fuel power plants or waste incinerators. It is also a process gas in many industries such as the electronic or textile industry. Industrial processes are often large-scale point sources with gas abatement systems which can be optimized to reduce the emissions. In the EU, HCl emissions from the industry are covered by Industrial Emissions Directive 2010/75/EU. The HCl emission measurement, as described in EN-standard 1911: 2010, does not fully extend down to concentration levels consistent with the increasingly stringent Emission Limit Values (ELVs) that will come into force [1]. The EMPIR project IMPRESS2 [2] is carrying out technical development work as well as underpinning spectroscopy in support of improved, and metrology backed monitoring and enforcing of ELVs in future European legislation for key air pollutants (i.e., HCl) and regulating emissions (i.e., HCl should be less than 1 umol/mol). The chlorine present in the fuels of combustion and gasification plants, appears in the slag [3] in biomass gasifiers but can also react to gas phase HCl. HCl, as a highly corrosive gas, influences the choice of the steel quality of the gasifier body material as well as the subsequent gas cleaning system to increase the plant lifetime, which is leading to considerable additional costs [4]. The content of HCl in the syngas of the biomass gasification must remain at ppb (10\({}^{-9}\) mol/mol, amount fraction) level in order to avoid catalyst poisoning [5]. Fast, non-invasive, highly sensitive and accurate HCl monitoring in the gasification process is important for active process control and optimization [6] and for ensuring staying below the legal emission values. Airborne molecular contamination (AMC) is a kind of chemical contamination in form of gases, vapors or aerosols that has adverse effects on products, processes or instruments in clean room manufacturing environments. The International Technology Roadmap for Semiconductors (ITRS) has made efforts to tighten specification for AMCs [7]. For example, the requirement of Cl\({}^{-}\) concentration in the wafer environment should be below 0.24 nmol/mol of gas phase Cl\({}^{-}\) concentration, as the acid gas HCl creates metal corrosion problems throughout fabrication [8] and can be critical to later damage the normal functionality of integrated circuits. AMC monitoring, and therefore reduction, has a direct financial benefit, through higher product yields and better process reproducibility from site to site. Within the EMPIR MetAMC2 project [9], optical techniques will be developed for trace level HCl detection in cleanrooms. Chemical reactions involving halogen radical species, such as chlorine, significantly influence the composition of the Earth's atmosphere. These species also play important roles in many atmospheric processes including the formation and destruction of ozone [10]. Chlorine also mainly comes from chlorofluorocarbons (CFC) and other refrigerants which are inert in the lower atmosphere and are then broken up and converted in to HCl in the upper atmosphere by the UV radiation. HCl is one of the important reservoir species for active halogens and the dominant gas phase contributor to the overall chlorine inventory in the lower atmosphere, as shown in the study of composite chlorine emissions [11]. Typical HCl concentrations are in the range of hundreds of ppt in remote marine environments, increasing to several ppb in coastal urban areas [11; 12; 13]. There is a strong need for highly sensitive measurement techniques for HCl due to estimations of HCl production varying considerably and challenging existing measurement methods. The knowledge of accurate spectral line parameters (such as pressure broadening coefficients) of HCl [14; 15; 16] is important and required for HCl measurements in atmospheric remote sensing, biomass gasification process, semiconductor manufactory cleanroom and precision industrial emission monitoring application.