## 2 Materials and Methods  
### Regulatory Background  
In the legislation of the EU, light-duty vehicles include passenger cars and light commercial vehicles, while heavy-duty vehicles include trucks and buses.  
Originally emission regulations for light-duty vehicles were introduced in the 1970s with Directive 70/220/EEC (only CO and HC) [1; 106]. In the years after, other pollutants were added, and various reductions of the emission limits were applied. In 2007 regulation 715/2007 defined Euro 5 and Euro 6 standards. The pollutants currently regulated are CO, HC (for positive ignition (PI) engines only), NO\({}_{x}\), HC + NO\({}_{x}\) (only for compression ignition (CI) engines), particulate matter (PM), and particle number (PN) (only for CI and PI direct injection engines). CO\({}_{2}\) has limits only as fleet-average. The measurements were conducted mostly from bags that collect a sample from a dilution tunnel with constant volume sampling (CVS), where the whole exhaust gas was diluted.  
In 2017, with Euro 6d-temp, additional on-road testing was added to the laboratory type approval and in-service conformity testing. Limits were defined only for PN and NO\({}_{x}\), while CO and CO\({}_{2}\) had to be measured. The limits were the laboratory type approval limits taking into account the additional measurement uncertainty of the on-board portable emissions measurement systems (PEMS) with temporary conformity factors [107; 108]. Since 2020, with Euro 6d the revised conformity factors were applicable. Regulation (EU) 2018/858 introduced a new EU type-approval framework (from September 2020), with an effective market surveillance system to control the conformity of vehicles already in circulation (new In-Service Conformity process from September 2019).  
Heavy-duty standards were originally introduced with Directive 88/77/EEC (applicable from 1992). Current Euro VI emission standards were introduced by Regulation 595/2009 followed by a number of amendments that specified technical details. Limits are applicable for CO, non-methane hydrocarbons (NMHC), CH\({}_{4}\) (for gas engines) NO\({}_{x}\), PM, PN, and NH\({}_{3}\). The measurements can be conducted from bags, directly from the full dilution tunnel, from a proportional partial flow system (PDFS), or directly from the tailpipe. Euro VI regulation introduced in-use testing with field measurement using PEMS. Conformity factors are applicable that take into account the additional measurement uncertainty of the PEMS.  
At a global level, GTR 15 (global technical regulation) for light-duty vehicles includes additional pollutants (only methodology, not limits), and the EU is highly likely to adopt most of them in future regulations. None of these pollutants, however, are prescribed as candidates for on-board testing. Table 1 summarizes the pollutants, their principle of measurement and their inclusion or not in the current EU regulations. Some of them were recently introduced in other countries (e.g., N\({}_{2}\)O in China 6, aldehydes in Brazil, Korea, USA) [109].  
Based on Table 1, FTIR is permitted to be used for the measurement of NH\({}_{3}\), N\({}_{2}\)O, C\({}_{2}\)H\({}_{5}\)OH, CH\({}_{2}\)O, and CH\({}_{3}\)CHO. However, with the current regulations, FTIR would have to be connected to the tailpipe for the measurement of NH\({}_{3}\), but at the dilution tunnel for the rest of the pollutants. As FTIR can determine simultaneously many of the regulated pollutants (CO\({}_{2}\), CO, NO, NO\({}_{2}\), CH\({}_{4}\)), and has the capabilities to be used at the tailpipe (e.g., for NH\({}_{3}\)), it is an attractive candidate for on-road testing.