###### Abstract  
Pollution from vehicles is a serious concern for the environment and human health. Vehicle emission regulations worldwide have limits for pollutants such as hydrocarbons, CO, and NOx. The measurements are typically conducted at engine dynamometers (heavy-duty engines) sampling from the tailpipe or at chassis dynamometers (light-duty vehicles) sampling from the dilution tunnel. The latest regulations focused on the actual emissions of the vehicles on the road. Greenhouse gases (GHG) (such as CO\({}_{2}\), CH\({}_{4}\), N\({}_{2}\)O), and NH\({}_{3}\) have also been the subject of some regulations. One instrument that can measure many gaseous compounds simultaneously is the Fourier transform infrared (FTIR) spectrometer. In this review the studies that assessed FTIR since the 1980s are summarized. Studies with calibration gases or vehicle exhaust gas in comparison with well-established techniques were included. The main conclusion is that FTIRs, even when used at the tailpipe and not at the dilution tunnel, provide comparable results with other well-established techniques for CO\({}_{2}\), CO, NOx, while for hydrocarbons, higher deviations were noticed. The introduction of FTIRs in the regulation needs a careful description of the technical requirements, especially interference tests. Although the limited results of prototype portable FTIRs for on-road measurement are promising, their performance at the wide range of environmental conditions (temperature, pressure, vibrations) needs further studies. F 2021 11 7416 11.11.  
that allows the collection of a broad range of wavelengths. By contrast, NDIR analyzers measure one compound due to the use of an optical filter that allows the selection of a narrow wavelength area, specific to the compound of interest. FTIR is used in geology, chemistry, materials, medicine, and biology research fields on solid, liquid, and gaseous samples [4]. FTIR has been used in a wide range of air pollution-related studies in both ambient air and environmental chambers [5; 6]. Already in the late 1970s, an FTIR system was installed in a van for air pollution measurements for the Environmental Protection Agency (EPA) of the United States of America (USA) [7; 8]. FTIR has also been applied for aerosol analysis [9; 10], for example, to assess the hygroscopicity of ambient particles [11] or analyze surface functional groups of particles [12]. Another application was the measurement of stack emissions from various processes, such as incinerators [13]. Other researchers have used open path FTIR to record CO, CO\({}_{2}\), and N\({}_{2}\)O [14] or NH\({}_{3}\)[15] emissions from high traffic roadside sites or even emissions in aircraft exhaust [16]. Extractive in cell FTIR has been used in many applications, e.g., trains [17]. Extractive measurement with an activated charcoal tube was used to measure volatile organic compounds (VOC) of a heavy-duty engine [18]. Applications in atmospheric and environmental studies were reviewed elsewhere [19; 20]. The application of FTIR spectroscopy to vehicles' exhaust analysis began in the early 1980s [21; 22; 23]. It received a lot of attention for the study of alternative fuels due to its ability to discriminate oxidized species in tailpipe gases [24; 25; 26; 27]. Other studies focused on TWC (three-way catalysts) [28; 29; 30] and low ambient temperatures [27; 29]. Later FTIR was used to measure emissions from NOx reduction systems [31] due to its ability to simultaneously measure separately the oxides of nitrogen [27; 32; 33]. However, the technique did not spread out in the industry due to several practical problems, such as complexity of calibration (due to cross sensitivity of, e.g., CO\({}_{2}\) and H\({}_{2}\)O), slow response, and poor concentration accuracy when compared to the regulated analytical techniques [31; 34]. The evolution of computers and algorithms made it possible to have an instrument that can measure and analyze the data in real time. Since then, and especially in the last decade, the use of FTIR spectrometers has widespread.