## 4 Discussion
### On-Board Applications  
In the previous paragraphs, it was shown that tailpipe application is possible, paying attention to analytical and sampling interferences. Are FTIRs robust enough for on-board applications? Portable systems were already available in the 1980s. However, portable does not necessarily mean suitable for on-board measurements. The main concerns are:  
* Size, weight, power consumption.
* Effect of environmental conditions (temperature, altitude, vibrations).
* Safety (liquid N\({}_{2}\) for cooling, other gases on-board, e.g., N\({}_{2}\) for purging).  
The importance of size, weight, and power consumption is different for light-duty and heavy-duty applications. The size and weight are very important for light-duty applications, especially for small city cars. Commercial portable FTIRs are split into units that can fit in the vehicle cabin and/or on the hook. The weight without accessories (e.g., pumps, batteries, and heated lines) is around 50 kg, but including accessories is around 100 kg, which is slightly heavier compared to commercial PEMS (portable emissions measurement systems). Even though in the 1980s the need for power generators of 10 kW was reported [186], today's portable systems are <0.5 kW (after warming up in the laboratory). These values are still higher or comparable to PEMS based on other principles. An important point for energy consumption is the location of the sampling pump. If it is located upstream of the FTIR, it needs to be heated, thus the energy consumption will be higher compared to a downstream location. Furthermore, at the downstream location, the pressure can be lower than atmospheric pressure, which is an advantage for on-board measurement because it can be maintained and fixed easier. The liquid N\({}_{2}\) on-board is a concern. One solution is keeping the liquid N\({}_{2}\) in a sealed container, with only one small tube connected, venting the evaporating N\({}_{2}\) to the atmosphere or to the rest of the exhaust gases from the FTIR pump.  
The effect of the environmental conditions should be well characterized. FTIRs are sensitive to vibrations: the better the resolution is, the longer the displacement of the moving mirror in the interferometer, and the higher the effect of the vibrations on the optical system. Vibration tests in the late 1990s concluded that FTIRs were best isolated by simply placing them on the rear seat of the vehicle [86]. Today FTIR suppliers claim that their systems are vibrations robust. For example, wire rope isolators can be used [187]. Static single mirror solutions have also been presented [188]. Experiments with NOPEMS (based on CLA or NDUV) showed that sudden temperature changes resulted in zero drift [189]. FTIR results are also sensitive to temperature. The recent CEN standard on PEMS performance prescribes appropriate testing procedures to properly assess the influence of temperature, pressure, and vibrations [190]. Long-term stability and robustness due to vibrations, contamination of optics, etc., should also be assessed.  
As with all portable systems, comparison with laboratory versions or other well-established techniques are necessary to increase their confidence in them. Sometimes portable systems might not have the appropriate spectral resolution, response time, or detection limits [104]. For example, in the past, drying of the sample has been used [86], slow response (30 s) [78], or the low optical resolution of 4 cm\({}^{-1}\)[110] might have negatively affected the accuracy of the results. It should also be emphasized that the studies in Appendix A were with prototype portable systems, thus further studies with commercial systems are needed.