## 1 Introduction  
The implementation of the vehicle emission regulations in Europe--commonly known as Euro standards--has resulted in a reduction in the emissions of a number of air pollutants from the road transport sector [1; 2; 3]. However, despite this positive development, additional efforts are being discussed to counteract the effects of certain trends (such as increases in population and in economic output) currently eroding the beneficial effects of the aforementioned regulations [1]. In order to meet the Euro VI NOx emission standards, heavy-duty (HD) Diesel vehicles have been equipped with Selective Catalytic Systems (SCRs), and HD compressed natural gas (CNG) vehicles use three-way catalytic converters (TWCs). While reducing the emissions of NOx, the use of SCRs and TWCs can lead to substantial emissions of other two pollutants, ammonia (NH\({}_{3}\)) and nitrous oxide (N\({}_{2}\)O). As a result, the concentrations of NH\({}_{3}\) and/or N\({}_{2}\)O measured from the exhaust of modern vehicles can be higher than those of NOx under some operative conditions [4; 5]. N\({}_{2}\)O is a long-lived greenhouse gas (GHG) with an atmospheric lifetime of 121 years and a 100-year time horizon global warming potential (GWP) of 298 CO\({}_{2}\) equivalent [6]. Moreover, it has been indicated that N\({}_{2}\)O released in the atmosphere from anthropogenic sources is the single most important ozone-depleting substance (ODS) [7]. NH\({}_{3}\), on the other hand, is a pollutant that leads to the formation of ammonium nitrate and ammonium sulfate [8]. It has been widely reported that the deposition of these ammonium salts leads to eutrophication of waters and acidification of soils, negatively affecting nitrogen-containing ecosystems [9; 10; 11]. It is important to mention that, according to the last SOER report of EEA [12], anthropogenic-induced alterations to the N-cycle are much more significant than those to the C-cycle, with there being significant concerns for their impact on biodiversity and ecosystems in general. Moreover, ammonium salts are the most abundant atmospheric secondary inorganic aerosols in many regions [13], and they deteriorate urban air quality [14], affect human health [15], and impact the global radiation budget [16]. The HD Euro VI Regulation (EC) No 595/2009 [17] prescribes emission limits of NOx, CO, PN, PM, NMHC and CH\({}_{4}\) from positive ignition engines (THC in cases of compression ignition engines). It introduced the in-use emission test using portable emissions measurement systems (PEMS) at type approval, and PEMS-based in-service conformity (ISC) verifications. The ISC test is carried out on-road and, since the introduction of the Euro VI Step E, includes the measurement of all the pollutants regulated in the laboratory, the only exception being NH\({}_{3}\). The U.S. Environmental Protection Agency (EPA) developed a GHG emissions program, under the Clean Air Act [18], that includes N\({}_{2}\)O emission standards of 0.10 g/php-h (i.e., 0.133 g/kWh). The measurements are conducted in the laboratory over the Heavy-Duty Engine FTP cycle. Therefore, although the European Union (EU) and the US EPA have developed standards to limit the emissions of NH\({}_{3}\) or N\({}_{2}\)O from HD vehicles, the release of these pollutants during real-world operation is not verified or regulated. The main reason for this was the absence of dedicated portable systems that allow for reliable measurements of these compounds under real-world driving conditions when the regulations were developed. A similar situation is experienced for light-duty vehicles, which face the same technical challenges. In addition, for these vehicles currently, there is no limit for NH\({}_{3}\) emissions. Previous studies have shown that on-road emissions of NH\({}_{3}\) and N\({}_{2}\)O can be reliably measured in HD Diesel vehicles using laboratory grade quantum cascade laser based analysers (QCL-IR) [19; 20].