# On the Issues of \(\mathbf{NO_{x}}\) as Greenhouse Gases: An Ongoing Discussion...
## 3 Global Warming Potential  
To estimate the impact of a particular substance on the GHE, the different climate metrics (coefficients) should be defined. One commonly used factor is global warming potential (GWP). According to the definition, \"GWP is based on the time-integrated radiative forcing due to a pulse emission of a unit mass of gas. It can be quoted as an absolute GWP (AGWP) (in units of Wm\({}^{-2}\) kg\({}^{-1}\) year), or as a dimensionless value by dividing the AGWP by the AGWP of a reference gas, normally CO\({}_{2}\)\" [33]. Furthermore, \"the relative (dimensionless) GWP signifies the cumulative radiative forcing relative to emission of the same mass of CO\({}_{2}\) within defined time horizons\" [38]. The time horizon is an important factor influencing the GWP value. Usually, the time horizon is defined as 100 years (commonly used in climate politics). However, in scientific analysis, three  
Figure 3: The division of GHGs including direct and indirect GHGs. Figure 2: Percentage of atmospheric absorption for radiation passing from the top of the atmosphere to the surface for different GHGs versus NO\({}_{\rm x}\) and the atmospheric window (based on [PERSON] [32]). different time horizons are suggested: 20, 100, and 500 years [33]. Thus, assuming such definitions, the GWP for CO\({}_{2}\) equals one. It should be stated that GWP can take a positive \"+\" or negative \"-\" value. A positive value indicates the considered compound influences global warming, while a negative value indicates an influence on global cooling. There are other factors for estimating the impact of gases on global warming such as the radiative forcing index (RFI) and global temperature change potential (GTP). However, GWP is the most popular and commonly used metric in climate politics, despite its limitations [33; 39]. Precise determination of GWP for NO\({}_{x}\) (as an overall value for all emission sources) is very difficult because in these gases, the GWP strongly depends on the type of source-grounded (surface), airborne, transportation, or from immobile sources. Furthermore, GWP can be affected by geographical location and time horizons [33; 45]. Moreover, the GWP for NO\({}_{x}\) varies substantially, according to the literature. For example, [PERSON] and [PERSON] [38] estimated that the GWP for NO\({}_{x}\) from surface sources (time horizons of 20 and 100 years) were in the range of GWP\({}_{20}\) = 30-33, and GWP\({}_{100}\) = 7-10. [PERSON] and [PERSON] determined [50] GWP\({}_{100}\) = 5 for surface Northern Hemisphere sources, GWP\({}_{100}\) = \(-\)10 for surface Southern Hemisphere sources, and GWP\({}_{100}\) = 456 for aircraft sources. The time horizon and source location strongly influence the GWP value. [PERSON], [PERSON], and [PERSON] [39] presented the evaluated GWP for NO\({}_{x}\) emitted from aircraft. They determined that the mean global values of GWP for time periods of 20, 100, and 500 years were GWP\({}_{20}\) = 322, GWP\({}_{100}\) = 59, and GWP\({}_{500}\) = 17, respectively. Researchers noticed that different values of GWP were obtained for the Northern and Southern Hemispheres and particular regions in the world. The highest value of GWP\({}_{20}\) (542) was an estimate for the area over Brazil. Such a potentially strong effect on the sensitive global climate was caused by the presence of O\({}_{3}\) and CH\({}_{4}\). Researchers have concluded that GWP has limitations, and the influence of NO\({}_{x}\)-O\({}_{3}\)-CH\({}_{4}\) should be considered. They also noticed that in aircraft sources of NO\({}_{x}\), a more precise estimation of global climate change was obtained when radiative forcing (RF), with a unit of mW m\({}^{-2}\) Tg(N)\({}^{-1}\), was used as a climate metric. The descriptive difference between RF and GWP is that \"while RF indicates the climate effect between past and present points in time, GWP gives the perspective for future impact on current emissions\" [39]. The values of GWP and other climate metrics were presented by [PERSON] et al. [33].