# On the Issues of \(\mathbf{NO_{x}}\) as Greenhouse Gases: An Ongoing Discussion...
## 5 Simplified Evaluation of NOx (Surface Sources) Impact on GHE  
Generally, unequivocal determination of the global impact of NOx on the GHE (cooling or warming) is very difficult. This is because of the mentioned differences in the GWP value and dualistic behavior of NOx as cooling or/and warming gases [33; 40; 41]. It was mentioned that the impact of NOx on climate change depends on many factors such as latitude, horizontal and vertical localization, the contribution of other gases (mainly CO, CH4, and SO2), conditions in the atmosphere (such as wind and sunlight irradiation), the processes occurring in the soil, and time horizons [7; 33; 38; 39]. [PERSON], [PERSON], and [PERSON] [39] noticed that the same amount of emitted NOx can cause different responses in terms of the GHE, depending on the emitter localization (aircraft in this case). Additionally, [PERSON] et al. concluded that \"GWPs are not a useful tool for calculating the complete suite of aircraft effects\" [33]. Particulate analysis of this issue has been presented by [PERSON], [PERSON], and [PERSON] [39]. It is commonly accepted that the emissions of NOx from aircraft (especially in the North Pacific and North Atlantic ocean zones) have a much stronger influence on global warming than the emissions of NOx from surface sources [33; 39; 50]. [PERSON], [PERSON], and [PERSON] [39] concluded that the global GWP\({}_{100}\) from NOx emitted from aircraft (based on 0.035 Tg(N)/yr incremental aircraft NOx emissions) equaled 59. However, depending on the region, this factor has a large range of values (25-110). The complexity of this scientific analysis is caused by the already mentioned conditions in the atmosphere [64] and the aircraft parameters (such as weight, engine type, and combustion conditions) [64; 65]. Moreover, NOx emissions from aircraft depend on the flight phase. For example, during landing and take-off periods (idle/taxi, approach, climb out, and takeoff), the highest NOx emissions were observed during the climb-out phase [66]. Thus, the presented analysis here did not include NOx emissions from aircraft because this has been covered by other researchers. Presented below is a simplified analysis, the aim of which is to estimate the impact of NOx emissions from stationary, surface sources on the GHE compared to the impact on the GHE from other direct GHGs. It is assumed that NOx emissions are from stationary, surface sources localized at central latitudes with a time horizon of 100 years. [PERSON] and [PERSON]\(\rm\breve{s}\)l [38] estimated a GWP\({}_{20}\) = 30-33 and GWP\({}_{100}\) = 7-10 for NOx as the global average value. However, in other references, the values of GWP for surface sources are extremely varied including positive and negative. For example, positive values in the range of GWP\({}_{20}\) = 43-130 for sources localized in tropical regions, and positive/negative values in the range of GWP\({}_{20}\) = \(-\)43 to +23. From all of the analyzed values of GWP\({}_{100}\) for surface sources, the highest global GWP\({}_{100}\) is 10. Thus, this value is assumed as the most pessimistic. The other assumed variant is GWP\({}_{100}\) = 1.6. This value seems to be most realistic because it was \begin{table}
\begin{tabular}{l c c} \hline \hline
**Warming** & **Cooling** & **Cross Impact** \\ \hline Water & & \\ \hline Nitrogen is a substrate for N2O production by nitrifying and denitrifying bacteria in water bodies [6]. & N- water can accelerate to grow algae growth. Nevertheless, the harmful (toxic, food-web altering, hypoxia-generating) algal blooms (HABs) have been linked to human nutrient (phosphorus (P) and nitrogen (N)) over enrichment [59] & NOx and SO\({}_{x}\) might be beneficial to the growth of microalgae as they can provide additional nutrients. However, this is true only when the culture pH is stably controlled and the NO\({}_{x}\)/SO\({}_{x}\) concentrations should be lower than the inhibitory level [63]. & \\ \hline \hline \end{tabular}
\end{table}