Nitrogen oxides (NOx = NO + NO2) emitted from a stationary combustion chamber (including waste to energy plants) or engines cause numerous undesirable environmental effects. These include negative influences on human and animal health, detrimental effects on plants and vegetation, acid rain, and smog. These negative influences are commonly accepted by the scientific community. However, the impact of NOx on the greenhouse effect (GHE) is not generally accepted by the scientific community.
1. Introduction
It has already been mentioned that global warming potential (GWP) can be negative or positive. Indeed, the warming and cooling effect of NO
x in the atmosphere is highlighted in the literature
[1][2][3]. The nature of this effect depends on the mentioned parameters such as NO
x source, horizontal and vertical location, and the co-existence of other compounds. In the next section, an explanation of the cooling and/or warming nature of NO
x is provided.
2. Warming Nature
The presence of NO
x can influence global warming. The results of investigations suggest that the main process responsible for this effect is the impact of NO
x on the conversion of tropospheric ozone (O
3)
[4], which is recognized as a GHG
[5]. Depending on the concentration of NO
x in the atmosphere and the equilibrium between other compounds contained in the atmosphere, O
3 can either be created or destroyed. If the concentration of NO
x are higher than the range of 10–30 pptv (parts-per-trillion (volumetric), 10
−12), O
3 can be created in the atmosphere. Furthermore, the rate of O
3 creation because of the presence of NO
x depends on the latitudes and seasons
[4]. Namely, it has been postulated that the presence of NO
x (NO/NO
2) influences the catalytic conversion of O
3, according to the following reactions ((2)–(5))
[6]:
Summarizing reactions (2)–(5), the overall process reaction (6) is
Thus, this proves and provides clear evidence that the presence of NO
x causes the creation of O
3 and CO
2 under sunlight irradiation. Hence, they influence global warming because of the creation of GHGs. The effect of the presence of NO
x on O
3 conversion in the atmosphere was confirmed by Renyi Zhang, Xuexi Tie, and Donald W. Bond
[7].
Another phenomenon potentially influencing global warming due to the presence of NO
x is their impact on N
2O conversion
[4][8]. Namely, NO
x emitted into the atmosphere can be converted into N
2O (a direct GHG) in the complex processes occurring in the soil. The simplified description of this complex mechanism of converting NO
x into N
2O is as follows: Emitted NO is transformed into NO
2, and next to nitrogen acids and other compounds in the form of aerosols. These compounds are then transferred into the soil by precipitation. Further transformation in the soil (such as by the denitrification process) leads to incidental emissions of N
2O from the soil to the atmosphere. It was estimated that the N
2O emissions from soil (as a consequence of NO
x transformation) are 1.2%–3.6% of the total N
2O emissions from other sources
[4]. Nevertheless, understanding the soil N cycling processes is still being discussed
[9].
3. Cooling Nature
It was previously mentioned that the presence of NO
x can lead (in some specific conditions) to global cooling. This is why the GWP values are sometimes negative. Furthermore, NO
x are sometimes termed as cooling gases
[10][11][12][13]. It was proven that the presence of NO can influence the increase in the concentration of OH radicals in the atmosphere, and OH radicals contribute to destroying methane, according to the following reactions
[12]:
Here, CH
4 belongs to the direct group of GHGs, thus destroying it causes a cooling effect. Moreover, CH
4 reduction results in a long-term reduction in tropospheric O
3, and a long-term reduction in stratospheric water vapor from the reduced oxidation of CH
4. Both of these phenomena are recognized as negative radiative forcing effects
[14]. It should be explained that the cooling effect of NO
x depends on the impact of other compounds existing in the atmosphere. Namely, the presence of CO can contribute to a decrease in the concentration of OH radicals. Consequently, the cooling effect of NO
x can be inhibited, and the GWP for NO
x is positive (a warming effect). Furthermore, the decrease in the OH concentration inhibits CH
4 destruction (being a direct GHG). If the impact of NO
x is considered without reference to the CO contribution, it would only be assumed that the cooling effect of NO
x is from surface sources. The increase in the CO concentration in the atmosphere causes NO
x to convert from cooling gases to warming gases with a positive GWP
[13]. One can have reasonable hope that the development of combustion technology by increasing the combustion efficiency and decreasing CO emissions will inhibit NO
x from having an effect as a warming gas.
Another phenomenon responsible for the cooling effect of NO
x is the formation of aerosols (dispersion of very fine liquid droplets) in the atmosphere. Increased aerosol formation and cloud reflectivity cause a decrease in sunlight radiation and enhance the cooling effect
[4][10]. The main process responsible for aerosol formation is the conversion of SO
2 into H
2SO
4 formations, which condensate as very fine droplets (aerosols). The contribution of NO
x in this process relies on OH formation. It has already been explained that an increase in NO concentration causes an increase in OH radical concentration in the atmosphere. Moreover, the presence of OH radicals intensifies SO
2 conversion into aerosols, thus directly causing a cooling effect
[10].
4. Summary
It has already been mentioned that the warming and cooling effects of NOx in the atmosphere are possible due to the impact of different processes. The warming and cooling effects are summarized in Table 1. These effects were divided into three groups in terms of the influence area (i.e., air, water, soil, and vegetation aboveground). Some processes seem to be opposing. Thus, examples of these cases are described in a “cross-impact” column.
Table 1. The summary of the warming and cooling effect of NOx in terms of the influence on the area.