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Dualistic Nature of NOx Impact on Greenhouse Effect
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This entry is adapted from the peer-reviewed paper 10.3390/app122010429

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.

NOx greenhouse effect (GHE) indirect greenhouse gas direct greenhouse gas

1. Introduction

It has already been mentioned that global warming potential (GWP) can be negative or positive. Indeed, the warming and cooling effect of NOx in the atmosphere is highlighted in the literature [1][2][3]. The nature of this effect depends on the mentioned parameters such as NOx 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 NOx is provided.

2. Warming Nature

The presence of NOx can influence global warming. The results of investigations suggest that the main process responsible for this effect is the impact of NOx on the conversion of tropospheric ozone (O3) [4], which is recognized as a GHG [5]. Depending on the concentration of NOx in the atmosphere and the equilibrium between other compounds contained in the atmosphere, O3 can either be created or destroyed. If the concentration of NOx are higher than the range of 10–30 pptv (parts-per-trillion (volumetric), 10−12), O3 can be created in the atmosphere. Furthermore, the rate of O3 creation because of the presence of NOx depends on the latitudes and seasons [4]. Namely, it has been postulated that the presence of NOx (NO/NO2) influences the catalytic conversion of O3, according to the following reactions ((2)–(5)) [6]:
OH + CO + O2 → CO2 + HO2     
HO2 + NO → NO2 + OH           
NO2 + hν → NO + O(3P)         
O(3P) + O2 + M → O3 + M       
Summarizing reactions (2)–(5), the overall process reaction (6) is
CO + 2O2 + hν → CO2 + O3     
Thus, this proves and provides clear evidence that the presence of NOx causes the creation of O3 and CO2 under sunlight irradiation. Hence, they influence global warming because of the creation of GHGs. The effect of the presence of NOx on O3 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 NOx is their impact on N2O conversion [4][8]. Namely, NOx emitted into the atmosphere can be converted into N2O (a direct GHG) in the complex processes occurring in the soil. The simplified description of this complex mechanism of converting NOx into N2O is as follows: Emitted NO is transformed into NO2, 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 N2O from the soil to the atmosphere. It was estimated that the N2O emissions from soil (as a consequence of NOx transformation) are 1.2%–3.6% of the total N2O 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 NOx can lead (in some specific conditions) to global cooling. This is why the GWP values are sometimes negative. Furthermore, NOx 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]:
HO2 + NO ↔ OH + NO2   
OH + CH4 ↔ H2O + CH3   
Here, CH4 belongs to the direct group of GHGs, thus destroying it causes a cooling effect. Moreover, CH4 reduction results in a long-term reduction in tropospheric O3, and a long-term reduction in stratospheric water vapor from the reduced oxidation of CH4. Both of these phenomena are recognized as negative radiative forcing effects [14]. It should be explained that the cooling effect of NOx 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 NOx can be inhibited, and the GWP for NOx is positive (a warming effect). Furthermore, the decrease in the OH concentration inhibits CH4 destruction (being a direct GHG). If the impact of NOx is considered without reference to the CO contribution, it would only be assumed that the cooling effect of NOx is from surface sources. The increase in the CO concentration in the atmosphere causes NOx 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 NOx from having an effect as a warming gas.
Another phenomenon responsible for the cooling effect of NOx 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 SO2 into H2SO4 formations, which condensate as very fine droplets (aerosols). The contribution of NOx 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 SO2 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.
Table
Warming Cooling Cross Impact
Air
In the short-term, NOx emissions contribute to warming by enhancing tropospheric O3 concentrations (on a daily time scale), which are recognized as GHG [2][5]. NOx enhances OH production. CH4 (GHG) is oxidized in the presence of OH [2][14].
NOx can lead to decreases in O3 concentration on a decadal time scale because it causes an increase in OH radical concentration, which decreases CH4 concentration, which decreases NO2 formation, which decreases O3 formation. [2][14].
The formation of fine particles called aerosols. Aerosols are powerful cooling agents, both directly by scattering or absorbing light, and indirectly by affecting the cloud formation, their lifetime, and brightness [2][10].		 NOx leads to O3 decreasing (on a decadal time scale) or increasing (on a daily time scale) [2].
Soil and vegetation aboveground
Nitrogen is a substrate for N2O production by nitrifying and denitrifying bacteria in soils. Thus, the deposition of nitrogen (Nr) onto ecosystems can increase N2O emissions and decrease the uptake of atmospheric CH4 by soil microorganisms. Soil microbes that consume CH4 often preferentially consume ammonium (NH4+), leading to reduced CH4 consumption rates in the presence of abundant NH4+ [2].
Inhibition of photosynthesis and a reduction of atmospheric CO2 sequestration by the plant biomass due to an increase of O3 concentration in the atmosphere (impacted by NOx). Reduction of aboveground C storage and reduction of belowground C assimilation and allocation [1][2]
In some cases, the excess of N leads to the enhanced mortality of plants due to nutrient imbalances or acidification [2].		 In some cases, inputs of Nr from atmospheric deposition enhance plant growth rates because of the fundamental constraint of N availability on plant productivity and CO2 uptake into plant biomass. N additions to soil typically increase C capture and storage [2].
Foliar N may also increase the albedo of the canopy, enhancing the reflectivity of the Earth’s surface, and hence contributing to cooling [2].		 Warming and cooling effects are possible. The effect of N on net C flux (both above and below ground pools) differs among ecosystems [1][2].
Water
Nitrogen is a substrate for N2O production by nitrifying and denitrifying bacteria in water bodies [2].
Denitrification occurring in water can emits N2O [15].
Nitrous oxide (N2O) can be emitted from wastewater treatment processes [15][16][17].
Both SO2 and NO inhibited algal growth at a high level of CO2 [18][19].		 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 [20]
The serious problem is cyanobacterial bloom formation. Decreasing P and N loads can counteract the direct positive effect of warming temperatures on bloom proliferation [21][22].
Some algae species can sequestrate the CO2 from the flue gas including SOX and NO [23].
In the case of some species (green alga Chlorella sp.), the presence of NOx can enhance algae growth [24]		 NOx and SOx 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 NOx/SOx concentrations should be lower than the inhibitory level [25].

References

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