Formation of Nitrogen Oxides (NOx)
1. Combustion of Fossil Fuels
Nitrogen oxides are formed mainly by the combustion of fossil fuels like coal, oil, and natural gas in high-temperature processes. Combustion combines atmospheric nitrogen with available oxygen to form mostly nitric oxide (NO) and some nitrogen dioxide (NO2), both together denoted by NOx. Power plants, industrial plants, and motor vehicles-particularly internal combustion engine-powered ones-emissions result in high NOx emissions. Indeed, it is estimated that more than half of the total world’s nitrogen oxide emissions are due to fossil fuel combustion.
2. Vehicle Emission
Another giant source of NOx pollution is transportation, especially within urban cities. All cars, trucks, buses, and other running vehicles with the combustion of gasoline or diesel engines emit NOx. Emissions from vehicles are one of the prime causes of air pollution within the cities because of the high vehicle density. The adoption of cleaner fuels and electric vehicles is being thought of to reduce NOx emissions from transport.
3. Agricultural Practices
Fertilizers, especially nitrogen-based ones, have been used in agriculture to increase yields; in the process, while nutrition for crops is provided, at the same time, they also tend to probably contribute to nitrous oxide emissions, an extremely potent greenhouse gas. The nitrous oxide is formed through microbial processes of nitrification and denitrification in the soils where synthetic fertilizers have been applied. Agriculture is a large contributor to the global sources of N2O, and thus it is one of the main sectors offering opportunities for reductions of greenhouse gas emissions.
4. Natural Sources
Besides volcanic emissions, lightning and wildfires in general also generate NOx. When lightning heats the air, nitrogen and oxygen can be sufficiently heated that they react to produce nitrogen oxides. Forest fires emit NOx, as do other forms of vegetation burning. Despite these natural sources, however, most NOx emissions globally are from human activities.
Environmental Impact of Nitrogen Oxides
1. Air Pollution and Smog
The most identifiable effect of nitrogen oxides on the environment contributes to the formation of ground-level ozone, otherwise known as smog. NOx combines with VOCs in the presence of sunlight to form ozone, a toxic pollutant irritating the respiratory system and thereby exacerbating diseases like asthma. This is usually worse during high-temperature months and areas characterized by high traffic and industrial densities within the urban setting.
2. Acid Rain
Nitrogen oxides are also responsible for acid rain. Atmospheric NOx gases react with water vapour and form nitric acid, HNO3, that may be deposited through rain, sleet, or fog. This type of acid deposition has caused widespread damage to ecosystems. Acid rain decreases the pH of soils and aquatic systems and may make them more acidic—harmful to plant growth and hazardous for fish and other wildlife. Some forests and lakes in areas receiving high loadings of NOx are acidified.
3. Ozone Layer Depletion
Nitrous oxide is the greatest contributor to stratospheric ozone depletion. While all NOx-related lower atmospheric effects tend to occur in a rather short time frame, that is, within a few days, N2O is longer-lived and can reach the stratosphere, where it degrades the ozone molecules. The depletion of the ozone layer allows more UV radiation to reach the Earth’s surface, resulting in increasing skin cancer, cataracts, and other human health problems, along with crop and ecosystem damage.
4. Climate Change
Nitrous oxide is an extremely harmful greenhouse gas, with a GWP roughly 300 times that of carbon dioxide over 100 years. Once N2O has been emitted, it remains in the atmosphere for over 100 years and enhances heating due to its heat-trapping characteristic in the atmosphere. Generally, synthetic fertilizer application and manure management are associated with very significant emissions of N2O in agriculture.
5. Human Health Risks
Nitrogen dioxide exposure raises serious health risks, notably in the respiratory system. NO2 causes irritation of the airway after exposition to a high concentration for a short period when symptoms like wheezing, coughing, and difficulty breathing can develop. Long-term exposure to nitrogen dioxide increases the risk of asthma and bronchitis and highly reduces lung function, especially among children and adults alike. The higher levels of NO2 fumigations usually occur with heavy circulation in the main urban areas.
References
“Nitrogen Oxides (NOx) Emissions.” United States Environmental Protection Agency (EPA).
[URL: www.epa.gov/nox]“Nitrogen Dioxide and Sulfur Dioxide.” World Health Organization (WHO).
[URL: www.who.int/airpollution/nitrogen-dioxide]Davidson, Eric A. “The Contribution of Nitrous Oxide Emissions to Climate Change.” Nature Climate Change, vol. 1, no. 4, 2020, pp. 186–191.
[DOI: 10.1038/nclimate1458]Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. Wiley, 2016.
“Ozone Layer Depletion.” United Nations Environment Programme (UNEP).
[URL: www.unep.org/ozone]
