In atmospheric chemistry, NOx (“knocks”) is shorthand for nitric oxide (NO) and nitrogen dioxide (NO₂)—the nitrogen oxides most relevant to air pollution. These gases contribute to the formation of several of the most harmful air pollutants. The European Union, the United States, and other countries regulate emissions of nitrogen dioxide as an indicator for the broader group of nitrogen oxides.
Exposure to high levels of NOx can cause a range of health impacts, including respiratory impairment, asthma exacerbation, decreased lung function, and cardiovascular effects.1 Children and the elderly are particularly susceptible to lung function impairments and chronic disease. NOx also contributes to the formation of particulate matter (PM) and tropospheric (ground-level) ozone, both of which are damaging air pollutants.
Anthropogenic NOx gases are primarily formed at high temperatures during combustion when nitrogen and oxygen from the air react. The formation of NOx is strongly temperature-dependent, with significant production occurring at temperatures above 2,000°F (1,093°C).2 Natural gas in a combined cycle power plant burns at 2,500°F to 3,600°F (1,370°C to 1,980°C). Gasoline in an internal combustion engine burns at 4,500°F to 4,800°F (2,480°C–2,650°C). This means that NOx emissions increased rapidly in the 20th century when fossil fuels powered rapid economic growth.
Diesel engines produce more NOx than gasoline engines because they burn fuel at higher temperatures and pressures. Heavy (residual) fuel oil is a high NOx emitter because it contains more nitrogen compounds than gasoline or diesel and because its combustion often involves longer burn times, which favor NOx formation.
Transportation by air, land, and sea accounts for the largest share of anthropogenic NOx emissions due to its reliance on fuels refined from oil. Most large cargo ships use heavy fuel oil, marine diesel engines have very high compression ratios, and they often operate continuously for long durations.3 The international shipping industry has also been subject to less stringent emissions regulations than passenger vehicles and trucks in most countries. These conditions lead to high NOx emissions from shipping.
Bacterial activity in soils releases NOx into the atmosphere through microbial processes called nitrification and denitrification. Agricultural practices enhance NOx emissions from both processes. The application of nitrogen fertilizers increases soil nitrogen levels, which amplifies both nitrification and denitrification. Land-use changes such as deforestation and tillage can also create conditions that promote NOx formation.
Countries that have developed large oil-based transportation systems, fossil fuel–powered electricity generation, and industrial agricultural systems reliant on fossil fuel inputs have contributed the most to NOx emissions. Just seven countries—the United States, China, Russia, India, Japan, United Kingdom, and Germany—account for half of all anthropogenic NOx emissions since 1750.
The time profile of NOX emissions varies considerably among countries. Emissions in some countries have peaked and markedly declined, while in other countries emissions continue to rise. Countries can be categorized as follows:
- · Early peakers: United States (1973), Japan (1979), Finland (1980), German (1980)
- · Mid peakers: Sweden (1986), Australia (1990),Russia (1990), Canada (1998)
- · Later peakers: Mexico (2008), China (2012), Brazil (2014)
- · No apparent peak: Indonesia, India, Argentina, Thailand, Saudi Arabia, Bangladesh
Countries that have successfully reduced NOx emissions have used a combination of policies, regulations, technologies, and fuel switching. The United States Clean Air Act and its amendments, European emissions control standards, China’s Air Pollution Control Plan, and Japan’s Air Pollution Control Law along with the Motor Vehicle NOx Law are examples of policies directly aimed at reducing NOx and other key air pollutants from vehicles and power plants. China, the EU, and the United States also operate emissions trading systems that include NOx emissions.
Selective catalytic reduction (SCR), exhaust gas recirculation (EGR), and low-NOx combustion burners and engines were developed in response to regulations requiring emission reductions from vehicles and power plants.
The transition away from coal toward natural gas for electricity generation has also contributed to NOx reductions. Compared to coal, natural gas contains less nitrogen, generally burns at lower temperatures, and combusts more completely and uniformly—conditions that reduce NOx formation.4
The rise of nuclear power in the 1970s and 1980s, along with the more recent surge in solar and wind power, has further displaced fossil fuel–based electricity generation, contributing to lower NOx emissions.
1 World Health Organization, “WHO global air quality guidelines,” 2021, https://www.who.int/publications/i/item/9789240034228
2 Boningari, Thirupathi, and Panagiotis G Smirniotis. “Impact of Nitrogen Oxides on the Environment and Human Health: Mn-Based Materials for the NOx Abatement.” Current Opinion in Chemical Engineering, Energy and Environmental Engineering / Reaction engineering and catalysis, 13 (August 1, 2016): 133–41. https://doi.org/10.1016/j.coche.2016.09.004.
3 Sea News, “Shipping Industry and the NOx Emissions,” February 22, 2019, https://seanews.co.uk/environment/shipping-industry-and-the-nox-emissions/
4 EPA Air Pollution Control Cost Manual (Sixth Edition, 2002), “Selective Catalytic Reduction, https://www3.epa.gov/ttncatc1/dir1/c_allchs.pdf