When boilers operate at high temperatures, oxygen interacts with nitrogen from the air and the fuel used, resulting in the formation of nitric oxide (NO) and nitrogen dioxide (NO2), collectively known as nitrogen oxides (NOx).
In the atmosphere, NO undergoes oxidation by ozone, converting it into NO2. In rural air, where there are no significant NO sources, most NOx exists in the form of NO2.
Inhalation of these gases in their pure form can be rapidly fatal. NO is both a potent greenhouse gas and a contributor to ozone layer depletion. It is a colorless and odorless gas.
Whereas NO2 is soluble in water, and characterized by its reddish-brown color and potent oxidizing properties. It is primarily formed via NO oxidation in combustion, including diesel engines, coal, oil, gas, wood, and waste plants.
Under UV light, NO2 is decomposed into NO and an oxygen atom, which then combines with O2 to form ozone.
NOx emission sources
The most significant sources of NOx emissions are fuel combustions (Figure 1). It originates from the combustion of natural gas, both indoors and outdoors.
On-road vehicles, such as trucks, buses, and cars also have a major contribution, followed by non-road vehicles. The latter includes diesel-powered machinery, industrial operations, such as oil and gas extraction, industrial boilers, and coal-fired power stations.
In the U.S. commercial sector, space heating, cooling, and ventilation collectively account for over 50% of total fuel usage. Within this category, space heating emerges as the dominant component, comprising 32%, while ventilation and cooling follow at 11% and 9% respectively.
Figure 2 shows the annual NOx emissions worldwide from 2010 until 2021. The introduction of gas as a primary fuel source in the early 1990s, displacing coal and oil, resulted in cleaner energy production and the operation of more advanced power plants.
Despite the consistent level of gas utilization for electricity generation post-2000, there was also a rising demand, which was met by an increased reliance on coal-fired power plants.
In 2021, global emissions of NOx peaked at 2.97 billion metric tons of CO2 equivalent (GtCO₂e). Looking at the figure between 2010 to 2021, the trend of NOx emissions has gone up by 12.5%.
Since the popularity of electric vehicles, NOx emissions stemming from road transport seem to have been controlled slightly but although they reduce exhaust emissions, they still have non-exhaust emissions to contribute to air pollution.
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Health effects of NOx pollution
NOx pollution poses health risks to those residing near emission sources. Additionally, certain vulnerable populations including pregnant individuals, children, older adults, people with pre-existing medical conditions such as asthma, chronic obstructive pulmonary disease, cardiovascular disease, diabetes, and lung cancer, and smokers are at higher risk from the exposure.
Figure 3 shows the latest available data on NOx emissions around the world as of writing this article. The emissions in India were measured as the highest in 2021, followed by the U.S. and China.
The health effects include various detrimental effects on lung health, such as heightened inflammation of the airways, worsened coughing and wheezing, diminished lung function, and asthma attacks.
Over the past few decades, diseases resulting from air pollution have become a growing global concern. Among all regions, East Asia has been disproportionately affected, with an estimated 3.11 million additional deaths annually attributable to air pollution. Figure 4 shows the yearly mortality excess due to ambient air pollution worldwide in 2020.
How to reduce NOx emissions?
Fortunately, in most of the countries, outdoor air quality has significantly improved in recent decades compared to previous years.
Individuals can take proactive measures to safeguard themselves on days when air pollution levels are high. Moreover, they can advocate for continued enforcement of policies mandating the reduction of air pollution at all levels of government.
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Here are some basic steps that companies and individuals can take to make an effort to reduce NOx emissions from their part:
- Heating, ventilation, and air conditioning manufacturers can adopt newer low-NOx technologies, offering higher thermal efficiency.
- Using public transport where applicable because NO2 levels tend to be higher in areas close to pollution sources, such as heavy traffic or industrial facilities.
- Practising energy conservation by using low-emission energy sources both at home and in the workplace.
- Using burners operating at lower combustion temperatures to reduce NOx emissions, thereby minimizing their impact on the atmosphere.
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Training and education play a huge role in raising public awareness about the health and environmental impact of NOx emissions. To some extent, businesses could additionally encourage their workforce by offering public transport passes or electric vehicle charging facilities.
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As individuals, they can keep up to date with air quality, enabling them to stay informed about pollutant levels in their surroundings. With ongoing joint efforts to mitigate emissions from these sources, NOx emissions are expected to decrease in the coming years.
