NOx, apart from affecting human health and the environment itself, has the potential to produce “secondary pollutants” such as ozone, smog, nitric acid, and particulate matter. Also, these pollutants have negative effects on health and the environment. NO_x monitoring is an efficient way to prevent the accumulation of high levels of NO_x as it helps detect the amount of NO_X we are breathing in and alerts us when a certain level is exceeded. This article covers information on nitrogen oxides, their sources, permissible levels in the ambient air, health, and environmental impact, possible corrective measures, the need for nitrogen oxide monitors as well as different methods of NO_x monitoring.

What is NO_x?

 

In atmospheric chemistry, NO_x is a collective term used to refer to the nitrogen oxides that are most relevant for air pollution i.e.  NO and NO₂. NO₂ (nitrogen dioxide) is a reddish-brown acidic gas having a pungent irritating odour comprising one nitrogen atom and two oxygen atoms. It is corrosive and strongly oxidizing. It can combust with compounds such as hydrocarbons, sometimes explosively. Its vapors are heavier than air and are toxic when inhaled. NO₂ gas itself is noncombustible, however, it can accelerate combustion. 

 

 

NO (nitric oxide or nitrogen monoxide) is a colorless, non-flammable, oxidizing, poisonous gas with a slightly irritating odour. It consists of one nitrogen atom bonded to one oxygen atom. It is highly reactive due to the presence of one unpaired electron. Thus, this results in rapid oxidation (within a few minutes) to form NO₂. 

Nitrogen oxides in the Atmosphere

 

About 1% of the total NO_X forms naturally in the atmosphere by lightning and some are produced by microbial processes in plants, soil, and water. It is typically produced from the reaction of nitrogen and oxygen when fossil fuels such as coal, oil, gas, or diesel are burned at high temperatures. From the nitrogen compounds released during combustion, generally, 10% is NO₂ while 90% is NO. 

Nitric oxide emitted from these sources rapidly reacts with oxygen in the atmosphere to form NO₂. In the presence of sunlight, it converts back to NO and forms ozone (another critical air pollutant). Therefore, during the day NO, NO_2, and ozone (O₃) exist in a quasi-equilibrium depending upon the amount of sunlight present. 

The atmospheric chemistry in the areas where vehicular emission is a predominant source generally follows the trend given below:

• There is often a higher concentration of NO₂ during peak traffic times
• The concentration of O₃ increases during the day as the sunlight increases, while the NO₂ concentration decreases. This happens because NO₂ reacts in the presence of sunlight to form O₃.
• In the late evening, NO₂ concentrations start to build up as there is no sunlight to convert NO₂.

 

 

The remaining NO₂ eventually reacts with other substances in the air to form particulate matter, gaseous nitric acid (HNO₃), and toxic organic nitrates such as PANs (peroxyacetyl nitrates). However, NO₂ may travel hundreds of kilometers before their eventual conversion to nitric acid or nitrates. 

Sources of Nitrogen Dioxide

 

NO_x is primarily released in the air from the burning of fuel. Emissions from vehicles such as cars, trucks, buses, boats, airplanes, etc. are the largest source of nitrogen oxides, followed by emissions from power plants, diesel-powered heavy equipment, and other movable engines, industrial boilers, etc. Other sources of NOx include petrol and metal refining, food processing, and manufacturing industries such as the production of fertilizers. Kerosene or gas heaters or stoves also produce substantial amounts of NOx, which when placed in poorly ventilated spaces would result in a build-up of high levels. Permissible levels of NO₂

The breakpoints concentrations describing the quality of air based on nitrogen dioxide concentrations for different countries are given below. In India, the satisfactory levels of daily NO2 are up to 80 µg/m³.

Table: Breakpoints of nitrogen dioxide (µg/m³)

India (8-hr)	US (24-hr)		China (24-hr)		EU (8-hr)
AQI Category	Breakpoint concentration	AQI Category	Breakpoint concentration	AQI Category	Breakpoint concentration	AQI Category
Good	1	Good	5	Excellent	2	Very low
Satisfactory	2	Moderate	11	Good	4	Low
Moderately polluted	10	Unhealthy for sensitive	14	Lightly Polluted	14	Medium
Poor	17	Unhealthy	18	Moderately Polluted	24	High
Very Poor	34	Very Unhealthy	35	Heavily Polluted	36	Very high
Severe	34+	Hazardous	58	Severely Polluted	36+

 

Health & Environmental Impact of NO_x

 

Nitrogen Oxide Effects on Health

 

The main effect of breathing in raised levels of nitrogen dioxide results in an increased likelihood of respiratory problems. Also, NO₂ inflames the lining of the lungs, and it can reduce immunity to lung infections.

Breathing air with a high concentration of NO₂ can irritate the airways of the human respiratory system. Exposures for a short period can aggravate respiratory diseases, particularly asthma, leading to respiratory symptoms (such as coughing, wheezing, or difficulty breathing), hospital admissions, and visits to emergency rooms. Meanwhile, Longer exposures may contribute to the development of asthma and potentially increase susceptibility to respiratory infections. 

Also, people with asthma are more affected as it can cause more frequent and more intense attacks. Meanwhile, children with asthma and the elderly with heart disease are more at risk.

Environmental Impact

 

• Ozone (Smog) formation – NOx plays a major role in atmospheric reactions and produces ground-level ozone, so it is one of the major components of smog that can have a significant impact on human health.
• Acid rain –  NO_x interacts with water, oxygen, and also with other substances in the air to form nitric acid which falls on the earth in the form of rain, fog, snow, or dry particles. Acid rain harms sensitive ecosystems such as lakes and forests by making them acidic and unsuitable. It also corrodes metals, fades fabrics and degrade rubber, and causes the deterioration of cars, buildings, historical monuments, etc. 
• Particulate Matter: NO_x also contributes to the formation of particulate matter which causes health concerns including problems in breathing and the respiratory system by penetrating deeply into the sensitive parts of the lungs resulting in respiratory diseases. 
• Water Quality Deterioration: Increased absorption of NO_x in the water bodies upsets the chemical balance of nutrients so it leads to oxygen depletion in the water bodies.
• Visibility Impairment: The nitrate particles that result from NO_x can block the transmission of light, hence, making the air hazy and difficult to see through. 

Possible corrective measures

 

The primary step of NO_x monitoring is to identify the areas with high NO₂ and NO levels or areas where air quality does not meet the NO_x national standards. In addition to this following corrective measures can be taken:

• Promote the use of alternative fuels and improve quality fuels with high emission rates. 
• Periodically maintain/service your vehicles to improve their performance and reduce emissions. 
• Also, avoid keeping your vehicle idle for long. 
• Have a walk, use bicycles, public transport, or carpool whenever possible.
• Avoid areas with traffic congestion, breathing such polluted air daily also leads to health effects in the long run.
• Also, limit outdoor activities and close windows and vents during times of high air pollution.

How to Measure NOx Emissions

 

Chemiluminescence for NO_x monitoring

 

It is the most commonly used conventional method of measuring the NO_x levels in the air and is based on a chemiluminescent reaction between nitric oxide (NO) and ozone (O₃). In the NO_x monitor, the air is drawn into two paths. In the first path, the NO₂ in the sample is quantitatively reduced to NO using a converter. Hence, this converted NO₂ along with NO already present in the air reacts with ozone (produced using an ozone generator) to generate activated NO₂. During the conversion, light energy at a specific wavelength is produced which is proportional to the amount of NO+NO₂ present in the air sample. In the second path, the air is directly reacted with O₃ i.e. without passing through the reactor and the light energy produced is directly proportional to the NO present in the air.

The light intensity is measured photometrically from both paths and recorded. The signal from the second path is the amount of NO present in the air sample while the electronic difference of the signal from both paths is the amount of NO₂ present in the air sample.

 

Cavity Attenuated Phase Shift Spectroscopy (CAPS) for NO₂ monitoring

 

The NO₂ monitor works with the CAPS principle which operates as an optical absorption spectrometer that measures NO₂ directly. Measurement is carried out in a temperature-controlled optical cell containing highly reflective mirrors on both sides to provide extensive optical path length, a blue UV light-emitting diode (LED) centered at 450 nm, and a  vacuum photodiode detector. Hence, the NO₂ present in the air drawn into the NO₂ monitor causes a phase shift in the signal received by a photodetector. As a result, this is proportional to the NO₂ concentration.

Semiconductor method for NO_x monitoring

 

When a metal oxide semiconductor-based NO_x monitor is exposed to an air sample, the nitrogen dioxide and nitric oxide molecules react on the metal oxide surface of the sensor and dissociate into charged ions which alter the resistance of the film. As a result, this interaction is measured as a signal and is converted to the respective gas concentration. However, when compared to other NO_x monitors, the energy consumption of such nitrogen oxide monitors is higher.

Electrochemical method for NO_x monitoring

 

NO_x monitors working on the electrochemical principle are operated based on the diffusion of nitrogen dioxide and nitric oxide gas into the respective sensor which also results in the production of electrical signals proportional to the NO₂ and NO concentration respectively. Hence, it allows accurate measurement of even low concentrations of NO_x, which is essential in NO₂ monitoring and NO monitoring for the ambient air. 

 

Among all the above principles of NO_x monitoring, applications like ambient air monitoring prefer nitrogen oxide monitors based on electrochemistry. This is because they yield more accurate NO_x concentrations and are inexpensive in comparison with the others.

Oizom’s working principle for NO_x monitoring

 

Oizom’s POLLUDRONE measures more than 30 parameters including gaseous pollutants like carbon monoxide, sulfur dioxide, nitrogen dioxide, nitric oxide, dust particulates like PM2.5, PM10, noise, and weather parameters like rainfall, wind speed, and wind direction. Most of the sensors measuring gases including NO and NO2 work on the principle of electrochemical sensing. Several cities, campuses, and universities have deployed the active sampling powered sensor-based air quality monitor POLLUDRONE for drawing actionable insights to tackle the rise in ambient nitrogen oxide concentrations. 

5 Reasons Why NOx Monitoring is Important

 

• NO₂ and NO are major pollutants from vehicular emissions and energy production, as a result, the poor air quality of almost all urban regions takes place.
• NO_x, apart from affecting human health and the environment itself, has the potential to produce “secondary pollutants” such as ozone, smog, nitric acid, and particulate matter which have negative effects on health and the environment. 
• When inhaled, NOx causes damage to lung tissue and a reduction in the functioning of the respiratory system making children, people with lung diseases such as asthma, and people who work or exercise outside more susceptible to adverse health impacts.
• NOx monitoring is an efficient way to prevent the accumulation of high levels of NOx as it helps detect the amount of NOX we are breathing in and alerts us when a certain level is exceeded.
• Real-time monitoring of NOx levels helps in calculating the air quality index to deliver health advisories as well as formulating an action plan to meet standards.