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. These pollutants have negative effects on health and the environment. 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 when a certain level is exceeded. This article covers information on nitrogen oxides, its 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 NOx monitoring. What is NOx? In atmospheric chemistry, NOx 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 vapours are heavier than air and are toxic when inhaled. NO2 gas itself is noncombustible, however, it can accelerate combustion. Source: https://www.gasdetection.com/the-tech-center/more-on-the-gases-we-detect/nitrogen-dioxide-and-nitrogen-dioxide-monitoring-instrumentation/ NO (nitric oxide or nitrogen monoxide) is a colourless, 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 NOX 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, NO2, and ozone (O3) exist in a quasi-equilibrium depending upon the amount of sunlight present.

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Nitrogen Oxides
All you need to know about...

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All you need to know about Nitrogen OxidesWHAT IS NOx?
In atmospheric chemistry, nitrogen oxides (NOx) is the generic term to refer to the highly
reactive poisonous gases composed of nitrogen and oxygen, i.e. NO and NO2
NO2 (nitrogen dioxide) is a reddish-brown acidic gas comprising one
nitrogen atom and two oxygen atoms.
It is corrosive, strongly oxidizing and has a pungent irritating odor.
Its vapors are heavier than air and are toxic in nature when inhaled.
NO (nitric oxide or nitrogen monoxide) is a colorless gas consisting
of one nitrogen atom bonded to one oxygen atom.
It is oxidizing, non-flammable, poisonous gas with a slightly
irritating odor.

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All you need to know about Nitrogen OxidesNOxin Atmosphere
NOx is typically produced from the reaction of
nitrogen and oxygen when fossil fuels are burned at
high temperatures. From total NOx released during
combustion, generally, 10% is NO2 while 90% is NO.
NO rapidly reacts with oxygen in the atmosphere to
form NO2. In the presence of sunlight, it converts
back to NO and forms O3 (another critical air
pollutant).
The remaining NO2 eventually reacts with other
substances in the air to form particulate matter (PM),
gaseous nitric acid (HNO3), and toxic organic nitrates
such as PANs (peroxyacetyl nitrates).
N2 + O2 2NO
NO + O2 NO2

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All you need to know about Nitrogen OxidesSOURCES
Natural Sources: Lightening, forest fires, microbial
processes
Anthropogenic / Industrial Sources:
● Burning of fossil fuels such as coal, oil, gas, diesel,
etc.
● Emissions from vehicles such as cars, trucks, buses,
boats, airplanes, etc.
● Emissions from power plants, industrial boilers, etc.
● Petrol and metal refining, food processing, and
manufacturing industries such as the production of
fertilizers.
● Kerosene or gas heaters or stoves

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All you need to know about Nitrogen Oxides
The breakpoints concentrations describing the quality of air based on the NO2
concentrations (μg/m3) for different countries are:
India (24-hr) US (24-hr) China (24-hr) EU (8-hr)
AQI Category
Break point
concentration
AQI Category
Break point
concentration
AQI Category
Break point
concentration
AQI Category
Break point
concentration
Good 40 Excellent 40 Very low 50
Satisfactory 80 Good 80 Low 100
Moderately
polluted
180
Lightly
Polluted
180 Medium 200
Poor 280
Moderately
Polluted
280 High 400
Very Poor 400 Very Unhealthy 2260
Heavily
Polluted
565 Very high 400+
Severe 400+ Hazardous 3760
Severely
Polluted
565+
Source: National Ambient Air Quality Index, CPCB (Oct 2014)
PERMISSIBLE LIMITS

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All you need to know about Nitrogen OxidesADVERSE IMPACTS
● NO2 inflames the lining of the lungs, and it can
reduce immunity to lung infections.
● Short-term exposure can aggravate respiratory
diseases, particularly asthma, leading to respiratory
symptoms (such as coughing, wheezing or difficulty
breathing), hospital admissions, and visits to
emergency rooms.
● Longer exposures may contribute to the
development of asthma and potentially increase
susceptibility to respiratory infections.
HEALTH IMPACTS OF NOx
● Children with asthma and the elderly with heart disease are more at risk.

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All you need to know about Nitrogen Oxides
Acid Rain
damages forests,
building and water
resources
ENVIRONMENTAL IMPACTS OF NOx
Smog formation
impacts human
health & vegetation
Visibility impairment
NOx block light,
reduces visibility
Water quality
deterioration
causes oxygen depletion
Particulate matter
penetrating deeply
into the lungs
ADVERSE IMPACTS

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All you need to know about Nitrogen OxidesCORRECTIVE MEASURES
The primary step is NOx monitoring to identify the areas with high NO2 and NO levels or
areas where air quality does not meet the NOx 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.
● Avoid keeping your vehicle idle for long.
● Have a walk, use bicycles, public transport or carpool whenever possible.
● Avoid areas with traffic congestions. Breathing such polluted air daily also leads to
health effects in the long run.
● Limit outdoor activities and close windows and vents during times of high air
pollution.

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All you need to know about Nitrogen OxidesMONITORING METHODS
Chemiluminescence
It is based on a chemiluminescent reaction between
nitric oxide (NO) and ozone (O3). NO2 in the sample is
converted to NO which reacts with O3 to form activated
NO2. During the conversion, light energy at a specific
wavelength is produced which is proportional to the
amount of NOx present in the air sample.
Semiconductor
When sensor is exposed to air sample, the NOx molecules
react on the metal oxide surface of the sensor and
dissociate into charged ions which alter the resistance of
the film which is measured and converted to the NOx
concentration.

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All you need to know about Nitrogen OxidesMONITORING METHODS
Electrochemical
NOx monitors working on the
electrochemical principle are operated
based on diffusion of NO2 and NO gas into
the respective sensor which results in the
production of electrical signals proportional
to the NO2 and NO concentration
respectively.
It allows accurate measurement of even low
concentrations, which is essential in NOx
monitoring in the ambient conditions.

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All you need to know about Nitrogen OxidesOIZOM’S PRINCIPLE
Oizom’s POLLUDRONE measures more than 30
parameters including gaseous pollutants like CO,
SO2, NO, NO2; dust particulates like PM2.5, PM10,
noise and weather parameters like rainfall, wind
speed and wind direction.
Most of the sensors measuring gases including
nitrogen oxides work on the principle of
electrochemical sensing.
Real-time monitoring of NOx levels helps in
calculating air quality index to deliver health
advisories as well as formulating an action plan to
meet standards.