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14 April 2022
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Understanding Air Pollutants: A Guide to Nitrogen Oxides (NOx)

What are Nitrogen Oxides (NOx)

Nitrogen oxides (NOx) are some of the most common air pollutants alongside carbon monoxide (CO), ozone (O3), particulate matter (PM2.5 and PM10) and sulphur oxides (SOx).

NOx are poisonous gases derived from nitrogen and oxygen combustion under high pressure and temperatures. The two most common nitrous oxides are nitric oxide (NO) and nitrogen dioxide (NO2). NOx can react with volatile organic compounds to form ground-level ozone and also reacts with atmospheric chemicals, producing PM2.5.

Sources

A major source of anthropogenic emissions of nitrogen oxides into the atmosphere, originates from the combustion of fossil fuels in stationary sources, such as heating and power generation, as well as in motor vehicles, such as internal combustion engines. According to figures from the UK’s Department for Environment, Food and Rural Affairs (DEFRA), road transport accounted for 33% of the emissions of nitrogen oxides in the UK in 2019, and other forms of transport such as aviation, rail, and shipping accounted for 14%. Whilst road transport does account for a third of NOx emissions, there has been a downward trend in emissions as newer vehicles meet more strict emissions standards.

Key industries which generate NOx through their activities are the energy, manufacturing and construction industries. The London Atmospheric Emissions Inventory (LAEI) found that NOx generated from construction accounts for 7.5% of all NOx emissions in the capital.

Environmental Impact

Rapid industrial development in the past 30 years has led to an increase in ambient levels of NOx and other pollutants. The impact of Nitrogen Oxides (NOx), is visible within the smog and typical brown clouds often covering larger cities, which provide poor air quality for the inhabitants. It can also lead to poor visibility, with nitrate particles making the air foggy. NOx emissions also contribute to the creation of acid rain and the formation of ground-level ozone that can damage ecosystems, animal and plant life.

Health Impact

There are both direct and indirect effects on human health as a result of NOx exposure.

High levels of Nitrogen Dioxide (NO2) can be specifically dangerous as they inflame the lining of your airways, and cause a flare-up of asthma or COPD, alongside symptoms such as coughing and difficulty breathing. Those most likely to be affected by these side effects, according to statistics from the British Lung Foundation, include children and older people.

The health effects of NOx become more severe with exposure to higher concentrations or longer term exposure. Prolonged exposure to high levels of NOx has the potential to increase vulnerability to respiratory infections, lead to decreased lung function and contribute to the development of asthma.

Whilst the effects are heightened with longer term and higher levels of exposure, particularly to children, for older people and those with pre-existing conditions, even short term exposure can cause symptoms such as irritation of the lungs in otherwise healthy people.

NOx exposure also has an impact on mortality, with the Department for Environment, Food & Rural Affairs estimating in 2015 that the effects of NO2 on mortality were equivalent to 23,500 deaths in the UK annually.

How Can NOx Be Measured

Nitrogen oxides can be measured through a process known as ‘chemiluminescence’ which is a chemical reaction emitting energy in the form of light. There are over 200 monitoring sites throughout the UK using chemiluminescence analysers.

The largest monitoring network in the UK is the Automatic Urban and Rural Network (AURN), with 127 sites across the country. The AURN is the main hub in the country for compliance reporting against the Ambient Air Quality Directives.

Nitrogen oxides can also be accurately measured in low concentrations using electrochemical sensors – often preferred over alternative methods due to their robustness.

Sensit by Crowcon RAMP

The RAMP is a robust, remote and reliable low-cost air quality monitoring platform. The device is capable of monitoring up to five gaseous chemical pollutants and the electrochemical sensors offer PPB, parts per billion, resolution for both NO and NO2 along with other common air pollutants.

The RAMP is suitable for use in a variety of industries including construction, transport, waste, oil and gas, chemical and petrochemical industries.

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