NO₂ is a reddish brown gas that is emitted from all combustion engines. There are two main nitrogen based compounds that are emitted from combustion engines: NO₂ and nitric oxide (NO). Collectively these two pollutants are referred to as NO_x or oxides of nitrogen.

Figure 1 Simplified cycle of NO and NO₂

To summarize:

• NO₂: Nitrogen Dioxyde
• NO: Nitric Dioxyde
• NO_x: Oxides of Nitrogen = {NO₂+NO}

At the point of emission (i.e. the exhaust pipe), the proportion of NO_x is around 90% NO and 10% NO₂ (1).

After a few hours in the atmosphere and in the presence of volatile organic compounds (VOCs) the NO is converted to NO₂. This reaction can occur over a couple of seconds to a few hours (2).

NO₂ reacts further with other substances in the air to form nitric acid, particulate matter and substances called PANs (peroxyacyl nitrates).

Also with sunlight NO₂ can convert back to NO and produce ozone (O₃) as a side pollutant. Because of the potential of NO₂ to produce these "secondary" pollutants it is important to monitor and regulate NO₂.

Short and long term exposure to NO₂ has been linked with an increased risk of respiratory problems. People with asthma, young children and older adults have an increased sensitivity to its effects (1).

The secondary pollutants caused by the presence of NO₂ in the atmosphere also have their own adverse effects. PANs are an irritant, nitric acid causes acid rain and particulate matter and O3 cause respiratory problems.

As previously mentioned, NO_x emissions comes from combustion engines. But what is happening in these engines is the high temperature reaction of hydrocarbons in the fossil fuel with air (which is 80% nitrogen). There are also natural sources of NO_x emissions such as wildfires, lightning strikes but the vast majority of NO₂ in the atmosphere is caused by human activity.

Figure 2 Sources of NO_x emissions in the USA – 2014

There are many different industries which emit NO_x. The figure (2) shows sources of NO_x emissions in the USA measured in 2014. The "mobile sources", i.e. road vehicles, boats, airplanes, farm vehicles etc, are easily the greatest contributor. In the USA and Europe, road vehicles are the main type of ‘mobile source’ of NO_x (1) (3).

In areas where road vehicles are the main source of NO_x there is often higher NO₂ concentrations during peak traffic times such as around 5pm weekdays shown in the NO₂ AQI below. Some additonal interresting notes:

• The ozone (O₃) concentration increases during the day while the NO₂ concentration decreases. That's because NO₂ transforms to NO_x in the presence of sunlight.
• After 6 PM, the NO₂ concentration builds up as there is no sunlight to convert NO₂ back to NO.

Figure 3 Example of AQI in London

In China, however a study conducted in 2013 found that industrial sources are the biggest (34% of the total) emitters of NO_x (4). This is why there is no strong link with NO₂ concentrations and rush hour traffic.

The NO₂ AQI is usually in the ‘Good’ range and shown in green. But because NO₂ is converted to other pollutants which have their own negative effects the NO₂ in the atmosphere still negatively affects our health and environment. If you would like a more detailed explanation into NO_x in our atmosphere have a look through these documents below.