What's so bad about Nitrogen Dioxide (NO2) as an air pollutant?

Nitrogen Dioxide 2009 tropospheric column density.

Nitrogen Dioxide (NO2) has taken most of the blame for reports that air pollution is rising in parts of Brighton and Hove. It is produced whenever fuel is burned - mostly by motor cars burning petrol and diesel, but also by domestic cookers and heaters burning gas. It is invisible, and is less harmful to healthy adults than it is to young and vulnerable people.

Brighton and Hove City Council (BHCC) published its Air Quality Action Plan (AQAP) 2011 a few days ago. The report says NO2 levels are rising in parts of Brighton and Hove - and has this to say about NO2:

"Nitrogen Dioxide is a respiratory irritant associated with both acute (short-term) and chronic (long-term) effects on human health. Repetitive exposure can inhibit lung tissue growth and repair increasing the risk of poor respiratory health later in life. Some of the research evidence suggests chronic exposure can make the respiratory tract more susceptible to disease including allergens. Children under six (especially infants born early) and people with existing respiratory illnesses such as asthma and bronchitis are more vulnerable to repeated inhalation of Nitrogen Dioxide. Healthy adults are less likely to have any detrimental health effects solely due to the NO2 at concentrations commonly found adjacent to European roadsides. Nitrogen Dioxide (NO2) and nitric oxide (NO) are both Oxides of Nitrogen (NOx). In the atmosphere oxides of Nitrogen also lead to the formation of other pollutants such as ground level ozone and particulate matter.

All combustion processes in air produce NOx emissions. Heat during combustion breaks the binary bonds of ambient Oxygen molecules (O2) releasing energy and allowing freed Oxygen atoms to oxidise plentiful atmospheric Nitrogen. It is expected that between 15 and 25% of NOx emissions from vehicles are readily formed as Nitrogen Dioxide prior to emission. This is referred to as primary NO2 and is released in the exhaust gas. The other 80% or so is Nitric Oxide which given time will convert to Nitrogen dioxide, in the atmosphere mainly as a result of reaction with ozone in the presence of sunlight. It is Nitrogen Dioxide that is associated with adverse effects upon human health.

The principal source of Nitrogen Oxides emissions in Brighton is road transport. In Brighton congested traffic in the city centre in combination with domestic and commercial heating and cooking are important sources.

NOx emissions can be mitigated by after-burning devices or recirculation of exhaust gasses. Emission of NOx are comparatively low for modern petrol cars travelling at intermediate speeds. There is some evidence to suggest that older petrol vehicles have less effective catalytic converters and consequently higher emission of NOx. In comparison diesel vehicles have higher emissions of NOx and a significantly higher ratio of primary NO2. As a general rule the older and heavier a vehicle the higher its emission of NOx, but the most modern diesel cars may be an exception to this rule. That said driving style, speed and engine temperature are also factors that influence the emissions rates. Furthermore dispersion of the emission is influenced by meteorology and the built environment."

Here's what Wikipedia has to say:

Nitrogen dioxide is toxic by inhalation. However, as the compound is acrid and easily detectable by smell at low concentrations, inhalation exposure can generally be avoided. One potential source of exposure is fuming nitric acid, which spontaneously produces NO₂ above 0 °C. Symptoms of poisoning (lung edema) tend to appear several hours after inhalation of a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.
Long-term exposure to NO₂ at concentrations above 40– 100 µg/m³ [as seen in some parts of Brighton and Hove - ed] causes adverse health effects.^[3]

The most important sources of NO₂ are internal combustion engines,^[4] thermal power stations and, to a lesser extent, pulp mills. Butane gas heaters and stoves are also sources. The excess air required for complete combustion of fuels in these processes introduces nitrogen into the combustion reactions at high temperatures and produces nitrogen oxides (NO_x). Limiting NO_x production demands the precise control of the amount of air used in combustion. In households, kerosene heaters and gas heaters are sources of nitrogen dioxide.

Nitrogen dioxide is also produced by atmospheric nuclear tests, and is responsible for the reddish colour of mushroom clouds.^[5]
Nitrogen dioxide is a large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthy levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone. A 2005 study by researchers at the University of California, San Diego, suggests a link between NO₂ levels and Sudden Infant Death Syndrome.^[6]
Nitrogen dioxide is also produced naturally during electrical storms. The term for this process is "atmospheric fixation of nitrogen". The rain produced during such storms is especially good for the garden as it contains trace amounts of fertilizer. (Henry Cavendish 1784, Birkland -Eyde Process 1903, et-al)

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitric oxide:

O₂ + N₂ → 2 NO

Nitric oxide can be oxidized in air to form nitrogen dioxide. At normal atmospheric concentrations this is a very slow process.

2 NO + O₂ → 2 NO₂

For analysis of the AQAP 2011 report: click here.