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. 

Deadly air pollution rising despite clean air technology, says Brighton and Hove Council report

Graph showing NO2 levels rising in Brighton and Hove after 2008

Nitrogen Dioxide (NO2) pollution has been increasing since 2008 in parts of Brighton and Hove, despite technology improvements that were supposed to reduce vehicle emissions.

The evidence is given in the graph above, taken from Brighton and Hove City Council (BHCC) Air Quality Action Plan 2011.

NO2 is linked to increased rates of death from all causes.

It was thought that improvements in engine design since the 1990s would lead to ongoing improvements in pollution levels. This appeared to be the case up to 2008 - but since then NO2 pollution has been increasing again.

The report says:

"It was thought that improvements to engine and fuel technology would reduce NO2 exceedences over time even if little action was taken locally. Nationally and internationally this has not happened to the extent that was anticipated.  In many cities estimates for air quality improvement made in the period 2000 to 2010 have been found to be over optimistic. That said locally in Brighton significant improvements have been recorded up to 2008.  It is likely that AQO compliance very close (<10 metres) to some heavily trafficked and congested roads is not likely to happen prior to 2015. Therefore further local intervention is required.  Brighton and Hove’s reports have consistently confirmed that road traffic is the primary (but not exclusive) contributing factor to poor air quality in the city. Therefore the majority of improvement measures described in the report are related to road traffic including the decisions people make when travelling.

In the main poor air quality in Brighton and Hove is a combination of stop-start heavy vehicles, accelerating engines, and the proximity of buildings adjacent to traffic." 

The report makes a strong case for the importance of this issue. Although some forms of modern air pollution are invisible, they remain deadly:

"Decision makers should be aware that reductions in ambient pollution led by investment in technology is a multiple win situation; for the urban environment, human-health and a positive growth area for the economy. Improvements in air quality aim to protect the most vulnerable members of society; most especially the very young and those with existing respiratory illness. Given that prevention is better than cure it is in the financial interests of government and the health service to save capital by investing in a healthier urban environment. Where the air is poor; airborne pollution can contribute to heart and lung conditions and influence life expectancy. Based on past figures the government white paper on sustainable transport1 estimated that poor air quality costs the UK economy £19 billion per year. As this assessment does not include all airborne pollutants it is likely to be an underestimate. Report by the committee on the medical effects of air pollution suggests fine particulate have a small contribution to 200,000 UK deaths in 2008  equivalent to a variable influence on one-third of annual mortality. A number of international studies also demonstrate a link between urban Nitrogen Dioxide (NO2) and all cause mortality. The latest evidence now suggests that poor air quality is a greater risk to our society than; alcohol abuse, passive smoking, obesity, flooding and road traffic accidents."

The report identifies modal shift as a key means for reducing air pollution. But in my opinion the report fails to highlight this or put it in the context of what other countries have achieved. This is a cautious approach to the problem in a country where we are reluctant to change our reliance on the private motor car. Modal shift means getting the population to choose sustainable transport en mass. For example, in Holland up to 33% of trips are made by cycle, in the UK this is less than 2%. There is much more discussion on elements of planning for modal transport in other parts of this blog. These points are made in he report, but in a muted way:

"Local business and the general public are both the cause and the solution for the problem of poor outdoor air. This action plan follows an open twelve-week consultation period and sets out how airborne pollution can be reduced in order to improve the outdoor air in Brighton. The final action plan takes account of consultee comments and includes new references to the London Mayor’s Air Quality Strategy and the Sussex Low Emission Strategy. In 2011 Brighton and Hove have been awarded a grant under the Local Sustainable Transport Fund (LSTF) for reductions in emissions and improvements to local air quality.

The former AQAP (Air Quality Action Plan 2006) and LTP1 (First Local Transport Plan) acknowledged that road traffic was main course and encouraged modal shift; namely avoidance of private car usage and reduction in total traffic numbers, promotion of walking cycling and public transport. It has been identified that most of the traffic within the city is locally generated, with two thirds of vehicles on the road at any one time making trips which begin and end in the city. Many of these shorter journeys could be achieved by more sustainable means.

A successful program to reduce car usage makes sense in terms of reducing fuel consumption, minimising carbon footprint, freeing up road space for all users and making neighbourhoods more conducive to; cycling, walking, social cohesion and play. In our city’s AQMA (Air Quality Management Area), private cars typically contribute between: one-quarter and a half of roadside NO2 pollution. Between three-quarters or a half can arise from other sources notably; trucks, vans, taxis and buses. The majority of locally derived pollution comes from either diesel vehicles or older petrol vehicles. Therefore a policy of travel choice has to be part of a much more comprehensive air quality action plan.

The main reasons for tackling poor air quality are the links with quality of life and the need to minimise the risk to human health."

The report also makes some interesting general observations on the poisons contained in vehicle emissions, and on transport in the city. But there is a tendency to look on the bright side. When the report says say there has been a 27% increase in cycling, and omits to mention that we started at 1.5% of all trips, we are still making less than 2% of trips by cycle. We are far behind our continental partners. On the positive side, the report does make the case that, when compared with the rest of Britain, Brighton and Hove is going in the right direction. Most of the nation is, apparently, increasing its reliance on the car, as B&H gradually reduces it.

"Since 2007 a decrease in traffic volumes has been observed at some locations (in B&H). Further analysis of traffic flow changes is included in the 2010 Air Quality Progress Report (AQPR)3.  Bus patronage has continued to increase every year since 1993. This
compares favourably to the national trend, which has shown a more substantial growth in car use between 1993 and 2007.

The city’s main commercial bus operator, the ‘Brighton and Hove Bus & Coach Company’ has achieved an increase in passenger journeys of approximately 5% each year since 1993. In addition, cycling in Brighton & Hove has substantially grown in recent years – a 27% increase was recorded in the 2006-2008 period.  The seafront cycle lane has one of the highest daily flows of bicycles anywhere in the UK.

Based on 2001 Census figures, vehicle ownership in Brighton & Hove is the lowest in the south-east region (comparable to a London Borough) and one of the lowest nationally. Across the city, there is an average of less than 0.9 cars or vans per household, compared to 1.3 in the South East.  Most traffic in the city is locally generated – some two-thirds of vehicles on the road at any one time are making trips, which begin and end within the city. Journeys from adjacent areas such as Shoreham, Worthing and Lewes also account for a significant proportion of the total traffic." 

Later, the report emphasises the point that many in Brighton still regularly use cars for short journeys (3 miles or less). That's 27,000 necessary journeys a day!

"Cycling in Brighton & Hove has seen substantial growth in recent years. (27% increase recorded in the 2006-2008 period.)  However, around 45% of workers employed in the city use a car to get to work drive less than three miles. This is estimated to be over 27,000 car journeys per day, so there is great potential for increasing safe bicycle use - and also to bring other benefits on both an individual and a wider level."

Link to BHCC AQAP 2011

Air pollution is above EU permitted levels - and getting worse in some areas of Brighton and Hove

21 July 2011. Brighton and Hove City Council today published air quality data that shows pollution is above EU legal levels - and getting worse in some areas of the city, despite efforts to reduce vehicle emissions.

The Council's second Air Quality Action Plan (AQAP) (link below) shows average Nitrogen Dioxide (NO2) concentrations were higher in 2010 than in 2008, and higher than 2009 in some streets. Continuous analysers at Preston Park and at Hove Town Hall recorded a small increase in levels during 2010 compared to previous years.

Green Councillor Ian Davey is cabinet member for transport and the public realm. He said: "This report shows how serious the problems with traffic related air pollution are in the city. Far from improving; air quality in some areas are clearly getting worse."

A council spokesperson said that in 2010, NO2 concentrations were above the legal limit, and worst at roadside locations in Central Brighton and Portslade. A similar situation occurs in other cities in Europe with examples of increasing ambient NO2 including Cambridge and Paris. Since 2007 some local road links have shown evidence of a decline in total traffic tallies.
The spokesperson said the increase in ambient NO2 in Brighton & Hove is likely to be due to the following contributory factors:
• An ageing vehicle fleet on the road
• A higher proportion of diesel vehicles that show no real performance improvement in emissions of NO and NO2
• A higher proportion of older petrol vehicles with catalytic converters that perform less well with time
• Higher regional background NO2 across South East England
• Higher domestic and commercial heating demand during the past two winters; 2009/2010 and 2010/11, and consequently a likely higher total emission from wood burning and gas fired central heating systems.

The second Air Quality Action Plan follows a 12-week public consultation and refers to key documents published during the past year.  It includes a comprehensive set of measures and recommendations aimed at improving air quality in Brighton.

Cllr Davey added: "We are delighted that the success of the bid to the government's Local Sustainable Transport Fund has given us £4 million to spend in the Lewes Road corridor which gives us an opportunity to make a quick start in one of the worst affected areas."

Together with external partnership and council funds, plus support from the local community, the Lewes Road Corridor project will be worth approximately £6 million over four years. Work is expected to begin this year.

The spokesperson added: "The council is also linking air quality into the city's Local Transport Plan and has joined with Sussex partners to initiate a low emission strategy pilot to address the problem county-wide. This will promote initiatives such as electric vehicle use and contribute to local planning policies.

"There has been progress in providing travel choice in the city; however a number of other measures require implementation if the EU and English limits for Nitrogen Dioxide are to be met."

Click here for link to: Brighton and Hove City Council Air Quality Action Plan 2011