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Nitrogen Dioxide
A mixture of nitrogen dioxide (NO2) and nitric oxide (NO) is emitted by combustion processes. This mixture of oxides of nitrogen is collectively termed NOx. NO is subsequently oxidised to NO2 in the atmosphere. NO2 is an irritant to the respiratory system, and can affect human health. Ambient concentrations of NO2 are likely to be highest in the most built-up areas, especially where traffic is congested, or buildings either side of the street create a “canyon” effect, impeding the dispersion of vehicle emissions. In this report, concentrations of nitrogen dioxide are expressed as microgrammes (i.e. 10-6 grammes) per cubic metre (µg m-3). To convert to parts per billion (ppb) if necessary, the following relationship should be used: 1 ppb = 1.91 µg m-3
for nitrogen dioxide at 293K (20°C) and 1013mb. Monitoring of NO2 is mainly carried out by two methods; automatic NOx analysers and NO2 diffusion tubes. Automatic analysers are based on the chemiluminescent method, and provide continuous monitoring of NO, NO2 and total NOx. The results can be directly compared with air quality objectives based on short-term measurements such as the hourly mean. This technique is the reference method for the EC 1st Daughter Directive. However, this automatic equipment is expensive and is commonly supplemented by a low-cost indicative method, diffusion tubes. These are passive samplers, which work by absorbing the pollutant direct from the surrounding air and need no power supply. Tubes are exposed for periods of typically 4-5 weeks, providing an average result for the exposure period. Although diffusion tube data cannot be compared directly with air quality limit values based on short-term averages, the low cost of diffusion tubes means they can be used to give wide spatial coverage, and are useful for screening studies, identifying areas with high concentrations of NO2, which can then be targeted for monitoring using more sophisticated techniques. NO2 is monitored using the automatic chemiluminescent technique at four sites in Northern Ireland, of which two belong to the Automatic Urban Network. The sites are listed in Table 4.1, and their locations are shown in Figure 4.1. Table 4.1
Automatic NO2 Monitoring Sites
Many District Councils in Northern Ireland carry out diffusion tube monitoring of NO2. Each District Council may operate several sites, which are in some cases part of the Nitrogen Dioxide Network. The total number of sites operated by each District Council, and the number belonging to the Nitrogen Dioxide Network, are listed in Table 4.2. As there are so many, full site details are provided in Appendix 1, Tables A1.1 (2000) and A1.2 (2001). Table 4.2
Diffusion Tube NO2 Monitoring Sites
NO2 diffusion tube monitoring sites in Northern Ireland are categorised as follows: · (A) Roadside; 1-5m from the kerb of a busy road. · (B) Intermediate; 20-30m from the same or equivalent busy road. · (C )Urban Background; >50m from any busy road and typically in a residential area. · (D) Rural Background; sites > 50m from any busy road, in a rural area. Locations of the NO2 diffusion tube monitoring sites in the Roadside category are shown in Figure 4.2, and NO2 diffusion tube monitoring sites in other categories are shown in Figure 4.3. These Figures show the locations of sites operating in 2001. District Councils that participate in the Nitrogen Dioxide
Network submit monthly measurements from typically four locations within their
area: two Roadside and two Urban Background. (Prior to 2001, the Network
included Intermediate sites. However, as from January 2001, most participating
District Councils now submit data from two Roadside and two Urban Background
sites, instead of one Roadside, one Intermediate and two Urban Background as
previously).
The World Health Organisation has set non-mandatory guide values for NO2. Within Europe, NO2 is covered by the 1st Daughter Directive, 1999/30/EC. In the UK, the Air Quality Strategy sets objectives for this pollutant, for protection of human health and ecosystems. Limit values and objectives for NO2 are shown in Table 4.3. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Table 4.3 Limit
Values and Objectives for Nitrogen Dioxide
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AUTOMATIC NITROGEN DIOXIDE RESULTS Table 4.4 below shows NO2 results from automatic monitoring sites in Northern Ireland, from 1992 onwards. The statistics shown are the maximum hourly mean, the annual mean, and the number of hourly means greater than 200 µg m-3 (the hourly EC limit value and AQS Objective). Annual data capture is at least 75% except where stated. Table 4.4 NO2
Results from Automatic Monitoring Sites
a)
Limited data capture: Belfast Centre 1992 (60%) b)
Limited data capture: Londonderry 1997 (25%) No sites exceeded the 1st Daughter Directive limit and AQS objective of 200 µg m-3 for the hourly mean more than the permitted 18 times per calendar year. The annual mean has been below the limit value of 40 µg m-3 at all sites since 1994. These sites are mostly Urban Centre and Urban Background (Newry Trevor Hill is a Roadside site, but in a smaller town). Higher values may be expected at roadside and kerbside locations in urban centres, and exceedences of the annual mean limit in such locations cannot be ruled out. The EC and AQS vegetation protection limit of 30 µg m-3 total NOx is not applicable to any of the above sites, as they are all in built up areas. Annual mean NO2 concentrations, for 2000 and
2001, from all NO2 diffusion tube sites in Northern Ireland,
are provided in Appendix 1. (No annual mean is shown where the site was in operation
for less than 6 months of the year). Table 4.5 provides a summary of average NO2
concentrations, for 2000 and 2001, at the four site types. The average
concentration for each site type is the arithmetic mean of all the individual
site annual means.
The averages in Table 4.5 are based upon all sites in Northern Ireland, whether or not they are part of the NO2 Network. The annual averages were found to be in most cases comparable with the averages reported by the NO2 Network for the Northern Ireland region, which are as follows: 2000: Roadside 29 µg m-3 , Intermediate 19 µg m-3 and Urban Background 13 µg m-3 . 2001: Roadside 27 µg m-3 , Urban Background 14 µg m-3 . During 2000, four sites had annual mean NO2 concentrations greater than the EC Directive limit and AQS objective of 40 µg m-3. These were as follows:
In 2001, the following six sites, all Roadside, exceeded the annual mean limit value of 40 µg m-3.
These findings indicate that exceedences of the annual mean limit for NO2 may occur not only in large urban centres, but also at roadside locations in smaller towns and cities with frequent heavy or waiting traffic, or street “canyon” effects. Emission inventory data for the UK are available from the National Atmospheric Emissions Inventory (NAEI), on the World Wide Web at www.naei.org.uk. Total estimated UK emissions of NOx have shown a steady, approximately linear decrease between 1990 and 2000, of around 5.4% per year. This is due to reduced emissions from two major sources: power generation, and road vehicles (the latter partly as a result of the introduction of catalytic converters). In greater Belfast, road traffic is the dominant source of NOx, accounting for 60% of the city’s total NOx emissions2; in this respect, Belfast might be expected to show a similar emission trend to the total UK. Hence, ambient NO2 should show some downward trend. Trends at Automatic Nitrogen Dioxide Sites Figure 4.4 shows how annual mean NO2 concentrations have changed at the automatic sites in Northern Ireland. Only two sites have been in operation long enough to assess trends: the Belfast Centre and Londonderry AUN sites. Regression analysis (Theil’s non-parametric analysis) shows a significant downward trend (at the 95% confidence level) in annual mean NO2 concentration at Belfast Centre, but not at Londonderry.
(data capture at least 75%) Trends at Diffusion Tube Nitrogen Dioxide Sites A substantial proportion (40%) of the NO2 diffusion tube sites operated by District Councils in Northern Ireland are also part of the NO2 Network. One of the objectives of this Network is to investigate long-term trends in concentrations of this pollutant. The Network has been in operation for nine years, thus it is possible to identify trends. Table 4.6 shows annual mean NO2 concentrations for Northern Ireland, based on data from the NO2 Network. These statistics are based upon the Network sites only, as very few of the non-Network sites had long-term datasets available. Table 4.6 Average
NO2 Concentrations, NO2 Network Diffusion Tube sites in Northern
Ireland
Average concentrations appear to have fallen slightly since the mid 1990s. Regression analysis shows that this downward trend in average NO2 concentration is significant (with 95% confidence limit) for all three site categories. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
