Measurement and Monitoring of Nitrogen Dioxide

9. Since 1973, nitrogen dioxide has been measured by the Department of the Environment on a regular basis in central London and, in 1985, following the implementation of the European Community Directive on nitrogen dioxide (85/203/EEC), a network of seven sites was established in traffic-related or industrially influenced 'hot spots'. In August 1996, 50 sites were in operation in the National Automatic Monitoring Networks, measuring nitrogen dioxide using a chemiluminescence method across the United Kingdom to an accuracy of ± 10-11% and a precision of ± 3.5 ppb². The majority of sites in the network are at central urban locations, with eight situated at roadside or kerbside locations and five sites measuring concentrations in rural areas. Figure 2 shows the location and classification of the sites in operation. The Government has made a commitment to continue the expansion of the National Automatic Monitoring Networks and over 80 continuously operating nitrogen dioxide sites should be in operation by the spring of 1997.
10. The information received from the automatic monitoring sites is made available to the public on an hourly basis, together with a daily air quality forecast. by the Department of the Environment via a freephone telephone helpline, CEEFAX, TELETEXT, the Tnternet and a range of other media. Hourly concentrations of nitrogen dioxide are described in bands ranging from 'Very Good' to 'Very Poor'. The Department of the Environment is currently considering amendments to this descriptive scheme.
11. Limit and guide values for the concentration of nitrogen dioxide in air have been specified by the European Community and incorporated into United Kingdom law by the Air Quality Standards Regulations (1989). The World Health Organization (WHO) in 1987 recommended in the Air Quality Guidelines for Europe, two guidelines for nitrogen dioxide expressed as an hourly and a 24 hour average concentration. A recent recommendation of a WHO working group has proposed a lowering of the hourly guideline, the removal of the 24 hour guideline, and the addition of an annual average guideline. The different limits, guidelines, and bands currently in use in the United Kingdom are illustrated in Table 2.
12. Exceedences of the European Community Directive limit value are very rare in the United Kingdom. During 1995 no site exceeded the limit value, although guide values were exceeded at 4 sites in London. Maximum hourly concentrations and average annual concentrations for nitrogen dioxide measured at United Kingdom automatic sites in 1995 are presented in Table 3. Table 3 also gives the number of occasions when the concentration of nitrogen dioxide exceeded 150 ppb, measured as an hourly average.
13. Some of the highest measured concentrations of nitrogen dioxide in ambient air in recent years occurred during a pollution episode in London in December 1991. The episode was exceptional in terms of the duration and magnitude of nitrogen dioxide concentrations, with a peak hourly concentration of 423 ppb, at Bridge Place, Victoria in central London. One interesting feature of this episode, illustrated in Figure 3, is that concentrations at two geographically separate urban background sites, Bridge Place and Earl's Court, West London and a kerbside site located between the two were almost identical for a period of several days, indicating uniformly elevated pollution across the whole of inner London.
14. There has been much debate about temporal trends in nitrogen dioxide concentrations. The longest available time series is for Victoria, central London (Figure 4), although this is not entirely homogeneous as the monitoring site has had to be re-sited twice during this period. The data indicate that in central London neither the annual mean nor the numbers of hours exceeding 150 ppb in each year have shown a significant trend between 1977 and 1995. It is likely that nitrogen dioxide concentrations within traffic-saturated city centre locations have changed little recently, but that concentrations have risen in suburban and rural areas as a result of increased emissions of oxides of nitrogen from road traffic up to 1991. Since then, emissions of oxides of nitrogen have reduced and there are indications of a downward trend in oxides of nitrogen and nitrogen dioxide at some sampling sites.

Table 2. Air quality standards and guidelines for nitrogen dioxide, ppb (µg/m³)

Source/Organisation	Annual average concentration	98th percentile (hourly means in a calendar year)	50th percentile (hourly means in a calendar year)	24-hour average concentration	1-hour average concentration
EC Directive
Limit Value		105 (200)
Guide Values		71 (135)	26 (50)
WHO
Current guidelines for Europe (1987)				80 (150)	210 (400)
Revised guidelines for Europe (1995)	21-26 (40-50)				105 (200)
Department of the Environment
Air Quality Bandings
Very Good					<50 (96)
Good					5099 (96-190)
Poor					100-299 (191-572)
Very Poor					<=300 (573)

Notes on conversion factors:

Values for nitrogen dioxide are given in ppb, the EC Directive (85/203/EEC) specifies values in µg/m³ and they have been converted in this table using a factor of 1µg/m³ = 0.52 ppb (assuming a temperature of 20^oC) The WHO guidelines specify values in µg/m³ (revised guidelines are quoted in both ppb and µg/m³) and they have been converted in this table using a factor of 1µg/m³ = 0.53 ppb (assuming a temperature of 25^oC)

Table 3. 1995 Calendar year statistics for nitrogen dioxide and the number of exceedences of the Panel's recommended Air Quality Standard

Site	Annual average concentration (ppb)	Maximum hourly average concentration (ppb)	Number of hours on which the recommended standard (150 ppb) was exceeded	Number of days on which the recommended standard (150 ppb) was exceeded
Rural
Strath Vaich, Highland	0.5	11	0	0
Ladybower, Derbyshire	8	40	0	0
Lullington Heath, East Sussex	8	43	0	0
Urban
Edinburgh Centre	26	120	0	0
Glasgow	26	81	0	0
Belfast Centre	21	199	6	3
Newcastle Centre	21	91	0	0
Billingham, Cleveland	19	90	0	0
Middlesbrough	17	97	0	0
Leeds Centre	26	109	0	0
Hull Centre	24	93	0	0
Manchester Town Hall	23	181	5	1
Liverpool Centre	26	90	0	0
Sheffield Tinsley	26	119	0	0
Leicester Centre	23	106	0	0
Walsall. West Midlands	24	119	0	0
Birmingham East	22	170	8	2
Birmingham Centre	25	177	4	2
Swansea	22	94	0	0
Cardiff Centre	22	86	0	0
London, Bloomsbury	35	176	2	1
London, Bridge Place	34	217	5	3
London, Cromwell Road	47	170	9	5
London, Earl's Court	28	131	0	0
London, Bexley	22	132	0	0
Southampton Centre	24	95	0	0