Urban population exposure
5.3.5 Nitrogen dioxide Urban population exposure
Data on annual mean NO2 concentrations at the end of the 1980s were available from 144
cities in 18 countries, covering a population of 91 million people or 29 % of the urban population of the Region west of the Urals.
The distribution of populations with data was fairly uniform, with the poorest coverage (21 %) in that part of the USSR west of the Urals (Map 5.4). For 19 % of the residents of cities with data, the annual average NO2 level exceeded 60 µg/m3. This is the level above which population studies show effects on the respiratory system from long-term ex-posure; it is used as a reference point be-cause there is no WHO guideline level for long-term exposure to NO2. Only one city, Belgrade, had levels above 100 µg/m3, which is the national ambient air quality standard in the United States. The available data indi-cated no large differences between groups of countries in the distribution of average long-term levels of NO2 and no changes in pollution levels over the 1980s (Table 5.5).
The daily average WHO guideline level for NO2(150 µg/m3) was exceeded in 25 cities inhabited by 21 million people, or by 23 % of people in cities with NO2 data (Table 5.6).
Such levels were more common in the west-ern than the eastwest-ern countries. The days with levels above 150 µg/m3, however, were fewer than 10 per year in most of the cities.
Only in three cities were such concentrations seen for longer periods in the second half of
Table 5.5: Exposure of the populations of cities with data to annual mean concentrations of nitrogen dioxide (NO2)
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the 1980s: almost 30 days per year in Mad-rid and Athens and as many as 88 days per year in Belgrade. In these three cities, the daily average concentrations occasionally ex-ceeded 300 µg/m3.
Rural population exposure
In general, estimated NO2 concentrations in rural areas do not seem to exceed the daily WHO guideline value of 150 µg/m3. This is suggested by EMEP long-range transport model calculations for three different years:
1985, 1988 and 1989 [48] and confirmed by the available NO2 monitoring data. On aver-age, therefore, the rural population of Eu-rope is not exposed to any NO2 concen-trations above the daily WHO guideline value. It should be noted, however, that the calculations as well as most of the measure-ments indicate only relatively large-scale, long-term trends; they do not imply that people living in rural areas never experience any elevated NO2 levels in ambient air.
These may well occur during local, high-level pollution episodes, and from indoor emission sources.
Total population exposure
The available data indicate that at least 7 % of the urban European population (21 mil-lion people) or 3 % of the total European population west of the Urals, experience concentrations of NO2 exceeding the daily
guideline level. If these data are taken as rep-resentative of all cities in Europe and extra-polated to provide exposure estimates, 20 % of the urban population (56 million people), or 8 % of the total population may be ex-posed to NO2 levels above the guideline level. On average, exposure to NO2 in am-bient air does not seem to differ in any im-portant way between eastern and western European cities, and no definite trend in concentrations can be indicated. Indoor sources such as gas stoves have an important impact on the patterns of exposure to NO2, affecting both short-term peak exposure as well as long-term average exposure, and may relate both to urban and to rural popu-lations.
The total European emissions of nitrogen oxides (NOx), which determine ambient NO2 levels, remained approximately stable between 1980 and 1990 at 21 million tonnes per year (Simpson, D., personal communi-cation, 1993). In 1980, 52 % of these emissions stemmed from western Europe and 48 % from eastern Europe; this picture had changed little by 1990, when 51 % came from western Europe and 49 % from eastern Europe.
In general, road traffic is the major source of NO2 exposure and levels above the WHO guidelines in cities in Europe. Motor ve-hicles contribute around 50 % of total emissions in Europe; the second largest source is power and heating plants, which ac-count for another 30 % [54]. In western
Eu-Table 5.6: Exposure of the populations of cities with data to daily nitrogen dioxide concentrations above 150µg/m3,a 1986–1991
rope, the contribution from road traffic is somewhat higher (about 60 %), while power and heating plants give rise to about 20 % of emissions. In eastern Europe, the shares are about equal at 40 % each. While emissions from the two main sources remained more or less constant between 1980 and 1990 in eastern Europe, an increase in traffic emissions in western Europe was offset by a comparative emission decrease from station-ary sources [54]. It should further be noted that, although traffic contributes on average 50 % to total NOx emissions, it contributes significantly more to ground-level NO2 con-centrations in cities because of the low height of these emissions [55]. Thus, reduc-ing population exposure in urban areas to below the daily WHO guideline value would require the implementation of strict emission controls on all road vehicles, and the limiting of traffic in congested areas of cities. Reducing emissions from stationary sources would require improved energy con-servation measures and the installation of NOx reduction equipment in all large power stations and heating plants. So far, there has been no substantial emission reduction on a European scale. The 1987 ECE Protocol on NOx emission reductions only requires emissions to remain stable, which does not appear to be sufficient to ensure the attain-ment of the WHO guideline level for daily exposure.
5.3.6 Ozone
In contrast to the other pollutants discussed in this chapter, O3 is a so-called secondary pollutant; that is, it is not directly emitted but is formed from other pollutants, particu-larly volatile organic compounds (VOC) and NOx, through chemical reactions in the at-mosphere. Although NOx emissions con-tribute to the formation of O3, elevated am-bient NO2 levels also act as an O3 sink. Thus, O3 levels tend to be lower in areas of intense NOx emissions, such as city centres with heavy traffic, and build up slowly downwind
from these sources. The actual amount of O3 formed mainly depends on the level and ratio of precursor emissions, ambient tem-perature, and the amount of sunshine. These factors also determine how quickly O3 is formed. In cities in southern Europe, where temperatures in the summer are high and sunshine is intense, photochemical smog builds up very quickly close to urban centres.
In contrast, the build-up is slower in north-ern Europe where temperatures are lower, and the highest O3 levels usually occur well away from city centres, in the suburbs and surrounding rural areas. Ozone pollution can, therefore, be both a local and a wider problem.
For these reasons, O3 monitoring is relatively rare in city centres and much of it focuses on concentrations in rural areas. In-sufficient urban data were available to allow a separate estimation of urban population ex-posure. Consequently, only rural concen-tration data were used to estimate combined urban and rural exposure. This probably does not lead to an underestimate of the total population exposure for two reasons. In northern Europe, population exposure is, in general, lower in cities than in the surround-ing suburbs and rural areas. Almost no ex-cess O3 is observed in city centres, as has been shown for London [47]. As a result, population exposure is somewhat overesti-mated in northern Europe. In southern Eu-rope, especially in Mediterranean cities, urban O3 concentrations can be high, as the limited monitoring data for Athens suggest.
In this case, the use of data on surrounding rural concentrations seems somewhat to underestimate O3 concentrations in south-ern cities. Population exposure may there-fore also be slightly underestimated. Only 13 % of the total European population, how-ever, lives in large cities in southern Europe.
Since O3 formation and thus O3 exposure depend very much on the weather condi-tions in the summer, observed annual levels vary with meteorological conditions; hot, sunny summers usually lead to greater expo-sure than do cool, wet summers. Expoexpo-sure calculations were therefore based on EMEP
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model results for two different years, 1985 and 1989; the summer of 1989 was particu-larly hot and sunny, while that of 1985 was comparatively cold and wet.
According to calculations based on the EMEP model, the lower limit of the range of the hourly WHO guideline values (150 µg/
m3) was exceeded during both years over most of Europe, the only exceptions being parts of northern Scandinavia. The upper limit of the range of the guideline values (200 µg/m3) was also estimated to have been exceeded in some places during both years.
In 1989, the area with high levels was relatively extensive, including parts of Aus-tria, Belgium, Denmark, France, the Ger-man Democratic Republic, the Federal Re-public of Germany, Greece, Hungary, Ire-land, Italy, Luxembourg, the Netherlands, Poland, Portugal, Spain, Switzerland and the United Kingdom, and extending north to southern Norway and Sweden. In 1985, on the other hand, hourly O3 concentrations ex-ceeded 200 µg/m3 over a much smaller area, affecting mostly central and southern Eu-rope. These patterns compare well with ob-served O3 measurements from the TORa and EMEP OXIDATEb rural monitoring net-works.
Owing to the lack of available data on urban concentrations, two different assump-tions were made about the population that could be exposed to O3. First, it could com-prise the total European (urban and rural) population, minus that of large cities north of the Alps (north of latitude 47º) with a total of 463 million people. This represents the lower estimate and was chosen because, as noted above, hourly concentrations of O3 in northern cities are not likely to exceed 200 µg/m3. Second, the exposed population could include the whole population within the EMEP grid, with a total of 656 million people. This is justified because, although
a Tropospheric Ozone Research (TOR) project under the EUROTRAC programme (a Eureka environmental project).
b EMEP oxidant data collection in OECD-Europe 1985–87 (OXIDATE).
people living in city centres in northern Eu-rope are not greatly exposed to excess O3, concentrations in the suburbs may often be more similar to those in rural areas, so that at least part of the urban population is prob-ably exposed.
On the basis of the EMEP model results, the annual average of maximum daily 1-hour O3 concentrations was calculated for 1985 and 1989. The long-term population expo-sure was calculated assuming that high con-centrations are not likely to last long in the cities of northern Europe (i.e. according to method 1 above).