4. Additional criteria for ozone sampling points

Type of sampling point

Objectives of measurement

Representativeness (1)

Macro-scale siting criteria

Urban background locations for ozone assessments

Protection of human health:

to assess the exposure of the urban population to ozone, i.e. where population density and ozone concentration are relatively high and representative of the exposure of the general population.

1 to 10 km2

Away from the influence of local emissions such as traffic, petrol stations, etc.;

vented locations where well mixed levels can be measured; where relevant and to the extent possible, locations frequented by sensitive population and vulnerable groups, such as schools, playgrounds, hospitals and homes for elderly;

locations such as residential and commercial areas of cities, parks (away from trees), wide streets or squares with very little or no traffic, open areas characteristic of educational, sports or recreation facilities.

Suburban locations for ozone assessments

Protection of human health and vegetation:

to assess the exposure of the population and vegetation located in the outskirts of the urban area, with the highest ozone levels to which the population and vegetation are likely to be directly or indirectly exposed.

10 to 100 km2

At a certain distance from the area of maximum emissions, downwind following the main wind direction/directions during conditions favourable to ozone formation;

where population, sensitive crops or natural ecosystems located in the outer fringe of an urban area are exposed to high ozone levels;

where appropriate, some suburban sampling points also upwind of the area of maximum emissions, in order to determine the regional background levels of ozone.

Rural locations for ozone assessments

Protection of human health and vegetation:

to assess the exposure of population, crops and natural ecosystems to sub-regional scale ozone concentrations.

Sub-regional levels

(100 to 1 000  km2)

Sampling points may be located in small settlements or areas with natural ecosystems, forests or crops;

representative for ozone away from the influence of immediate local emissions such as industrial sites and roads;

at open area sites.

Rural background locations for ozone assessments

Protection of human health and vegetation:

to assess the exposure of crops and natural ecosystems to regional-scale ozone concentrations as well as exposure of the population.

Regional/national/continental levels

(1 000 to 10 000  km2)

Sampling points located in areas with lower population density, e.g. with natural ecosystems, forests, at a distance of at least 20 km from urban and industrial areas and away from local emissions;

avoid locations which are subject to locally enhanced formation of ground-near inversion conditions;

coastal sites with pronounced diurnal wind cycles of local character are not recommended.