| |
|
The air pollution
concentrations in an urban area are the combined result from emissions within
the urban area itself, and the air pollution coming from outside. The
contribution from outside – the extra-urban contribution – is the result of
man-made emissions from near-by activities, from neighbouring cities, as well
as the combined effect of emissions from upwind areas
sometimes up to several thousand km away. Even contributions from the
hemispheric/global scale may be important, which is the case for instance for
ozone. Natural pollutants may also be important, such as for particulate matter
(e.g. Saharan dust, marine aerosols-"sea spray") and for ozone
(influx of stratospheric ozone into the troposphere and boundary layer).
The extra-urban (often
called "regional") pollution contribution is particularly important
for particulate matter PM (PM2.5, PM10) and for ozone.
The high regional PM and ozone is mainly a result of secondary particle
formation from precursor gases (for PM: mainly SO2, NOX,
NH3 and VOC; for ozone: mainly NOX and VOC). Considerable
secondary PM and ozone formation is due to extensive precursor gas emissions in
large upwind areas with air flow transport over several hours and more, such as
between European regions. During the air transport certain meteorological
conditions must prevail, such as strong sunshine for ozone formation, and no
precipitation.
Apart from this
contribution from secondary formation of pollution, large
sources/emitters/industrial areas of primary pollutants (primary PM, NOX,
etc.) quite near, but outside the urban area may, when it is upwind, of course
contribute significantly to the urban concentrations.
The importance of the extra-urban
(regional) contribution is demonstrated in Figure 1. The figure shows
statistics from the data in AirBase http://air-climate.eionet.eu.int/databases/airbase.html,
the air pollutant data base held by the European Topic Centre for Air Quality
and Climate Change (ETC/ACC) for the European Environment Agency (EEA) http://org.eea.eu.int/. AirBase
contains data from several hundreds of monitoring stations across Europe, of
different types (Rural, Urban, Traffic, etc.). The figure represents data for
the year 2000 and shows, for each compound (NO2, PM10,
Ozone) the average, and 10th and 90th percentile of
concentrations for all of each of the station types (rural, urban, traffic).
Figure 1: Data in AirBase on pollutant
concentrations at monitoring stations in Europe, 2000, showing the typical
concentrations at rural, urban and traffic stations, annual average and
short-term percentiles (LV: limit value; TV: target value; NO2
max19: 19th highest hour in a year; PM10 max36: 36th
highest day in a year; Ozone max26: 26th highest daily max 8-hour
average)..
Note that the rural,
urban background and traffic stations do not in general represent adjacent
areas (that is, not for all cities there is a near-by rural station), although in many cases this is true. Still, the large
number of stations implies that the figures give a good indication of the
average rural, urban and traffic concentrations in Europe.
The information in the
figure can be summarised as follows:
For NO2,
the average rural background concentrations in Europe
are typically 50-60% of the average urban concentrations. For PM10,
the rural contribution is about 90%! For ozone, the figure demonstrates the
well-known effect that the urban concentrations are typically lower than the
rural ones, since ozone is most often reduced due to chemical reaction with NO
inside the city, which produces NO2.
In order to work effectively on
the air pollution situation in the city, it is thus obviously important for
local authorities and air pollution control departments to know how to assess
the extra-urban contribution and to consider the possibilities to abate the
extra-urban contribution (by contacting/working with regional/national
authorities).
|