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Introduction
Under the initial air quality objectives laid
out under the UK National Air Quality Strategy in 1997, the target objective
for PM10 was 50mg/m3 as the 99th percentile
of daily maximum running 24-hour means.
This allowed for 4 days every year when the 50mg/m3
concentration could be exceeded (in contrast to the 35 days allowed in the EU
limit Value and the, later, loosened UK objective). This put many of the local authorities in East Anglia in danger of
breaching the objective, whether they covered urban areas or rural, due to the
high contribution of secondary particles in this part of the country associated
with its proximity to the European mainland.
Norwich City Council was already funding a PhD
studentship to help them take a wider perspective on their work under the UK
Local Air Quality Management regime.
Part of the work carried out was to help assess the contribution of
urban sources of pollution within the boundaries of Norwich in contrast to
regional sources, from the rest of Norfolk, the UK and mainland Europe. As all the Las in Norfolk appeared unlikely to
achieve the PM10 objective, including the predominantly rural
authorities that had little air quality management capacity, the regional
grouping of Air Quality Officers raised sufficient funds to undertake a
detailed study of particles in the region.
Discussion
The problems caused by transboundary
pollution are almost always an issue to more than one local authority by their
very nature. It is often more of a
problem for heavily populated urban areas which also have a significant
quantity of domestically generated pollution in addition to the ‘imported’
element.
In order to properly assess the magnitude of regional
scale pollution it is necessary to carry out a detailed study consisting of a
mixture of monitoring,
modelling and potentially chemical analysis (at least when
dealing with particles).
Ideally automatic/continuous monitoring stations need
to be identified or set-up both within an urban area and the surrounding rural
locations. In conjunction with
meteorological data (primarily wind direction) this can be used to identify the
differences in concentration between the city centre and upwind and downwind
locations. This in itself should
identify the difference contributions from the transboundary
and domestic contributions. With
particles, monitoring data for sulphate and nitrate
concentrations can also be very helpful in identifying the secondary particle
component which is almost always transboundary due to
the length of time that it takes these particles to form.
Modelling can be used in a variety of ways to help
identify transboundary components:
Local modelling at a city scale can identify local
contributions to total pollution concentrations. However, especially with particles, it is
very hard to set-up models to be sure that the model is correctly predicting
the local contribution as some idea of what this is is
needed in order to tune the model. One potential strategy is to set the model
up and verify and adjust it on a non-transboundary
pollutant such as nitrogen dioxide, and then simply change the emission inputs
to model primary particles. There is a
reasonable amount of evidence to show that this is not ideal as the magnitude
of the errors in multiple pollutant models frequently vary from pollutant to pollutant.
Regional modelling can be carried out in two different
ways. The simplest is to carry out
trajectory modeling where the path of air parcels arriving at the city is
traced backwards in order to identify the geographical source of the pollutants
that they bring with them. When this is correlated with monitoring data it can
produce a crude but still useful indication of the
variations in pollution levels depending on the source airsheds,
and therefore an indication of the magnitude of increase in concentrations when
the incoming air comes from polluted areas.
More complex regional-scale modeling can
be carried out using Eulerian or Lagrangian
models (the latter was used in the Norfolk study). These allow the input of
very large emissions inventories – even at a European scale. Then using detailed meteorological
information they basically combine a trajectory model with the emissions inputs
to calculate how much pollution is being picked up by air parcels as they
travel across the region. Some of these
models contain chemistry modules which allow them to calculate the formation of
secondary pollutants such as ozone, and sulphate and
nitrate particles. These models are
generally very complex and specialized and may be beyond the general reach of
most local authorities, however, as shown by the Norfolk example, this type of work has a large degree of research interest and
links can often be made with academic institutions to help examine the
problems.
Finally, chemical analysis of particles
will help to some extent in separating locally sourced pollution from transboundary. The
basic split being that the transboundary component is
predominantly secondary particles.
However, there is some evidence that there may be a significant element
of primary (combustion-related) particles within the transboundary
component. In addition to this, ‘coarse’
or natural particles such as wind blown dust can travel significant
distances. Saharan dust events have been
known to cause pollution episodes in the United Kingdom, and dust from storms in China have recently been found to be crossing the pacific and being measured
in the United States.
Recommendation
In dealing with transboundary
pollution the initial step has to be to identify the magnitude of the problem so that costs of
any research can be weighed against the potential benefits. This is best done by examining monitoring
data from as many different types of site within a region as possible, and
where pollution patterns at sites follow each other closely, using
meteorological information to try and identify whether these are regional
pollution episodes or whether it is just regionally similar whether affecting
local pollution emissions.
Further Reading
Chatterton, T., 2001, "Regional and
Urban Scale Modelling of Particulate Matter: Can PM10 be Managed at
a Local Level?",
Ph.D. Thesis, School of Environmental Sciences, University of East Anglia http://www.uwe.ac.uk/aqm/research/tc/
Chatterton, T., 2000, "The Relative
Contribution of Local and Distant Sources of Particulates to Eastern England", Report on regional-scale
modelling of PM10 commissioned by a consortium of district councils
in Norfolk.
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