Modelling the dispersion of particulate matter in
air

There are significant pressures for EU-wide
modelling— but what are the current capabilities?

Europe is poised to establish regulations to limit the concentration of
particulate matter (PM) in ambient air under the auspices of the Air
Quality and Assessment Directive. PM10, PM2.5 and PM0.1 are defined as
particulate matter with a diameter smaller than 10 µm, 2.5 µm and 0.1 µm
respectively. These regulations are intended to set mandatory air quality
standards with which Member States will be expected to comply, yet a
number of recent studies suggest that elevated particulate concentrations
in urban areas during pollution episodes are related to non-local or even
transboundary pollution. These studies raise serious concerns about how
best to control ambient concentrations of PM, and about the effectiveness
of national air quality regulations to control exposure to particulate
matter in urban areas. At the same time, certain Member States have
expressed a wish to carry out modelling exercises to explore the
cost-effectiveness of vehicle emission abatement measures as part of the
Auto/Oil-2 Programme.

This article provides an overview of new initiatives in the area of
particulate matter modelling that are aimed at unravelling these difficult
issues.

PM MODELLING—THE ESSENTIAL BUILDING BLOCKS

As with the modelling of other pollutants, robust modelling of future
particulate concentrations initially requires the following input data:

base year emission inventories and spatial distribution;

base year validation data; and

projections for future emission trends.

The particulate issue is greatly complicated by the need to consider
not only man-made and natural sources of particulates that are emitted
directly to the atmosphere (primary particulates), but also particulates
formed in the atmosphere through physical and chemical reactions
(secondary particulates). Secondary particulate matter includes inorganic
aerosols (ammonium sulphate and ammonium nitrate), and organic aerosols,
in which case changes in non-PM precursor emissions are important.
Resuspension of dust that had settled onto surfaces and transboundary
transport of PM add further layers of complication.

EMISSION INVENTORIES

Emission inventories are only available from a small number of
countries including Germany, The Netherlands and the UK. Each has
different source categories and none are comprehensive in their coverage
of all type of sources. They may also be limited in their spatial
coverage, e.g. confined to single cities. The estimated relative
proportions coming from each source type also vary between different
inventories. As yet, the relationship between emission estimates and
ambient air quality is largely unconfirmed by source attribution analysis.
Table 1 provides a comparison of the status of various prominent
inventories.

Comparison of prominent particulate matter
inventories

emission inventory

spatial coverage
and
disggregation

primary
PM

secondary
PM

resuspension
of PM

TNO

approximately 50x50 km Europe grid

man-made
PM10, PM2.5 & PM0.1

none

none

UK

national

man-made PM10

none

none

Birmingham

1x1 km West Midlands grid

man-made PM10

none

none

London

1x1 km Greater London grid

man-made PM10

none

none

Germany

national

man-made PM10

none

none

Netherlands

national

man-made PM10

none

none

Table 1
Emission inventories are only
available from a small number of countries; so far their reliability
remains questionable.

Limited PM emission data means that, for Europe as a whole, PM source
apportionment relies heavily on judgement and it is widely recognized that
current estimates are subject to significant error. Where emission
inventories are derived from energy consumption figures, assumptions also
need to be made concerning the extent to which abatement measures are
applied. The basis for forward projections of emissions is also a source
of uncertainty.

MODELLING OF AMBIENT PM
CONCENTRATIONS

A number of models are beginning to be examined as possible candidates
for modelling particulate matter across the European Union. These range
from complex dispersion models to simple empirical relationships derived
from the comparison of emissions and air quality. The detailed
capabilities of the models are important. In all cases they rely on
questionable emission inventories. Some only model certain types of
particulate, whilst others are not validated or validated for only a
limited number of cities. Although much work is currently under way to
improve this situation, so far, no single model can be considered reliable
and comprehensive in its treatment of PM at the EU scale. Thus, the
results from any such modelling work need to be treated with extreme
caution.

VALIDATION OF MODELLING RESULTS

Direct measurements of PM10 are necessary to
validate the models. PM10 data are scarce in all
countries and, to the best of our knowledge, data have only been collated
at the EU level from France, Germany, Luxembourg, Netherlands, Spain,
Sweden and the UK, totalling 70 monitoring sites across the EU as a whole
(all types of sites). PM2.5 data are even more
scarce, with data collated at EU level limited to only a few sites in a
few countries. In addition, there are no standard methods for measuring
PM10 or PM2.5 and, as a
consequence, measurements using different methods are not directly
comparable. For instance those measurements which employ heated elements
may lose the volatile particulate fractions, and certain measurement
methods capture a narrower range of particles than other methods, e.g.
more of the smallest size fractions escape through the sampling
device.

CONCLUSIONS

No single model is comprehensive in its treatment of particulate matter
and there remains a real need to structure models that can address all
man-made and natural sources of primary and secondary particulate
matter.

Moreover, the uncertainties associated with current modelling
capabilities for particulate matter are considerable, and these
uncertainties will be added to the uncertainties surrounding the health
impacts of particulates. Validation of the emission inventories and models
will be crucial in the early stages of attempting to explore this complex
area using air pollution models.

Under these circumstances, the legislator should set air quality limit
values for PM at a pragmatic level. In particular, some allowance should
be made in the initial stage for occasionally exceeding limit values. Once
further research has reduced the current uncertainties, a tightening of
this allowance might be justified.