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METHODOLOGICAL DESCRIPTION
(a) Underlying Definitions and Concepts: Air pollution stems from gases and
airborne
particles that, in excess, are harmful to human health, artefacts and
ecosystems. Emissions of air pollutants from anthropogenic activities are often
directly related to the combustion of fossil fuels for energy. However, non-energy-
related emission sources are also significant for some pollutants — for example,
NMVOCs. Emissions of greenhouse gases, or GHGs (e.g. carbon dioxide [CO
2
],
nitrous oxide [N
2
O] and methane [CH
4
]), are excluded from the scope of this indicator
and are described separately in the GHG emissions indicator.
Sulphur Dioxide (SO
2
): The primary product from the combustion of sulphur is SO
2
.
However, other sulphur oxide compounds can also be produced; thus,
when reported,
these compounds are to be jointly referred to as SO
x
(sulphur oxides).
Nitrogen Oxides (NO
x
): The primary combustion product of nitrogen is nitrogen
dioxide (NO
2
). However, several other nitrogen compounds are usually emitted at the
same time, such as nitrogen monoxide (NO), nitrous oxide (N
2
O), etc., and these may
or may not be distinguishable in available test data. Total NO
x
is to be reported on the
basis of the molecular weight of NO
2
.
Volatile Organic Compounds (VOCs): VOCs are defined as any compound of carbon
(excluding CO, CO
2
, carbonic acid, metallic carbides or carbonates, and
ammonium
carbonate) that participates in atmospheric chemical reactions. In some cases, the term
non-methane volatile organic compound (NMVOC) is used to indicate that methane is
exempt from the VOC categorization.
Carbon Monoxide (CO): CO is formed from the incomplete combustion of fossil
fuels. In most countries the transport sector is the main source of CO emissions.
Emissions of NO
x
, VOCs, CO and CH
4
contribute to the formation of ground-level (or
tropospheric) ozone. These ozone precursors can be aggregated on the basis of their
ozone-forming potential to assess the combined impact of the different pollutants. The
relative weighting factors are as follows: NO
x
, 1.22; NMVOCs, 1.0; CO, 0.11; and
CH
4
, 0.014. This methodology is routinely used by the European Environment
Agency (EEA) for its reporting of ozone formation, but the use of such factors does
not yet have broad international acceptance. The
factors are assumed to be
representative for Europe as a whole, but on the local geographical scale, the factors
may vary (for further information regarding uncertainties in these factors, see De
Leeuw 2002).
Particulates: Terms commonly associated with particulate matter are particulate
matter with a diameter less than 10
μm (PM10), total suspended particulate (TSP),
primary particulate and secondary particulate. PM10 in the atmosphere can result
from direct particulate emissions (primary PM10) or from emissions of gaseous
particulate precursors that are partly transformed into particles by chemical reactions
in the atmosphere (secondary PM10). TSP consists of matter emitted
from sources in
solid, liquid and vapour forms, but existing in the ambient air as particulate solids or
liquids.
Secondary PM10 precursors include SO
2
, NO
x
, NH
3
and VOCs. Reliable information
on the relative contribution of VOCs to particulate formation is not available. For
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estimations of quantities of secondary particulates, aerosol formation factors could be
used to assess the aggregated particulate formation potential arising from emissions of
the different secondary pollutants (see De Leeuw 2002). The factors are as follows:
SO
2
, 0.54; NO
x
, 0.88; and NH
3
, 0.64. It should be noted that, as for the tropospheric
ozone
formation factors, these factors are only a best approximation of the relative
contribution of the different pollutants and significant local variations may actually
occur in both urban and rural areas.
Since the objective of this set of indicators is to describe the impact of human
activities on the environment, emissions from natural sources (such as forest fires and
volcanic eruptions) should be excluded from the indicator.
The indicator should therefore present annual air pollutant emissions and their
percentage changes. Emission intensity expressed as quantities of pollutant emitted
per unit of gross energy use could be used to assess sustainability. It would also be
useful if policy-relevant information on emission targets were included in the
indicator (if such targets exist for a given country). This would
allow an assessment of
the ‘distance to target’ for a country, and hence whether existing pollution abatement
measures are sufficient to meet existing national or international targets.
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