Air quality expert group - Fine particulate matter (PM2.5) in ... - Defra

PM2.5 in the UK
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4.4 A critical assessment of emission inventories for
modelling of PM2.5 concentrations
54. Modelling concentrations of PM2.5 from emissions data is complicated by
the fact that it requires inventories for a range of pollutants, including direct
emissions of PM2.5 itself as well as its precursor gases SO2, NOx, NH3 and
NMVOCs. These pollutants are emitted in varying amounts from different
sources and exhibit different spatial and temporal behaviour. With contributions
from four different precursor gases and direct emissions to concentrations of
ambient PM2.5, there are therefore more emission sources to include in the
inventories used in modelling PM2.5 concentrations than are required to model
other pollutant concentrations. Many of these, especially non-combustion
and diffuse sources, are very difficult to quantify. Many of the major sources
of PM2.5 in the atmosphere, both primary and secondary, are also difficult to
regulate and control. Because of the wide variety of pollutants and sources that
contribute to direct PM2.5 emissions and to its formation in the atmosphere,
the uncertainties in the emission inventories for each of these pollutants and
sources are compounded in the models used to estimate concentrations.
55. Modellers turn to emission inventories as the primary source of emissions data.
For anthropogenic emissions these are developed mainly by national inventory
agencies. There is a well-established mechanism for reporting national emission
inventories driven by the requirements of international bodies, such as the UN
and EU, under various protocols. The emphasis is on providing inventories using
common methodologies so that inventories provided by different countries are
comparable and are also consistent with other national statistics, for example
on energy consumption. This leads to a method of accounting for emissions
which is ideal for policy-makers, who can identify which are the major sources
and assess the costs and benefits of alternative control strategies. This approach
works particularly well for greenhouse gases where the long-lived nature of
these emitted gases means they can be treated almost as a commodity that
can be traded; this is evident in the concept of “emission trading schemes”.
Using common inventory approaches that define what sources are and are
not included in national inventories is also ideal for tracking progress against
national emission reduction targets. For this, most national inventories are fit
for purpose. They may also be adequate for air quality modelling of primary
pollutants.
56. However, national inventories may fall short of what is required by models for
PM2.5 concentrations. Simply knowing the annual rate of emissions for a given
pollutant and source sector is not sufficient, nor is it sufficient to consider
only anthropogenic sources. The spatial and temporal variation in emissions is
important and this is far less well-understood than the annual rate of emissions
at a national level which is usually derived from national statistical datasets.
The question is how do emissions vary with time of day and day of the week
and season, where exactly do they occur and what are the influences of
meteorological factors? Whilst some of these aspects can be assessed, e.g. for
combustion sources, many cannot, especially for fugitive sources of dust and
many agricultural processes.