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

5.3.1 PM2.5 modelling for public information and forecasts
20. PM2.5 was added to the UK’s Daily Air Quality Index (DAQI) and public
information systems in January 2012. Models are required to support the issuing
of daily air quality forecasts for this pollutant. The requirement for the DAQI is
to forecast a 24-hour mean concentration of PM2.5. The range of concentrations
and breakpoints for the PM2.5 index are illustrated in Table 5.2.
Table 5.2: Daily Air Quality Index (DAQI) bands and breakpoints for PM2.5.
Modelling PM2.5 and the future
Index 1 2 3 4 5 6 7 8 9 10
Band Low Low Low Moderate Moderate Moderate High High High
µg m -3 0-11 12-23 24-34 35-41 42-46 47-52 53-58 59-64 65-69
Very
High
70 or
more
21. In addition, the following information is required from modelling for the PM2.5
daily forecasts in the UK:
• prediction of the headline “worst-case” pollutant level in each of the
nominated zones and agglomerations;
• within each zone further determination of the differences in pollutant
concentrations between rural, background and roadside locations; and
• public forecasts for at least 24 hours ahead and model runs to support a
longer term outlook.
22. The operational concerns for modelling PM2.5 for the daily air quality forecasts are
similar to those for forecasting other pollutants such as PM10, nitrogen dioxide
(NO2) and SO2. In particular, the accuracy of the PM2.5 forecast is dependent on
the accuracy of the weather data and the emissions inventory data or chemical
schemes which are required to configure the model. Models used for forecasting
also need to be able to operate subject to the following constraints:
(a) Run-time. The model needs to be able to run on a daily basis and reliably
provide the results in time for the daily air quality forecast to be issued.
(b) Sufficient temporal resolution. As a minimum the model is required to
output daily mean concentrations for the following day for the forecast.
(c) Ability to cover the domain required. The model must run over the
required domain and allow the forecasts for different location types to be
determined. For PM2.5 forecasting a model run for a small domain will need
to include the impacts of emissions and transport over a much wider area.
(d) Sufficient output information. If a pollution episode is forecast then further
details of the likely source or components of the PM2.5 may be required in
order to issue further advice to the public on possible health effects or on
how to reduce the emissions which are contributing to the episode. To help
in providing this further information, a chemical transport model such as
CMAQ can provide the breakdown of PM2.5 components in a speciation
plot such as that illustrated in Figure 5.4.
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