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

PM2.5 in the UK
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26. This was investigated using a trajectory model and OpenAir to define the
possible source of SO4 2- during the period (Carslaw, 2011). The trajectories were
three-hourly and were coloured according to the measured SO4 2- concentration
(assumed to be constant for each hour of the day) (see Figure A2.3.6). A
preliminary inspection of the trajectories and emission inventories suggests
a potentially significant contribution from ships. Furthermore, comparison
between TNO and EMEP emissions estimates show that for ships, SO2 emission
rates can differ by up to a factor of two. The combination of errors in model
trajectory estimates, emissions sensitivity tests and further data mining should
provide an important insight into these periods of high PM concentrations and
the role of shipping emissions.
Figure A2.3.6: Trajectory of observed daily SO4 2- in April 2008.
PM10 elemental carbon
27. The daily analysis of EC predictions for the final five months of 2008 indicates
that the model underpredicts observed EC at London North Kensington
in 2008, giving an average concentration of 0.44 µg m -3 compared with a
measurement of 1.5 µg m -3 . There are a number of possible reasons for this,
including underestimation of the contribution that EC makes to overall PM10
emissions. For road traffic in particular, and other sources in general, a more
comprehensive calculation of the emission of EC (and primary organic carbon
(POC)) should be made as part of future model runs.
Estimating POC and SOC from OC measurements
28. Prior to comparing the CMAQ predictions of POC and secondary organic
carbon (SOC), it was necessary to split the measurement of OC into primary and
secondary components. To do this we used the EC tracer approach described in
Jones and Harrison (2006), giving average estimates of observed POC and SOC
at London North Kensington of 1.9 and 1.3 µg m -3 respectively.