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

Concentrations and composition of PM2.5
3.6.2 Relationship of hourly mean PM2.5 concentrations with air mass
trajectory
23. Back trajectories provide useful information on the origin of an air mass and
have been used to understand better the origins of PM2.5 concentrations in the
UK. An analysis has been carried out using HYSPLIT 96-hour back trajectories
calculated for every three hours derived from NCEP/NCAR Reanalysis data
(Draxler and Hess, 1997). The whole of each back trajectory, i.e. one every
three hours, has been assigned the hourly mean PM2.5 concentration for the
arrival time at the arrival site. The same concentration is therefore associated
with all the 96 hours preceding the arrival time, giving 96 latitude/longitude
locations along the trajectory with the arrival time concentration. For a large
number of trajectories that cover widely different geographic regions, this
technique provides an indication of the source origin of the pollutant being
analysed. There are of course uncertainties in these calculations, for example in
the accuracy of the back trajectory calculations themselves. It must, however,
be borne in mind that any elevated concentrations at the arrival site will be
assigned to all points along the trajectory, even though the source may have
only contributed at some intermediate point along the trajectory. The analysis
of a large number of trajectories can, nevertheless, provide a good indication of
the geographic locations most strongly associated with elevated concentrations
of PM2.5.
24. As an example, Figure 3.12a shows the time series of daily mean PM2.5
concentrations at the Bexley Slade Green site in East London for 2009. It
is apparent from this figure that there are clear periods when the PM2.5
concentrations are elevated, particularly in the first half of the year. The bottom
plot, Figure 3.12b, shows the 96-hour back trajectories month by month at the
Bexley site. The low concentrations in June-August tend to be associated with
trajectories from the Atlantic and the north and the absence of trajectories from
continental Europe. Almost all the high concentration periods are associated
with air mass trajectories from the east, although in some cases the pattern
is complex. For example, in March 2009 there was a period of 4-5 days when
anticyclonic conditions dominated to the east of the UK which set up circulatory
air masses from continental Europe. Such conditions would ensure that
precursor emissions would have accumulated as air masses passed over areas of
high precursor emission source strength, and this would have been combined
with sufficient time for secondary aerosol to be produced.
25. The trajectory analysis has been extended to seven sites across the UK, as
shown in Figure 3.13 for 2009 (the sites are broadly ordered from north to
south). For clarity, the trajectory point concentrations have been smoothed
to indicate approximate source origins, similar to the work of Seibert et
al. (1994) and Stohl (1996). The purpose of Figure 3.13 is to highlight the
approximate spatial origins that contribute the highest PM2.5 concentrations
measured at the UK sites. There is consistency across the different sites in that
the highest concentrations of PM2.5 are dominated by air masses from the east
and south-east. The higher background concentrations seen for the urban
background Edinburgh site in comparison with the rural Auchencorth Moss
site will be due to local urban sources elevating the overall concentrations
measured in Edinburgh, irrespective of the trajectory direction. However, it is
clear, for 2009 at least, that the contribution from mainland Europe to high
PM2.5 concentrations at these northern UK sites is less than for sites further
59