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

PM2.5 in the UK
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3.2.1 Diurnal variations of PM2.5 at urban background sites
4. The diurnal cycle of PM2.5 during 2010 is summarised in Figure 3.1a for four
areas of the UK. Concentrations have been averaged by hour of the day
using local time, i.e. they take account of the change from winter to summer
time. The plots show average concentrations for the hour beginning the
measurement period, i.e. the value shown for 23:00 is the average over the
period 23:00 to 24:00. The concentration scale is the same on all the urban
background plots. The delineation of the four areas is shown in Figure 3.1b.
The diurnal cycle is fairly limited in scale, spanning just a few µg m -3 . The central
UK, southern UK and London sites have a very similar pattern, with the lowest
concentrations occurring during the afternoon, between 13:00 and 17:00.
They also show a morning peak, around 08:00-09:00, and a late evening peak,
around 21:00. The concentrations at the northern UK sites are lower overall than
those in the other three areas. There is also no afternoon dip in concentrations
at the northern UK sites; the lowest concentrations thus occur during the early
morning period, around 04:00-06:00. This observation is consistent with the
lesser role played by secondary nitrates in the north of the UK.
5. Figure 3.2 shows distinct differences in the diurnal patterns for winter and
summer at southern UK sites in 2010, although the lowest concentrations still
occur during the afternoon in both seasons. The winter concentrations show
more substantial morning and late evening peaks than do the summer values,
which may be related to greater domestic heating requirements at these times
of day. These patterns are essentially the same as those reported for 2009
by Laxen et al. (2010), who also analysed the diurnal patterns for nitrate, as
well as those for the volatile component of PM2.5, as defined by the FDMS
analysers. They concluded that the afternoon dip was probably due to the loss
of the semi-volatile nitrate particles during the warmer afternoon period. The
morning peak was probably related to road traffic and domestic emissions,
with the late evening peak probably related to domestic sources, both from
heating and cooking. The link with heating can also be seen in the results for
black carbon in Figure 3.23. Laxen et al. (2010) also considered that the evening
peak may in part be due to reduced dispersion arising from the more stable
atmospheric conditions during the night, together with a contribution from
volatile particulate matter (PM) (i.e. nitrate and organic compounds) condensing
on ambient particles at lower night-time temperatures. The average diurnal PM2.5
ranges are nevertheless relatively small, around 5 µg m -3 in winter and 2 µg m -3
in summer, relative to the seasonal mean concentrations of around 17 µg m -3 in
winter and 11 µg m -3 in summer, suggesting that local sources are not dominant.