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

PM2.5 in the UK
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3.4.2 Relationship of PM2.5 with PM10, NOx and NO2
17. The diurnal pattern for PM2.5 has been discussed above in isolation. The
pattern in relation to PM10, nitrogen oxides (NOx) and nitrogen dioxide (NO2)
is examined in this section. Figure 3.9 shows the pattern for all four pollutants
during 2010 at eight urban background sites across the UK where data capture
was > 90% for all four pollutants. The PM2.5 pattern is similar to that for PM10,
with the latter concentrations being around 5 µg m -3 higher, although it is
noticeable that the lowest PM2.5 concentrations occur at around 15.00, while
those for PM10 are around 05.00. The minimum for NOx and NO2 levels is also
around 04.00-05.00. Both PM2.5 and PM10 show much smaller diurnal variation
than is the case for NOx and NO2. This is consistent with a more limited role for
local emissions in the case of PM2.5 and PM10, and, conversely, a more important
role for regional emissions of PM2.5 and PM10.
18. NOx patterns will be dominated by motor vehicle emissions; this is consistent
with the strong peak in concentrations during the morning rush hour (around
08.00-09.00) and to a lesser extent during the evening rush hour (around
17.00). There is evidence of a contribution of the morning rush hour to PM2.5
concentrations, but not the evening rush hour.
19. It is also evident in Figure 3.9 that PM2.5 and PM10 concentrations increase
during the late evening to a peak around 20.00-21.00, at a time when NOx
and NO2 concentrations are decreasing. Fuller et al. (2011) have suggested
that domestic biomass combustion may result in elevated PM concentrations
in the late evening and weekends based on their work using tracers for wood
smoke (levoglucosan). The late evening peak may thus reflect a contribution
from domestic sources, including both heating and cooking (Laxen et al., 2010),
although evidence on the diurnal cycle of emissions related to domestic heating
from gas and coal does not support a late evening peak from this source (Jenkin
et al., 2000). The late evening peak will to some extent reflect the more stable
atmospheric conditions at this time of the day, but this cannot be the main
reason, otherwise NOx emissions would not be decreasing over this period. The
late evening peak may also reflect a contribution from volatile PM condensing
on ambient particles, which is also evident in higher night-time nitrate
concentrations (Laxen et al., 2010).