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

PM2.5 in the UK
48
PM2.5/µgm -3
20
15
10
5
0 5
12
hour
18 23
Winter Summer
20
15
10
5
0 5
12
hour
18 23
Figure 3.2: Variation of PM2.5 concentrations (µg m -3 ) at urban background sites
by hour of the day in 2010 at southern UK sites (n=10) in winter and summer.
The shading represents the 95% confidence interval of the mean.
3.2.2 Diurnal variations of PM2.5 at roadside and industrial sites
6. The diurnal patterns for roadside and industrial sites during 2010 are shown
in Figure 3.3. The diurnal range is slightly larger at roadside sites than at urban
background sites, while the variation at industrial sites is broadly similar to
that at urban background sites (cf. Figure 3.1a). However, at roadside sites
the morning increase is greater and the afternoon dip is less prominent than
at the urban background sites, with concentrations remaining above the early
morning (03:00-05:00) minimum (cf. Figure 3.3 roadside and Figure 3.1a).
While the morning peak is consistent with emissions during the morning
rush hour, there is no clear peak during the evening rush hour (16:00-18:00),
although concentrations during this period are somewhat higher than those
seen in the pattern at urban background sites, suggesting an influence of the
higher traffic flows at this time that is being masked by changing background
concentrations, i.e. the traffic increment during the rush hour is superimposed
on concentrations that are increasing at this time due to sources contributing to
the late evening peak, including the condensation of semi-volatile species.
A smaller evening traffic-related rush hour peak in PM2.5 is to be expected,
owing, in part, to fewer lorries on the road at this time of day, and to the
greater atmospheric turbulence at this time of day compared to the morning
rush hour. These effects are also seen in the nitrogen oxides (NOx) diurnal
pattern in Figure 3.9 later in this chapter.
7. The range of diurnal concentrations is much lower at industrial sites. The
morning and evening peaks are still evident, suggesting that the background
pattern is playing a dominant role at these sites with only a minimal
contribution from industrial activities.