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

PM2.5 in the UK
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3.7 PM2.5 episodes
3.7.1 Winter episodes
27. Episodes during the autumn and winter months are well known in the UK,
occurring when a high pressure system is dominant and stagnant atmospheric
conditions prevail, with little vertical mixing and low wind speeds. These
conditions can allow local emissions to build up over several days. If conditions are
particularly cold then local emissions from a heating plant for example will also be
much higher. Such conditions arose during December 2010, when extreme (for
the UK) cold persisted for a number of days across much of the country.
28. During the seven day period 19-25 December 2010, temperatures were below
freezing throughout the day and wind speeds were low in Derry in Northern
Ireland and Glasgow in Scotland, especially on 22 December (dropping to -12°C
at night). Both temperatures and wind speed increased on 26 December. There
were high PM concentrations in both locations, although they were particularly
high in Derry (Figure 3.14). The main part of the PM episode was due to PM2.5,
with little contribution from the coarse PM2.5-10 fraction. Also shown in Figure
3.14 are the NOx and sulphur dioxide (SO2) concentrations, which followed a
similar pattern to the PM, although SO2 was more dominant in Derry and NOx in
Glasgow. The peak concentrations in Derry occurred during the evening (higher
concentrations occurred between around 13:00 and 24:00, peaking between
16:00 and 22:00), while in Glasgow the peaks were broader and occurred
earlier, during the afternoon and early evening (higher concentrations occurred
between around 09:00 and 22:00, peaking between 17:00 and 18:00). The
linkage with SO2 in Derry is consistent with the burning of solid and/or liquid
fuels in domestic premises. The timing of the peaks suggests the use of these
fuels during the evening, but not during the early morning. In Glasgow, it is
likely that road traffic played a more important role, as seen in the greater NOx
concentrations. The average concentration of PM2.5 over the seven days of the
episode 19 to 25 December 2010 was 96.9 µg m -3 at Derry and 40.6 µg m -3
at Glasgow Centre. At Derry, this episode contributed 1.9 µg m -3 to the annual
mean, amounting to 9.6% of the annual mean of 19.3 µg m -3 ; at Glasgow
Centre, this episode contributed 0.8 µg m -3 to the annual mean, amounting to
6.3% of the annual mean of 12.4 µg m -3 . PM2.5 episodes related to stagnant
winter conditions, which allow a build-up of local emissions, can last
several days and represent a not insignificant contribution to annual
mean concentrations.