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

PM2.5 in the UK
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43. The UK has initiated a monitoring programme to meet the Directive
requirements in respect of PM composition. MARGA instruments were installed
at Auchencorth Moss (located to the south of Edinburgh) in 2006 and at
Harwell (located to the south of Oxford) in 2009. These instruments specifically
measure the species listed above in the PM2.5 and PM10 fractions on an hourly
basis (but not Fe). The data have been examined carefully by AQEG, but
concerns about their quality make them unsuitable for analysis. These concerns
include concentrations being higher in the PM2.5 fraction than the PM10 fraction
and a lack of ionic balance. It is therefore recommended that the suitability
of these instruments for meeting reporting obligations under the
Directive is reviewed.
44. Monitoring of total ambient nitrate, sulphate and ammonium concentrations
has been undertaken over a number of years at sites across the UK (see Chapter
2). Although these measurements are not specifically for the PM2.5 size fraction,
the results are still useful for an analysis of seasonal patterns and trends.
3.9.3 Seasonal pattern of nitrate and sulphate
45. The UK Acid Gases and Aerosols Monitoring Network (AGANet) has been in
operation since 1999, with 12 original sites expanded to 30 sites in 2006. This
Network records monthly mean concentrations. The seasonal pattern across
all sites and all years is shown in Figure 3.18. The shaded area represents the
uncertainty in the mean value for each month at the 95% confidence level,
it does not represent the overall measurement of uncertainty. There is clear
evidence of a nitrate peak in the early spring (~3 µg m -3 ), in particular in March
and April, before a rapid drop to relatively constant concentrations for the rest
of the year (~1.5 µg m -3 ). The sulphate concentrations are also highest in March
and April (~1.4 µg m -3 ), but show only a slow decline over the summer, with the
lowest concentrations occurring in the winter months (0.8-1.0 µg m -3 ).
NO 2
30
25
20
15
Nitrate
J F M A M J J A S O N D
month
SO 4
14
12
10
08
Sulphate
J F M A M J J A S O N D
month
Figure 3.18: Variation of nitrate and sulphate concentrations (µg m -3 ) by month
of the year (data from 30 AGANet sites, 1999-2009). The shading represents
the 95% confidence interval of the mean. Note different scales.