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

Concentrations and composition of PM2.5
46. The chemical equivalence of nitrate and sulphate from AGANet with
ammonium from the National Ammonia Monitoring Network (NAMN) has been
tested by examining the ionic balance. The 1:1 equivalence between ammonium
and the sum of “nitrate + 2 x sulphate” seen in Figure 3.19 confirms the
neutrality of the particles and that nitrate and sulphate are present mainly as
ammonium nitrate (NH4NO3) and ammonium sulphate ((NH4)2SO4) respectively.
The units are in nmol m -3 to allow equivalence to be demonstrated.
-3
PM NH4, nmol m
4
140
120
100
80
60
40
20
0
-20
y = 0.9741x - 2.5706
R 2 = 0.9665
0 20 40 60 80 100 120 140 160
PM NO 3 + 2xPM SO 4 , nmol m -3
Figure 3.19: Ammonium concentrations versus nitrate plus two times sulphate
concentrations in nmol m -3 (data from 30 AGANet sites, 1999-2009).
3.9.4 Trends in nitrate, sulphate, ammonium, black smoke and
black carbon
Nitrate
47. Measurements of nitrate began at acid precipitation network sites in 1990 and
the 1990-2000 measurements at Eskdalemuir show a -20% to -30% decrease
over this period (Lovblad et al., 2004) in line with the observations for EMEP
sites in Norway and Denmark. Nitrate measurements switched to the Delta
denuder method in 2001 and RoTAP (2012) reports the spatial distribution of
the annual mean PM nitrate concentrations using these data. No information
was provided in RoTAP (2012) for the presence of any trends in nitrate, which
may reflect the large annual variability in concentrations.
48. The pattern of nitrate concentrations measured between September 1999
and December 2009 across the 12 long-term sites in the AGANet programme
is shown in Figure 3.20. Concentrations increase from 2000 to 2003, then
decrease to 2009, leading to little overall change over this period (Mann–
Kendall trend analysis at 95% confidence level shows a slight downward trend
of -0.04 µg m -3 yr -1 ). There is a notable peak in the spring of 2003 (February-
April), although nitrate concentrations are typically high at this time of the year
(Figure 3.21).
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