Chemistry Reference
In-Depth Information
1.0
0.5
0.0
80
90
100
110
120
50
60
70
40
-0.5
1990-2009
1993-2009
Non sig
-1.0
-1.5
-2.0
-2.5
Annual maximum 8-hr mean O
3
in 1990, ppb
Fig. 4 Scatter plot of the trend in the maximum 8-h mean ozone in ppb per year against the
extrapolated annual maximum at the start of the 1990 or 1993-2009 period for 73 EMEP
monitoring stations. The
large filled symbols
are for the stations whose downwards trends are
highly statistically significant (
a
0.1) and the
small filled symbols
(Non sig) are for those that are
less than highly statistically significant (
a
>
0.1)
precursor emissions. There are two monitoring stations with long-running time series
covering the three decades from the 1980s to the 2000s. These stations GB0036R
Harwell and GB0039R Sibton showed highly statistically significant downwards
trends of
0.9 ppb per year, respectively.
Examination of the residuals showed no evidence of any let up in these downwards
trends during the 2000s compared with the 1980s or 1990s. These downwards trends
are fully consistent with expectations based on computer modelling studies utilising
published European emission inventories for man-made NO
x
and VOCs over the
1990-2009 period. These modelling studies focused on the GB0036 Harwell moni-
toring station in the southern United Kingdom [
23
]. However, the results should be
similar for other monitoring stations in northwest Europe, particularly those in
Belgium, the Netherlands, Denmark, northern Germany and northern France.
A simple explanation of why the trends are more pronounced for the stations
with the highest initial episodic peak levels would be that these stations show the
highest ozone responses to VOC and NO
x
emission reductions. Whilst this is
certainly feasible, it is not a common finding in photochemical ozone modelling.
Ozone responses to changing VOC and NO
x
emissions in photochemical models
are usually illustrated as isopleth plots. Ozone isopleths are commonly seen as a set
of parallel curves, see Fig.
5
for one such isopleths diagram constructed from a
European photochemical trajectory model [
23
]. It is not at all common for the
isopleths to become closer together as would be implied by the ozone responses to
1.2
0.7 ppb per year and
1.4
