Chemistry Reference
In-Depth Information
6
5.5
5
104
100
96
92
88
84
80
76
72
68
64
60
56
52
48
44
40
4.5
4
3.5
Best Estimate
VOC emissions
Best Estimate
O
3
= 88 pp
b
3
OZONE
ISOPLETH
DIAGRAM
FOR 18th
JULY 2006
2.5
2
Best Estimate
NO
x
emissions
1.5
1
Harwell,
Oxfordshire, UK
0.5
2
4
6
8
10
12
NO
x
emissions, arbitrary units
Fig. 5 Ozone isopleth diagram for 18th July 2006 at the EMEP GB0036R Harwell station plotted
from the results of VOC and NO
x
emission sensitivity experiments performed with a Photochemical
Trajectory Model [
23
]
VOC and NO
x
emission reductions increasing with increasing ozone levels and
decreasing with decreasing ozone levels.
There must be a further reason why the apparent ozone trends decline with
decreasing initial ozone level in Fig.
4
. If the experience of the ozone isopleth
diagram is accepted, then ozone trends should be independent of the initial ozone
level. Other factors must be acting to offset the influence of the regional NO
x
and
VOC emission reductions at the stations with the lowest initial ozone levels. It is
likely that the rising hemispheric ozone levels are this offsetting influence. The
regional photochemical ozone production is superimposed on top of the hemi-
spheric ozone level. That is to say, the air masses that cross the North Atlantic
Ocean and arrive at the western seaboard of Europe already contain ozone and
regional photochemical ozone production adds further to this as the air masses
travel eastwards into continental Europe. Any rise in ozone level in these air masses
due to hemispheric ozone increase will offset any reduction in episodic peak ozone
brought about be regional-scale NO
x
and VOC emission reductions. It is apparent
that the stations which show the least ozone trends in Fig.
4
are those most likely to
be influenced by any growth in hemispheric ozone levels.
