Geology Reference
In-Depth Information
in Figure 12.
18
In an isopleth diagram, initial mixture compositions
giving rise to the same peak O
3
concentration are connected by the
appropriate isopleth. An isopleth plot shows that ozone production is a
highly non-linear process in relation to NO
x
and VOC, but picks out
many of the features already highlighted in Figure 11, i.e. when NO
x
is
''low'' the rate of ozone formation increases with increasing NO
x
in a
near-linear fashion. On the isopleth, the local maximum in the ozone
formation rate with respect to NO
x
is the same feature as the turn over
in N(O
3
) in Figure 11. The ridgeline along the local maximum separates
two different regimes, the so-called NO
x
-sensitive regime, i.e. N(O
3
) p
(NO
x
) and the VOC-sensitive (or NO
x
saturated regime), i.e. N(O
3
) p
(VOC) and increases with increasing NO
x
. The relationship between
NO
x
, VOCs and ozone embodied in the isopleth diagram indicates one
of the problems in the development of air quality policy with respect to
ozone. Reductions in VOC are only effective in reducing ozone under
VOC-sensitive chemistry (high NO
x
) and reductions in NO
x
will only be
effective if NO
x
-sensitive chemistry predominates and may actually
increase ozone in VOC-sensitive regions. In general, as an air mass
moves away from emission sources, e.g. in an urban region, the chem-
istry tends to move from VOC-sensitive to NO
x
-sensitive chemistry.
100
50
20
10
1
5
20
10
2
2
5
1
10
100
1000
VOC (ppbC)
Isopleths giving net rate of ozone production (ppb h
1
) as a function of VOC
(ppbC) and NO
x
(ppbv) for mean summer daytime meteorology and clear
skies
18
Figure 12
