Geology Reference
In-Depth Information
pptv observe the P(O
3
) dependency predicted by the model, but above
[NO
x
]
B
300 pptv P(O
3
), computed from the measured HO
2
and NO,
continues to increase with NO
x
, suggesting a NO
x
-limited regime.
Case Study II - Photochemical control of ozone in the remote marine
boundary layer (MBL) - An elegant piece of experimental evidence for
the photochemical destruction of ozone comes from studies in the
remote MBL over the southern ocean at Cape Grim, Tasmania
(411S).
20
In the MBL, the photochemical processes are coupled to
physical processes that affect the observed ozone concentrations,
namely deposition to the available surfaces and entrainment from
the free troposphere. The sum of these processes can be represented in
the form of an ozone continuity equation (a simplified version of
Equation 2.6), viz
d
½
O
3
dt
¼
C
þ
E
v
ð½
O
3
ft
½
O
3
Þ
H
þ
v
d
½
O
3
H
ð
2
:
39
Þ
where C is a term representative of the photochemistry (the net result
of production, P(O
3
), minus destruction, L(O
3
) Equation (2.32)), E
v
the entrainment velocity (a measure of the rate of ozone transport
into the boundary layer), [O
3
]
ft
the concentration of free-tropospheric
ozone, v
d
the dry deposition velocity (a measure of the physical loss of
ozone to a surface) and H the height of the boundary layer. In
general, the MBL is particularly suitable for making photochemical
measurements owing to its stable and chemically simple nature.
Figure 14 shows the average diurnal cycle of ozone and total peroxide
(mainly H
2
O
2
) in clean oceanic air as measured at Cape Grim during
January 1992. During the sunlit hours an ozone loss of about 1.6
ppbv occurs between mid-morning and late afternoon. This loss of
ozone is followed by an overnight replenishment to a similar starting
point. In contrast, the peroxide concentration decreases overnight
from 900 to 600 pptv and then increases from 600 to 900 pptv between
midmorning and late afternoon. It is worth noting that the magnitude
of this anti-correlation of ozone and peroxide is dependent on season.
The daytime anti-correlation between O
3
and peroxide can be inter-
preted as experimental evidence for the photochemical destruction of
ozone, as the ozone is destroyed via reactions (2.7, 2.15 and 2.16)
while simultaneously peroxide is formed from chemistry involving the
odd-hydrogen radicals OH and HO
2
(reactions (2.11, 2.12 and 2.17)).
