Geology Reference
In-Depth Information
under urban conditions OH may be formed from secondary sources
such as
HONO
þ
hn(l
o
400 nm)
-
OH
þ
NO
(2.44)
where the HONO can be emitted in small quantities from automobiles
or formed from a number of heterogeneous pathways,
25
as well as gas-
phase routes. OH produced from HONO has been shown to be the
dominant OH source in the morning under some urban conditions,
where the HONO has built up to significant levels overnight. Another
key urban source of OH can come from the photolysis of the aldehydes
and ketones produced in the NMHC oxidation chemistry, in particular
formaldehyde, viz.
HCHO
þ
hn(l
o
334 nm)
-
H
þ
HCO
(2.45)
H
þ
O
2
þ
M
HO
2
þ
M
-
(2.12)
HCO
þ
O
2
-
HO
2
þ
CO
(2.46)
Net
HCHO
þ
2O
2
þ
hn
-
2HO
2
þ
CO
(2.47)
Smog chamber experiments have shown that the addition of aldehyde
significantly increases the formation rates of ozone and the conversion
rates of NO and NO
2
under simulated urban conditions.
3
A marked by-product of oxidation in the urban atmosphere, often
associated, but not exclusive to, urban air pollution is PAN. PAN is
formed by
OH
þ
CH
3
CHO
-
CH
3
CO
þ
H
2
O
(2.48)
CH
3
CO
þ
O
2
-
CH
2
CO
d
O
2
(2.49)
addition of NO
2
to the peroxyacetyl radical (RCO
d
O
2
) leading to the
formation of PAN.
CH
3
CO
d
O
2
þ
NO
2
þ
M
CH
3
CO
d
O
2
d
NO
2
þ
M (2.50)
"
PAN is often used as an unambiguous marker for tropospheric chem-
istry. The lifetime of PAN in the troposphere is very much dependant on
the temperature dependence of the equilibrium in reaction (2.50), the
lifetime varying from 30 min at T
¼
298 K to 8 h at T
¼
273 K. At mid-
troposphere temperature and pressures PAN has thermal decomposition
