Geology Reference
In-Depth Information
lifetimes in the order of 46 days. It is also worth noting that peroxyacyl
radical like peroxy radicals can oxidise NO to NO 2
CH 3 CO d O 2 þ NO
-
CH 3 CO d O þ NO 2
(2.51)
PAN can be an important component of NO y in the troposphere (see
Section 2.5.1) and has the potential to act as a temporary reservoir for
NO x and in particular it has the potential to transport NO x from
polluted regions into the background/remote atmosphere. 26
There are other side-products of urban air pollution that are giving
increasing cause for concern. These include the production of both
aerosols (both organic and inorganic) and particles.
The effects of photochemical smog/urban air pollution remain on the
political agenda owing to their potential impact on human health and
the economy. In summary, urban photochemistry is not substantially
different from tropospheric photochemistry. It is the range and concen-
trations of the VOCs involved in oxidation coupled to the concentration
of NO x and other oxidants that lead to a large photochemical turnover.
2.6 NIGHT-TIME OXIDATION CHEMISTRY
Though photochemistry does not take place at night, it is important to
note, within the context of tropospheric oxidation chemistry, the potential
for oxidation chemistry to continue at night. This chemistry does not lead
to the production of ozone, in fact the opposite, but has importance
owing to the potential for the production of secondary pollutants. In the
troposphere, the main night-time oxidant is thought to be the nitrate
radical formed by the relatively slow oxidation of NO 2 by O 3 , viz.
NO 2 þ O 3
-
NO 3 þ O 2
(2.52)
The time constant for reaction (2.52) is of the order of 15 h at an ozone
concentration of 30 ppbv and T ¼ 290 K. Other sources include
N 2 O 5 þ M
-
NO 3 þ NO 2 þ M
(2.53)
but as N 2 O 5 is formed from
NO 3 þ NO 2 þ M
-
N 2 O 5 þ M
(2.54)
the two species act in a coupled manner. Dinitrogen pentoxide, N 2 O 5 ,is
potentially an important product as it can react heterogeneously with
water to yield HNO 3 . During the daytime the NO 3 radical is rapidly
photolysed as it strongly absorbs in the visible, viz
NO 3 þ hn
NO þ O 2
-
(2.55)
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