Lecture 4
The chemistry of ground level pollution
The chemistry of
ground level air
Photochemical smog
CHAPTER 03: Figure 3.1
NO2 can be photolysed in throposphere
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© 2012 W. H. Freeman & Co.
The general cycle of OH/HOO formation
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© 2012 W. H. Freeman & Co.
Formation of oxygen atoms and ozone
Within a few minutes, most of the nitrogen dioxide, NO2, produced
in OH/HOO cycle absorbs UV-A from sunlight and photochemically
decompose
NO2 + UV-A --> NO + O
The oxygen atoms produced in this cycle quickly react with
molecular oxygen to form ozone
O + O2 --> O3
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© 2012 W. H. Freeman & Co.
In its reaction with otherwise-stable gases whose
molecules contain multiple bonds OH radical adds itself to
them, for example
OH + CO --> HOCO
Molecular oxygen reacts quickly with transient free
radicals:
HOCO + O2 --> HO2 + CO2
The hydroperoxy radical, HO2, is in turn converted back to
hydroxyl by its oxidation of nitric oxide:
HO2 + NO --> OH + NO2
CHAPTER 03: Unnumbered Figure 3.1
Hydroxyl radical
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© 2012 W. H. Freeman & Co.
CHAPTER 03: Unnumbered Figure 3.2
Structure of some atmospheric molecules
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© 2012 W. H. Freeman & Co.
CHAPTER 03: Unnumbered Figure 3.3
NO and NO2
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© 2012 W. H. Freeman & Co.
CHAPTER 03: Figure 3.2
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© 2012 W. H. Freeman & Co.
Principles of reactivity
• The usual first step in the oxidation of an
atmospheric gas is its reaction with the OH
radical
• OH addition does not occur to O=O and
C=O bond
• OH does not add to multiple bond in any
fully oxidised species CO2, SO3, N2O5
Principles of reactivity
•
If molecule have reactive double bond OH will
add to it (CO, CH2=CH2)
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For molecules that do not have a reactive
multiple bond, but do contain a hydrogen atom,
OH reacts with them by the abstraction,
forming water and a new reactive free radical
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Peroxy radical reacts with NO forming NO2
Principles of reactivity
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Free radicals that contain nonperoxy oxygen
atom and contain H atom, react with oxygen
with abstraction of H atom by O2:
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CH3-O + O2 —-> CH2=O + HO2
CHAPTER 03: Figure 3.3
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© 2012 W. H. Freeman & Co.
Early morning traffic increases the emissions of both nitrogen oxides and VOCs as
people drive to work.
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Later in the morning, traffic dies down and the nitrogen oxides and volatile organic
compounds begin to be react forming nitrogen dioxide, increasing its concentration.
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As the sunlight becomes more intense later in the day, nitrogen dioxide is broken
down and its by-products form increasing concentrations of ozone.
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At the same time, some of the nitrogen dioxide can react with the volatile organic
compounds to produce toxic chemicals such as PAN.
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As the sun goes down, the production of ozone is halted. The ozone that remains in
the atmosphere is then consumed by several different reactions.
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CHAPTER 03: Figure 3.4
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© 2012 W. H. Freeman & Co.
CHAPTER 03: Figure 3.5
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© 2012 W. H. Freeman & Co.
Formation ozone in troposphere
O+O2=O3
The blend of air pollution, high temperatures and sunlight -which lead to ozone formation -- is particularly relevant to
local Israeli conditions. The following studies provide some
understanding of the ozone levels around Israel.
• A report from 2008 shows increasing ozone concentrations
across the country, including the Western Galilee, the Haifa
region, Afula, Modi’in and Jerusalem. It is worth noting
that in these areas there was a deviation from the Israeli 8hr
standard (set at 82 parts per billion - ppb) (4). The
American standard is 75 ppb and the American Lung
Association is lobbying for setting an even lower standard,
since studies show that ozone is hazardous at 75 ppb as well
(5). • Air quality monitoring has shown that people living in areas
with lower traffic-related air pollution (compared to Gush
Dan) -- such as Gush Etzion, Bet Shemesh, Beer Sheva, and
Afula -- are exposed to higher levels of ozone than people
in the large metropolitan zone of Gush Dan (4). This is
because most pollutants that cause ozone formation are
emitted in the coastal region and then swept eastward by
western wind.
Israel standards
Catalytic convertors in cars
Green chemistry: Supercritical and liquid
CO2 as solvent