Transcript Environmental Chemistry part 4

Environmental chemistry
E. 10 smog
smog

State the source of primary pollutants
and the conditions necessary for the
formation of photochemical smog.

Outline the formation of secondary
pollutants in photochemical smog.
smog
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many different chemicals
primary pollutants
secondary pollutants such as nitric acid,
aldehydes, organic peroxides and PANs that
have been formed from the primary
pollutants by free radical reactions brought
about by UV from the Sun.
photochemical smog: conditions (1)

sunshine:

windless day, hot, dry
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thermal inversion (cold air below warm air so
sunlight converts primary pollutants
into secondary pollutants; visible sunlight is enough
to break bonds in NO2 as there is delocalisation
which weakens a bond;
the air cannot rise)
photochemical smog: conditions (2)

relief: city in a valley surrounded by hills

primary air pollutants: nitrogen oxides
and volatile hydrocarbons – VOCs
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source: internal combustion engines
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time: morning ‘rush hour’
formation of secondary pollutants
In the car: N2 + O2  2NO
NO + ½ O2  NO2
NO2 +
O +
H2O +
RH +
hv
 NO
+
O
O2  O3
O3
O
 O2

R
+
2OH
+
OH
Nitric acid:
HO + NO2  HNO3 /OH +
NO2

HNO3
Hydroxyl free radicals react with VOC to form PANs
PANs : RCO3 + NO2  RCO3NO2
smog
primary
pollutants


volatile
hydrocarbons VOC
nitrogen
monoxide
secondary
pollutants

nitrogen
dioxide
ketones,
aldehydes,
PANs,
peroxides,

ozone,


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
photochemical smog
(http://www.eco.pref.mie.jp/english/data-syu/data-sokuhou/img/R06-01_e.gif)
smog: variation in day
thermal inversion

Normally, in the troposphere, temperature decreases as you go
up; this makes warm and less dense air rise taking pollutants
with it. The warmer air is replaced by cooler air which is again
warmed up continuing the process;

Thermal inversion occurs when cooler air slips underneath
warmer air so that a layer of warm air is trapped between 2
layers of cool air. The bottom cool layer of air, which is
denser than the warm layer of air above it, remains close to the
surface. Any pollutants released in that layer of cooler air stay
close to the Earth’s surface; this allows the primary pollutants
to react and produce secondary pollutants and smog.
thermal
inversion

thermal inversion
animation
smog: effects
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PAN’s cause eyes to water and respiratory
problems, also catalyses formation of sulphuric
acid,
ozone in the troposphere is:



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an eye irritant and also irritates respiratory system
causes deterioration of rubber as it breaks –C=C- and bleaches
dyes
affects leaves and therefore growth/damages crops
reduced visibility
secondary pollutants are toxic to plants
corrosion of building materials by acids and
ozone.