Photochemical Smog
•
What is photochemical smog?
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How does smog form? How big of a
problem is it?
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What factors influence the formation of
smog?
What is smog?
Air pollution in large industrial cities (or urban regions) is called smog.
Term ‘smog’ is derived from the words ‘smoke’ and ‘fog’.
At least two distinct types of smog are recognized:
1) Sulfurous smog (‘London-type’ smog)
1) Photochemical smog (‘Los Angeles-type’ smog)
London Smog - This type of smog comes from coal smoke
combining with the water vapor and liquid water in cool, humid or
foggy air.
L.A. smog has been identified as coming from auto exhaust
primarily
Smog - smoke + fog
London smog:
requires humid/foggy, stagnant air
have lots of coal burning
SO2 + H2O
H2SO4
L.A. smog:
requires clear, sunny skies (since L.A.
photochemical smog requires sunlight
for at one of the key chemical reactions).
NOx + ROG + sunlight
O3 + NO2
ROG are reactive organic gases from
unburned gasoline
NOx are oxides of nitrogen
Favorable conditions for high
concentrations
London smog:
• temperature inversion
• Humid, foggy, stagnant air
• air will look sooty, dirty, foggy
L.A. smog:
• temperature inversion
• hot sunny, stagnant weather
• air looks hazy, brownish in color
Primary pollutants in L.A. smog:
CO - carbon monoxide
NO - nitric oxide
ROG - reactive organic gases
(unburned gasoline)
These are mainly direct combustion
products from gasoline or diesel
burning
internal
combustion
engines.
There is a significant source of
ROGs from stationary industries
and small businesses.
Secondary pollutants in
smog:
O3 - ozone
NO2 - nitrogen dioxide
Particles
PAN - peroxyacetyl nitrate
L.A.
'These are products of reactions in
the atmosphere. There are NOT
directly emitted.
The main secondary pollutant is
ozone....... near the surface, it is
bad for us, but up in the
stratosphere, it's a good thing....
Source of the Primary Pollutants
For Photochemical Smog
Mobile
sources
(such
as
automobiles) are the largest
sources of CO to the atmosphere.
Stationary industrial sources are
the largest source of particulate
matter (PM).
ROGs seem to be shared between
the stationary and mobile sources.
LA smog photochemistry - the
null cycle
A null cycle neither produces nor
destroys anything overall.
so, how do ozone and nitrogen
dioxide concentrations build up
during the day?????????
Ozone Production - Null Cycle
Observations of O3 concentration
vs. time show that there is a
significant increase in O3 during the
afternoon.
Reactive Organic Gases
Reactive organic gases (ROG) undergo a series of reactions to
form radicals.
The alkyl peroxy radical (RO2*) reacts with and oxidizes NO to
form NO2 faster than NO reacts with O3 to produce the same
result.
Thus, when ROGs are present, it is likely that O3 will not be
destroyed to produce NO2, and the null cycle is broken.
Note that each time an NO2 molecule is formed (by whatever
method), it very quickly results in the production of O3 (via photo
dissociation and a recombination).
NEW CYCLE WITH REACTIVE HYDROCARBONS
THE NULL CYCLE WITH NO HYDROCARBONS
Sulfurous smog
Sulfurous smog results from a high concentration of sulfur oxides in the
air and is caused by the use of sulfur-bearing fossil fuels, particularly
coal.
Photochemical smog
Photochemical smog, which occurs most prominently in urban areas
that have large numbers of automobiles, requires neither smoke nor
fog.
This type of smog has its origin in the nitrogen oxides and
hydrocarbon vapors emitted by automobiles and other sources,
which then undergo photochemical reactions in the lower
atmosphere.
Classical (‘London-type’) smog:
London-type smog occurs in the regions where
1) emission of the sulfur-containing compounds is high (due to
burning of coal to generate heat and energy);
2) air contains high liquid water contents (e.g., fogs).
Fog is the dispersed water drops.
Fogs can be viewed as clouds that are in contact with the Earth’s
surface.
Fogs are created during cooling of air next to the Earth’s surface
either by radiation to space (radiation fogs) or by a contact with a
surface (advection fogs).
Burning of coal produces sulfur dioxide, soot and other gases and
particulates, which are called smoke.
The typical London smog results from the accumulation of smoke from
coal burning, which has a high sulfur content.
It leads to the production of high concentrations of sulfuric acid in fog
droplets. These acidic particles, along with high densities of smoke,
inhibit the normal functioning of the lungs and can cause death.
In the atmosphere, sulfuric acid particles can be formed in two ways:
1. Direct dissolution of SO2 into water drop and subsequent aqueousphase oxidation to sulfate.
2. Gas-phase conversion of SO2 to sulfuric acid gas (H2SO4) which
has a low surface vapor pressure, and therefore, easily condenses
onto particles. The gas phase conversion requires three steps:
Photochemical (‘Los Angeles-type’) smog:
Los-Angeles-type smog occurs in the regions where
1) high emissions of automobiles
2) large concentrations of reactive hydrocarbons (RH) (from
automobile exhaust or from other natural or anthropogenic sources)
3) plenty of sunlight (high level of UV radiation)
Photochemical smog forms primarily as a result of interactions
among nitrogen oxides (NOx = NO + NO2), reactive hydrocarbons
and sunlight.
Photochemical (‘Los Angeles-type’) smog:
How it works:
Primary pollutants NO and reactive organic vapors, RH, are emitted
from automobiles.
First, RHs are chemically transformed to radicals, denoted R.
NOTE: The organic radical
can be composed of many atoms and
have a complex molecular structure.
Then organic radicals react with nitric oxide, NO, to form nitrogen
dioxide NO2:
Photochemical (‘Los Angeles-type’) smog:
However, in urban air, the sun breaks NO2 back to NO and O
Atomic oxygen produces ozone, O3, which is of primary concern in
photochemical smog:
Thus, the overall process of smog formation can be summarized as
Most of the world’s energy comes from the burning of organic
compounds, whether they represent the organic matter of wood or the
hydrocarbons of natural gas, coal, petroleum (oil), and other fossil
fuels.
Photochemical (‘Los Angeles-type’) smog:
.
Ethane is emitted from automobiles.
First, a hydroxyl radical attacks ethane, CH3CH3, by abstracting
a hydrogen atom to form ethyl radical,
Ethyl radical, CH3CH2. reacts quickly with oxygen in the
presence of a third body to form an ethylperoxy radical via
The ethylperoxy radical then reacts with nitric oxide to form
nitrogen dioxide and ethoxy radical:
Then NO2 goes to form O3 and ethoxy radical goes into a chain
of reactions.
Photochemical (‘Los Angeles-type’) smog:
.
Hundreds of different compounds may be produced by the
reactions of RH.
One of the most important is peroxyacetylnitrate (PAN).
Thus, secondary hydrocarbons are important components of
photochemical smog.
Along with NOx and organic gases, carbon monoxide is a key
component of smoggy air. However, its atmospheric chemistry
is very simple:
•
Time is important in determine the density of photochemical
smog.
•
Concentration of NOx, VOC and ozone generation depends on
time.
•
Early morning traffic increases the amount of both nitrogen oxides
and VOCs in atmosphere as people drive to work.
•
Later in the morning, traffic dies down and NOx and VOC begin to
react forming nitrogen dioxide thereby increases its
concentration.
•
As sunlight become more intense latter in the day, NO2 is broken
down which increase the concentration of O3.
At the same time NO2 + VOC
PAN
•
•
As sun disappears, the production of O3 is halted. The ozone that
remains in atmosphere is then consumed by several different
reactions and concentration goes down.
Photochemical (‘Los Angeles-type’) smog:
Photochemical smog formation:
Initial stage: NO, CO, RH
Final stage: O3, NO2, PAN (and other hydrocarbons), haze
(aerosols)
NOTE: photochemical smog evolves in time
Many big cities exhibit photochemical smog, including Mexico
City, Tokyo, Johannesburg, and Athens, among others.
Photochemical Smog
Fig. 20-5 p. 440
Percent of CO, NOx, SOx = SO2 + SO3 and reactive organic gases
(ROG) emissions by source category.
Comparison of general characteristics of sulfurous ‘Londontype’ smog and photochemical ‘Los Angeles-type’ smog.
Health Impacts of Smog
Smog Impacts:
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•
•
•
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Headache
Eye, nose and throat irritations
Breathing Problems
Coughing and wheezing
Aggravates
asthma,
heart
problems
Speeds up aging of lung tissue
Damage plants
Reduce Visibility
It can cause rubbers and fabric to deteriorate.
It can damage plants, leading to the loss of crops.
Reduce visibility.
Factors Influencing Smog Formation
Smog Levels are Influenced by:
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•
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Local climate
Topography
Population Density
Amount of industry
Transportation
Factors Influencing Smog Formation
Natural
Smog:
1.
Factors
Can
Reduce
Precipitation: Pollutants are
washed out with rainfall (Rain
or snow can “wash” air)
2. Wind can blow smog away
and replace clean air
Factors Influencing Smog Formation
Natural Factors Can Increase Smog:
1.
Tall urban buildings slow air
exchange
2.
Hills or Mountains do the same
3.
High temperatures
Factors Influencing Smog Formation
Factors affecting smog concentration
•
Smog concentrations vary over time
and space according to environmental
conditions and sunlight (the source of
energy for photochemical reactions).
•
The higher the sunlight intensity, the
greater production rate of O3.
•
The greater the wind speeds and
mixing heights the lower the smog
concentration.
•
In addition, the direction of the wind will
control the areas where smog is
transported.
Factors Influencing Smog Formation
Geographical Factors
•
Mountains stop the horizontal
transport of smog or divert it in
another direction, unless the wind
is strong enough to blow over the
mountain (not likely to happen in
L.A., due to the inversion that
prevents vertical mixing).
•
Since secondary pollution forms
after the emission of primary
pollution, we are likely to find the
higher
secondary
pollution
concentrations downwind of the
source regions.
Diurnal Smog Variation
•
Due to the time it takes for ozone to
build up in the afternoon, the highest
peaks of ozone occur inland, because
the sea breeze transports pollutants
inland during the afternoon.
•
Without
ROGs,
the
ozone
concentration would remain low most
of the day.
•
At night, there is no sunlight to photo
dissociate NO2, so O3 is not produced.
The
O3
from
the
daytime
photochemistry dissipates overnight.
Seasonal Variations
•
Variations in sunlight intensity cause
variations in O3 production rate.
•
When the sun is most intense (i.e.,
Summer), O3 should reach highest
levels, and primary pollutants should
be at low concentrations.
•
In the winter, when the sun is weak,
there will be reduced production of O3.
Primary pollutants, such as CO, reach seasonal maxima during the winter,
but more from the lower mixing heights during this season than from the
reduced sunlight intensity.
The effect of low mixing heights would be to reduce the dispersion volume
in which pollutants can mix, which increases the concentration if the
source rate is the same.