Photochemical smog

Photochemical smog
Photochemical smog is a mixture of pollutants that are formed when nitrogen
oxides and volatile organic compounds (VOCs) react to sunlight, creating a
brown haze above cities. It tends to occur more often in summer, because that
is when we have the most sunlight.
Primary pollutants
The two major primary pollutants, nitrogen oxides and VOCs, combine to change
in sunlight in a series of chemical reactions, outlined below, to create what are
known as secondary pollutants.
Secondary pollutants
The secondary pollutant that causes the most concern is the ozone that forms at
ground level. While ozone is produced naturally in the upper atmosphere, it is a
dangerous substance when found at ground level. Many other hazardous
substances are also formed, such as peroxyacetyl nitrate (PAN).

The chief original reactants in an episode of photochemical smog are:
1. Molecules of nitric oxide, NO, and of unburned.
2. Partially oxidized hydrocarbons that are emitted into the air as pollutants
from internal combustion engines.
3. Nitric oxide is also released from electric power plants.
The concentrations of these chemicals are orders of magnitude greater than
are found in clean air. Gaseous hydrocarbons and partially oxidized
hydrocarbons are also present in urban air as a result of the evaporation of
solvents, liquid fuels, and other organic compounds. Collectively, the
substances, including hydrocarbons and their derivatives, that readily
vaporize into the air are called volatile organic compounds, or VOCs.

PAN (Peroxyacytyl nitrate) is a kind of air pollution. It is part of smog. PAN makes
people's eyes hurt and it is bad for your lungs. It also damages plants.

VOCs break
Cycle, allowing
PAN to form from
NO +VOC
Photochemical Smog

Chemical Equation for
Photochemical Smog
light
NO x + VOC
reactants
Ozone + Pan
products
PAN = Peroxyacetyl nitrate

Examples of Smog

Pollution by Carbon monoxide (CO)
Carbon Monoxide pollution occurs primarily from emissions produced by fossil
fuel powered engines. The incomplete reaction of air with fuel produces the
colorless, odorless and highly toxic gas.

Physical and chemical properties
Carbon monoxide (CO) is a tasteless, odorless, colorless, noncorrosive and quite
stable diatomic molecule that exists as a gas in the Earth’s atmosphere. Radiation
in the visibleand near-ultraviolet (UV) regions of the electromagnetic spectrum is
not absorbed by carbon monoxide, although the molecule does have weak
absorption bands between 125 and 155 nm. Carbon monoxide absorbs radiation
in the infrared region corresponding to the vibrational excitation of its electronic
ground state.

Symptoms of CO poisoning
The initial symptoms of CO poisoning are similar to the flu (but without the
fever) They include:
Headache
Fatigue
Shortness of breath
Nausea
Dizziness
If you suspect that you are experiencing CO poisoning, get fresh air immediately.
If you suspect that you are experiencing CO poisoning, get fresh air immediately.
Leave the home and call for assistance from a neighbor’s home. You could lose
consciousness and die from CO poisoning if you stay in the home.

How does Carbon Monoxide Poisoning Work?
Enters the body through the lungs and is delivered to the blood
• Red blood cells pick up CO instead of oxygen
– Hemoglobin likes CO 250 times more than oxygen
• CO prevents the oxygen that is present from being readily released to
and used properly by tissues.

Effect = function of CO concentration
times duration of exposure
• 200 ppm for 2-3 hours
• 400 ppm for 1-2 hrs
• 800 ppm for 45 minutes
• 1600 ppm for 20 minutes
• 3200 ppm for 5-10 min.
Mild headache; fatigue, nausea, dizziness
Serious headache, other symptoms intensify
Dizziness, nausea, convulsions, unconscious
within 2 hours
Death within 1 hour
Death within 1 hour

Standards or Guidelines
The OSHA standard for workers is no more than 50 ppm for 1 hour of
exposure. NIOSH recommends no more than 35 ppm for 1 hour. The U.S.
National Ambient Air Quality Standards for CO (established in 1985) are 9 ppm
for 8 hours and 35 ppm for 1 hour. The Consumer Product Safety
Commission recommends levels not to exceed 15 ppm for 1 hour or 25 ppm
for 8 hours.

Water Pollution

What is water pollution?
Water pollution means one or more substances have built up in water to such an
extent that they cause problems for animals or people. Oceans, lakes, rivers, and
other inland waters can naturally clean up a certain amount of pollution by
dispersing it harmlessly.
If you poured a cup of black ink into a river, the ink would quickly disappear
into the river's much larger volume of clean water. The ink would still be
there in the river, but in such a low concentration that you would not be able
to see it. At such low levels, the chemicals in the ink probably would not
present any real problem. However, if you poured gallons of ink into a river
every few seconds through a pipe, the river would quickly turn black. The
chemicals in the ink could very quickly have an effect on the quality of the
water. This, in turn, could affect the health of all the plants, animals, and
humans whose lives depend on the river.

Thus, water pollution is all about quantities: how much of a polluting substance is
released and how big a volume of water it is released into. A small quantity of a
toxic chemical may have little impact if it is spilled into the ocean from a ship. But
the same amount of the same chemical can have a much bigger impact pumped
into a lake or river, where there is less clean water to disperse it.

SOURCES OF WATER POLLUTION
Water pollutants are categorized as point source or nonpoint source, the
former being identified as all dry weather pollutants that enter watercourses
through pipes or channels. Storm drainage, even though the water may enter
watercourses by way of pipes or channels, is considered nonpoint source
pollution.
If pollution comes from a single location, such as a discharge pipe attached to a
factory, it is known as point-source pollution. Other examples of point source
pollution include an oil spill from a tanker, a discharge from a smoke stack (factory
chimney), or someone pouring oil from their car down a drain. A great deal of
water pollution happens not from one single source but from many different
scattered sources. This is called nonpoint-source pollution.

Any change in the dynamic equilibrium in aquatic ecosystem (water body/
biosphere/atmosphere) disturbs the normal function and properties of pure water
and gives rise to the phenomenon of water pollution.
The symptoms of water pollution of any water body/ground water are:
• Bad taste of drinking water.
• Offensive smells from lakes, rivers and ocean beaches.
• Unchecked growth of aquatic weeds in water bodies (eutrophication),
• Dead fish floating on water surface in river, lake, etc.
• Oil and grease floating on water surface.

Water Pollutants
The large number of water pollutants are broadly classified under the
categories:
1. Organic pollutants.
2. Inorganic pollutants.
3. Sediments.
4. Radioactive materials.
5. Thermal pollutants.

Oxidation-Reduction Chemistry in Natural Waters
Dissolved Oxygen
By far the most important oxidizing agent (i.e., substance that extracts electrons
from other species) in natural waters is dissolved molecular oxygen, O 2 , Upon reaction,
each of the oxygen atoms in O 2 is reduced from the zero oxidation number to -2, in H 2 0
or OH . The half-reaction that occurs in acidic solution is
whereas that which occurs in basic aqueous solution is:
Because the solubilities of gases increase with decreasing temperature, the
amount of O 2 that dissolves at 0 o C (14.7 ppm) is greater than the amount that
dissolves at 35°C (7.0 ppm). The median concentration of oxygen found in
natural, unpolluted surface waters in the United States is about 10 ppm.

Oxygen Demand
The most common substance oxidized by dissolved oxygen in water is organic
matter having a biological origin, such as dead plant matter and animal wastes.
If, for the sake of simplicity, the organic matter is assumed to be entirely
polymerized carbohydrate (e.g., plant fiber) with an approximate empirical
formula of CH 2 O, the oxidation reaction would be Dissolved oxygen in water is
also consumed by the oxidation of dissolved ammonia, NH 3 , and ammonium
ion, NH 4+ -substances that, like organic matter, are present in water as a result
of biological activity-eventually to nitrate ion, N0 - 3- .

The capacity of the organic and biological matter in a sample of natural water to
consume oxygen, a process catalyzed by bacteria present, is called its biochemical
oxygen demand, BOD. It is evaluated experimentally by determining the
concentration of dissolved 0 2 at the beginning and at the end of a period in which a
sealed water sample seeded with bacteria is maintained in the dark at a constant
temperature, usually either 20°C or 25°C. A neutral pH is maintained by use of a
buffer consisting of two ions of phosphoric acid, namely H 2 P0 4- and HP0 4
2-
:
The BOD equals the amount of oxygen consumed as a result of the oxidation of
dissolved organic matter in the sample. The oxidation reactions are catalyzed in the
sample by the action of microorganisms present in the natural water. If it is suspected
that the sample will have a high BOD, it is first diluted with pure, oxygen-saturated
water so that sufficient O 2 will be available overall to oxidize all the organic matter; the
results are corrected for this dilution. Usually the reaction is allowed to proceed for
five days before the residual oxygen is determined. The oxygen demand determined
from such a test, often designated BODs, corresponds to about 80% of that which
would be determined if the experiment were allowed to proceed for a very long timewhich
of course is not very practical. The median BOD for unpolluted surface water in
the United States is about 0.7 mg O 2 per liter, which is considerably less than the
maximum solubility ofO 2 in water (of8.7 mg/L at 25°C). In contrast, the BOD values for
sewage are typically several hundreds of milligrams of oxygen per liter.

Decomposition of Organic Matter in Water
Dissolved organic matter will decompose in water under anaerobic (oxygen free)
conditions if appropriate bacteria are present. Anaerobic conditions occur naturally in
stagnant water such as swamps and at the bottom of deep lakes. The bacteria operate
on carbon to disproportionate it; that is, some carbon is oxidized to carbon dioxide,
CO 2 , and the rest is reduced to methane, CH 4 :
This is an example of a fermentation reaction, which in chemistry is defined as
one in which both the oxidizing and the reducing agents are organic materials.
Since methane is almost insoluble in water, it forms bubbles that can be seen
rising to the surface in swamps and sometimes catches fire.

Since anaerobic conditions are reducing conditions in the chemical sense,
insoluble Fe 3+ compounds that are present in sediments at the bottom of lakes
are converted into soluble Fe 2+ compounds, which then dissolve into the lake
water:
It is not uncommon to find aerobic and anaerobic conditions in different parts of
the same lake at the same time, particularly in the summertime when a stable
stratification of distinct layers often occurs, figure 13-3. Water at the top of the
lake is warmed by the absorption of sunshine by biological materials, while that
below the level of penetration of sunlight remains cold. Since warm water is less
dense than cold (at temperatures above 4°C ), the warm upper layer "floats" on
the cold layer below, and little transfer between them occurs. The top layer, called
the ePilimnium, usually contains near-saturation levels of dissolved oxygen, due
both to its contact with air and to the O 2 produced in photosynthesis by algae.
Since conditions in the top layer are aerobic, elements exist there in their most
oxidized forms:

•Carbon, with an oxidation number (O.N.) of +4, as CO 2 or H 2 CO 3, or HC0 3- ;
• Sulfur, O.N. of +6, as SO 4 ;
• Nitrogen, O.N. of +5, as NO 3 ; and
• Iron, as Fe(III) , in the form of insoluble Fe(OH) 3.
Near the bottom, in the hypolimnium, the water is oxygen-depleted since it has no
contact with air and since 02 is consumed when biological material, such as the dead
algae that have sunk to these depths, decomposes. Under such anaerobic conditions,
elements exist in their most reduced forms:
• Carbon, with an O.N. of -4, as CH 4 ;
• Sulfur, O.N. of - 2, as H 2 S;
• Nitrogen, O.N. of -3, as NH 3 and NH 4 +; and
• Iron, as Fe(II), in the form of soluble Fe 2+ .
Anaerobic conditions usually do not last indefinitely. In the fall and winter, the top
layer of water is cooled by cold air passing over it, so that eventually the oxygen-rich
water at the top becomes more dense than that below it and gravity induces mixing
between the layers. Thus in the winter and early spring the environment near the
bottom of a lake usually is aerobic.

FIGURE 13-3 The stratification of a lake in the summer, showing the typical forms of the
major elements it contains at different levels.