ASSIGNMENT 1 Environmental Management

SAYED ASADULLAH UNISEL, FACULTY OF ENGINEERING, CIVIL DIVISION

Photochemical smog is a major local or regional air pollution phenomenon characterized by

oxidants, irritating vapors, and visibility-obscuring particles that occurs in urban areas where

the combination of pollution-forming emissions and appropriate atmospheric conditions are

right for its formation. Though not a treat to the global atmosphere as such, in some urban

areas photochemical smog is highly detrimental to health and to the quality of life. Smog

originally was used to describe the unpleasant combination of smoke and fog laced with sulfur

dioxide, a chemically reducing atmosphere, which was formerly prevalent in London when high-

sulfur coal was the primary fuel used in that city.

One of the most common urban air pollution problems in the production of photochemical

smog. This condition occurs in dry, stagnant air masses, usually stabilized by a temperature

inversion, that are subjected to intense sunlight. A smoggy atmosphere contains ozone, organic

oxidants, nitrogen oxides, aldehydes, and other noxious species. In latter stages of smog

formation, visibility in the atmosphere is lowered by the presence of a haze of fine particles

formed by the oxidation of organic compounds in smog.

The chemical ingredients of smog are nitrogen oxides (NOx) and volatile organic compounds

(VOCs), both are released from the automobile, as well as from other sources. The driving

energy force behind smog formation is electromagnetic radiation with a wavelength ar around

400 nm or less. In the ultraviolet region, just shorter than the lower limit for visible light. Energy

Absorbed by a molecule from this radiation can result in the formation of active species, thus

initiating photochemical reactions that lead to the noxious products characteristic of smog.

The process by which photochemical smog is formed is shown in Figure 1. Methane (CH4) is one

of the least active hydrocarbons in terms of forming smog, it will be used here to show the

smog formation process because it is the simplest hydrocarbon molecule. Smog is produced in

a series of chain reactions. The first of these occurs when a photon of electromagnetic radiation

with a wavelength less than 398nm is absorbed by a molecule of nitrogen dioxide (NO2), to

produce an oxygen atom, O.

NO2 + hv ⇾ NO + O

The oxygen atom is very reactive species that can abstract a hydrogen atom from methane,

CH4 + O ⇾ CH3⋅ + OH⋅

ASSIGNMENT 1 Environmental Management

SAYED ASADULLAH UNISEL, FACULTY OF ENGINEERING, CIVIL DIVISION

produce a methyl radical (CH3⋅) and a hydroxyl radical (OH⋅). In these formulas, the dot shows a

single unpaired electron. The Hydroxyl radical is especially important in the formation of smog

and in a wide variety of other kinds of photo chemical reactions. The methyle radical car react

with an oxygen molecule,

CH3 ⋅ + O2 ⇾ H3COO ⋅

To produce a methylperoxyl radical (H3COO⋅) This is a strongly oxidizing, reactive species. One

of the very important reactions of peroxyl radicals are there reaction with NO, produced in the

photochemical dissociation of NO2.

NO + H3COO⋅ ⇾ NO2 + H3CO⋅

To generate NO2, which can undergo photo-dissociation, re-initiating the series of chin

reactions by which smog is formed. Literally hundreds of other reactions can occur, leading

eventually to oxidized organic matter that constitutes the small particulate matter

characteristic of smog.

As the process of smog formation occurs, numerous noxious intermediates are generated. One

of the main ones of these is ozone and it is the single species most characteristic of smog.

Whereas ozone is an essential species in the stratosphere, where it filters out undesirable

ultraviolet radiation, it is a toxic species in the troposphere that is bad for both animals and

plants. Another class of materials formed with smog consists of Oxygen-rich organic

compounds containing nitrogen of which peroxyacetyl nitrate PAN

C – C – O – O – NO2

Is the most common example. This compound and ones similar to it are potent oxidizers and

higly irritating to eyes and mucous membranes of the respiratory tract. Also associated with

smog are aldehydes, which are irritants to eyes and the respiratory tract. The simplest

aldehyde, and one commonly found in smoggy atmospheres is Formaldehyde:

H–C–H

O

H

H

Peroxyacetyl Nitrate (PAN)

O

H

Formaldehyde

ASSIGNMENT 1 Environmental Management

SAYED ASADULLAH UNISEL, FACULTY OF ENGINEERING, CIVIL DIVISION

Figure 1. Outline of the process by which photochemical smog is formed. “R”

represents hydrocarbon groups. The automobile emits nitrogen oxides and

hydrocarbons, the two main ingredients required to produce Photochemical Smog.

Stanley E. Manahan, Environmental Science and Technology: A Sustainable Approach to Green

Science and Technology, Second Edition.

Lawrence P, Bevy Trends in Catalysis Research [Editor].