Environmental Engg.

Lesson-30 Types of secondary air pollutants and their properties

The primary pollutants often react with one another or with water vapour, in the presence of sunlight to form entirely a new type of pollutants called / termed as secondary air pollutants.  These types of pollutants are the chemical substances, which are produced from the chemical reactions of natural or anthropogenic air pollutants or due to their oxidation caused by the energy of the sun.

There are two main reasons for making distinction between primary and secondary air pollutants.  First, in order to perform and interpret atmospheric chemical research, one must distinguish between primary and secondary air pollutants.  The second reason is that emission controls can only be effectively treated at primary anthropogenic air pollutants, their formation process must be understood and somehow interrupted.  Controlling the air concentrations of primary anthropogenic pollutants is much easier than controlling the concentrations of secondary pollutants.  The distinction between primary and secondary air contaminants is not always clear, as the same chemical can either be directly emitted into or formed by reactions in the air.

As secondary air pollutants are mainly formed by chemical reactions, and chemical reactions usually produce products that are less reactive than their reactants, it would be convenient to assume that secondary pollutants are more inert than primary pollutants.  In many cases, it is true.  However, as sunlight drives many atmospheric reactions, additional energy can be found in some secondary pollutants.

The most important secondary air pollutants are:

• Sulphuric acid

• Ozone

• Formaldehyde

• Peroxy-acetyl-nitrate (PAN)

Sulphuric acid

It is formed by the simple chemical reaction between sulphur dioxide and water vapour, and is much more toxic pollutant than sulphur dioxide having far reaching effects on environment since it causes acid rain.

Ozone

Ozone is a primary example of a very reactive secondary air pollutant.  Thus photoactivation can produce highly reactive products.  The peak concentration of ozone is built late in the day, after the sun has had time to drive their formation.

Since ozone has been generally found to occur in the highly motorized areas, particularly during day time, it is believed that it is produced by the photochemical reaction of hydrocarbons and nitrogen oxide.  Possibility of formation of such photochemical smog is quite high in places where number of plying automobiles is too high and where inversion smog conditions prevail in the atmosphere.

The presence of ozone gas in the air may cause irritation in the respiratory tract, reaching much deeper into the lungs than the oxides of sulphur. 

Formaldehyde

Formaldehyde is an organic chemical that is very prevalent in our environment. It has colorless gas with a pungent odor from a family of gases called aldehydes. Commonly known as a preservative in medical laboratories and mortuaries, formaldehyde is also found in other products such as chemicals, particle board, household products, glues, permanent press fabrics, paper product coatings, fiberboard and plywood.  It is a sensitizing agent that can cause an immune system response upon initial exposure. It is also a suspected human carcinogen that is linked to nasal cancer and lung cancer. Formaldehyde exposure is most common through gas-phase inhalation.

Peroxy-acetyl-nitrate (PAN)

It is a secondary pollutant present in photochemical smog. It is thermally unstable and decomposes into peroxyethanoyl radicals and nitrogen dioxide gas. It is a lachrymatory substance.

Peroxyacetyl nitrate, or PAN, is an oxidant more stable than ozone. Hence, it is better capable of long-range transport than ozone. It serves as a carrier for oxides of nitrogen (NOx) into rural regions and causes ozone formation in the global troposphere.

The formation of PAN on a secondary scale becomes an issue when ethanol is used as an automotive fuel. Acetaldehyde emissions increase, which subsequently react in the atmosphere to form smog. Whereas ethanol policies solve domestic oil supply problems, they drastically exacerbate air quality conditions

Peroxy acetyl nitrate irritates the eyes resulting in blurred vision and eye fatigue.  It decreases vital capacity due to decrease in both inspiratory capacity and expiratory reserve volume.