Nearly 50 % of people living in the United States—an estimated 150 million—live in areas that don’t face air quality standards. Public vehicles and heavy-duty trucks are a major source of this pollution, which includes ozone, particulate matter, and other smog-forming emissions.

Automobile such as cars, motors, taxies, trucks, scooters, buses, Lorries, aeroplane etc., are considered as primary sources for transportation. CO, NO, NO2 gases released by the internal combustion engines of cars, buses, trucks, aero planes etc. cause air pollution. The released of pollutant gas by the internal combustion engine where petrol is used as a fuel.

Let us consider petrol is used as fuel in car engine. Petrol contains hydrocarbons as main constituent. These hydrocarbons have the general formula C8H18 and hence called octanes. petrol burns quickly in a car engine .Due to the short time available for burning, incomplete combustion of petrol takes places and so CO, CO2, H2O vapor alcohol, and acid, unburnt carbon particles are released. CO and carbon particles emitted into the air and make it polluted.

The automobile exhaust are responsible for more than 75 per cent of total air pollution. The automobiles such as cars, scooters, taxis etc. release smoke which contain huge amount of poisonous gases .These gases are released as a result of incomplete combustion of petrol and diesel, causing air pollution. Is named as anti – Knocking petrol. Anti-knocks reduce the rate of combustion of the fuel. The Chloro Fluorocarbons released from supersonic aeroplanes are responsible for ozone depletion.

It was assumed that throughout the world the total number of vehicles like cars, buses, motor vehicles running on the road per a day may be 500 million (5*108). Out of these vehicles 2.5 million are running on the roads of our country India. Due to this reason per one day 800 to 1000 pollutant gases are entering into the atmosphere causing air pollution.

Transportation and Climate Change

   Burning of fossil fuel such as Coal, Oil, Gasoline release carbon monoxide, a greenhouse gas, carbon monoxide, hydrocarbons, nitrogen oxide, and particulate matter into the atmosphere. The release of carbon monoxide  and other greenhouse gases like methane Nitrous oxide and hydro fluorocarbons is causing the atmospheric earth to warm , that the result are  changes to the climate .

   The Emitted pollutants from Fuels, which are harmful as smoking 10 cigarettes a day. When the vehicle pollution is high in the atmosphere, it creates a hole in the ozone layer contributing to smog and causing various health issues.

            Greenhouse gas (GHG) emissions from transportation account for about 28 percent of total U.S. greenhouse gas emissions, making it the largest contributor of U.S. GHG emissions. Between 1990 and 2018, GHG emissions in the transportation sector increased more in absolute terms than any other sector.

All the gaseous pollutants are primary pollutants. These pollutants are emitted directly and such are not found in the atmosphere. Instead, they form as a result of the pollutants emitted from these sources reacting with molecules in the atmosphere. Pollutants that are emitted into the environment from a source called Primary pollutants. There are also secondary pollutants which are formed in the atmosphere.

Secondary pollutants are those which are formed in the air due to interaction of primary air pollutants among themselves or by reaction with normal atmospheric constituents like sunlight, water vapor etc. With or without photo activation. For example ozone, formaldehyde, peroxy – acetyl nitrate and acidic mist like H₂SO₄ (SO₂ + moisture), smog, photochemical smog.

Evidences and experiments indicate that exhaust gases of automobiles have particular importance in the formation of secondary pollutants For example

                               Nitrogen oxide produced in the combustion of petroleum and other fuels emitted to the atmosphere, yield ozone in the presence of sunlight. It is to be noted that ozone is not emitted as such to the atmosphere but formed only from primary pollutants by the interaction in the atmosphere and thus constitute of secondary pollutants. The ozone produced reacts with hydrocarbons to form a series of compounds such as aldehydes, ketones, organic acids, acyl nitrates and epoxy compounds. This types of photochemical reactions usually occur in smog. The term ‘ smog’ denotes a mixture of smoke and fog .it is reported that during a smog , the concentration of ozone and oxidant material are found to be high . According to the Haagen – smit theory , ultraviolent rays in the sunlight break the nitrogen dioxide molecule to form nitric oxide and atomic oxygen . These products react with molecular oxygen to form ozone. The reaction may be represented as follows

1. NO₂              –   NO + O
2. O₂ + O         –    O₃
3. NO + O₂       –   NO₃
4. NO₃ + O₂     –   NO₂ + O₃
5. 2NO₂ + O₃   –   N2O₅ + O₂
6. NO + O₃        –  NO₂ + O₂

The liberated molecular oxygen reacts with molecules, of aldehydes and Oxy- genated organic compounds. which are activated by the absorption of light .

      RCOOOH+O₂                    –          RCOOH + O2

Thus once again ozone is liberated as a by product. On absorption of light energy, some of the aldehydes and ketones become free radicals. They react with molecular oxygen as follows

HCO + O₂                   –       HCO₃

The free radical HCO3, reacts further with molecular oxygen, nitric oxide and hydrocarbons as Follows

     HCO₃ + O₂                          –              HCO₂ + O₃

     HCO₃ + NO                          –            HCO₂ + NO₂

The cycle reaction is set in motion and numerous products are formed. It is to be noted that ozone and nitrogen dioxide are evolved continuously throughout the cycle of reaction. Free radicals lead to several other types of free radicals reaction and polymerization and oxidation chain reaction.                            

Though these reaction are studied well, the chemistry of photochemical smog is still not clearly understood. Many attempts have been made to reduce the level of ozone in the atmosphere. Actually, if the amount of hydrocarbons released in the atmosphere is reduced, then the ozone formed will be utilized in the oxidation of nitric oxide to nitrogen dioxide. But unfortunately the level of ozone have never decreased and the puzzle has not yet been solved .