Ground Level Ozone

The most important secondary pollutant is the Ground Level Ozone or Tropospheric Ozone. Ground Level Ozone is formed by the reactions of the oxides of Nitrogen (NOx), Carbon Monoxide (CO) and Volatile Organic Compounds (VOCs) and NMVOCs such as Xylene in the atmosphere in the presence of sunlight. Thus culprits for Ground Level Ozone are NOx, CO, VOCs and NMVOCs.

In the last 100 years, the emission of Methane (a Volatile Organic Compound) has increased dramatically and it has contributed to the increased concentration of Ground Level Ozone.

Formation of Ground Level Ozone

The formation of the Ground Level Ozone involves a long complex series of the reactions in which carbon monoxide and VOCs are oxidized to water vapour and carbon dioxide. The series of the reactions begins with the Hydroxyl OH radicals, which is one of the main chemical species controlling the oxidizing capacity of the global Earth atmosphere. They are produced by many pathways but most notably they are formed from the decomposition of hydro peroxides (ROOH) and by reaction of excited atomic oxygen with water. The reaction involves the following steps:

The Carbon Monoxide reacts with the Hydroxyl Radical, producing a Hydrogen atom.

OH + CO →  H + CO₂

The hydrogen atom formed by this reacts rapidly with oxygen to give a peroxy radical HO₂

H + O2 →  HO₂

Peroxyradical then reacts with the NO and gives NO₂ which, in presence of Sunlight is photolysed to give atomic oxygen and through reaction with oxygen a molecule of ozone.

HO2 + NO →  OH + NO₂

NO2 + hν →  NO + O

O + O2 →  O₃

In total, the reaction is as follows:

CO + 2O₂ →  CO₂ + O₃

The above reaction is simple demonstration. The Chemical processes that involve the VOCs are the complex ones. But the result of these reactions is the Ozone. From the above, kindly note that Carbon Dioxide DOES NOT play a role in formation of Ground level Ozone.

Sources and Health Effects of Ground Level Ozone

Breathing ozone can trigger a variety of health problems, particularly for children, the elderly, and people of all ages who have lung diseases such as asthma. Ground level ozone can also have harmful effects on sensitive vegetation and ecosystems.