Why Tropospheric Ozone is Bad for the Environment?

 

Three oxygen atoms when bound together forms ozone (O₃). It is very unstable and highly reactive relative to oxygen and often used as a bleach, a deodorizing agent, and a sterilization agent for air and drinking water.

Ozone can be ‘good’ or bad for health and the environment depending on the place of its occurrence. Stratospheric ozone( at an altitude of 25-30 Km) is good because where it is found naturally and acts as a protective radiation shield for living organisms on earth. It strongly absorbs ultraviolet light in the region 220-330 nm and thereby protects life on earth from severe radiation damage.

Ozone found in tropospheric region is termed as bad ozone or ground level ozone. When nitrogen oxides (NO_x), volatile organic compound (VOCs) (emitted by cars, power plants, refineries, chemical plants etc.) and sunlight reacts together to form various secondary pollutants, which are also known as photochemical oxidants. One of the most abundant photochemical oxidants is ozone and is the one for which ambient air quality standards has been set. It is one of the six common ‘criteria air pollutants’ identified in the clean air act. This is mostly found in the urban and suburban areas but can also be transported in rural areas by wind and most likely to reach unhealthy levels on hot sunny days. Please refer Montreal Protocol for more Info.

How does tropospheric ozone form?

It begins with the formation of nitric oxide (NO) during combustion of fuel in vehicles and power plants.

N₂ + O₂  →2NO

The nitric oxide thus formed reacts with oxygen and gets converted into nitrogen dioxide (NO₂).

2NO + O₂ → 2NO₂

In presence of sunlight and with a photon of required energy NO₂ can dissociates in a process called photolysis.

NO₂ + hv → NO + O

Here hν represents a photon with a wavelength of less than λ < 0.39µm. The single o atom can then combine with diatomic oxygen (O₂) to form ozone.

O + O₂ + M → O₃ + M

Where M represents a molecule (usually N₂ or O₂) which absorbs excess energy emitted in the reaction.

Ozone can then convert NO into NO₂.

NO + O_{3 →} NO₂ + O₂

NO₂ have a tendency to form ozone whereas NO tends to destroy it.

The equations given above represents the sequence of stages through which atmospheric NO, NO₂ and O₃ progress on a typical smoggy day.

These reactions suggests that in early morning traffic concentration of NO rises. Then, at mid –morning with increased sunlight the level of NO₂ increases as NO gets oxidized into NO₂. O_{3 ­}builds up as sun’s intensity increases toward noon and NO₂ begins to drop. During evening traffic rush again there is a formation of new nitric oxide but ozone is so effective in its reaction with NO that as long as O_3­ is present in the atmosphere, NO level don’t rise up.

Effects of ground level ozone on human health and environment

Ground level ozone is responsible for variety of health problems. People with asthma, children, elders and outdoor workers are mostly at risk from breathing air containing ozone. It triggers variety of health problems such as chest pain, coughing, throat irritation and airway inflammation. It affects lung function and also leads to worsen bronchitis, emphysema and asthma.

It leads to cracking of rubber products and affects sensitive vegetation and ecosystem, including parks, forests and wildlife.

 

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