Tropospheric ozone, also known as ground-level or "bad" ozone, forms when nitrogen oxides and volatile organic compounds react with sunlight in the air. It is a major air pollutant that poses risks to human health and the environment. Ground-level ozone is the main component of smog and can cause respiratory issues in humans as well as damage crops and other plants. While it is naturally occurring in small amounts, emissions from vehicles, power plants, chemical solvents and other sources are increasing tropospheric ozone levels globally with harmful effects. Reducing NOx and VOC emissions through international cooperation is needed to address the growing threat from tropospheric ozone pollution.

1. Tropospheric
Presented to: Prof. Umesh Kulshrestha
Presented by: Chandrashekhar Azad V.

2. Ozone
• Ozone (O3), or trioxygen, is a triatomic
molecule, consisting of three oxygen atoms.
• It is an allotrope of oxygen that is much less
stable than the diatomic allotrope(O2).
• Ozone is a gas that occurs both in the Earth's
upper atmosphere and at ground level.
• Ozone can be "good" or "bad" for people's
health and for the environment, depending on
its location in the atmosphere.

3. Ozone
OZONE
TROPOSPHERIC STRATOSPHERIC
• “Bad” ozone is  “Good” ozone is
found found
• Act as pollutant  Act as protective laver
• Measured in ppb,  Measure in DU
µg/m3 (Dobson unit)

5. Tropospheric ozone
• In the troposphere, the air closest to the Earth's
surface, ground-level or "bad" ozone is a
pollutant that is a significant health
risk, especially for children with asthma.
• It also damages crops, trees and other
vegetation.
• It is the main ingredient of urban smog.

6. Chemistry

7. Chemistry
Ground-level or “bad” ozone is not emitted directly into the air,
but is created by chemical reactions between oxides of nitrogen
(NOx) and volatile organic compounds (VOC) in the presence of
sunlight.
Ozone is made in the troposphere by the same reaction
responsible for its formation in the stratosphere. However, the
source of O is different than in the stratosphere where the O
atoms are made by photo dissociation of O2 at UV wavelength
<240 nm. But in the troposphere only UV radiation with >290 nm
is available which can yield oxygen from NO2.

8. Sources
• The sources of tropospheric ozone are:
– Influx from stratosphere
– Generation by photochemical reaction in troposphere
• As mentioned earlier VOC and NOx are the main precursors of
ozone in the troposphere. These pollutants are emitted from
industrial facilities and electrical utilities, motor vehicle exhaust,
gasoline vapors and chemical solvents.

9. Impact of tropospheric ozone
• A strong oxidant, tropospheric ozone is harmful to
humans and ecosystems.
• Breathing ozone can trigger a variety of health
problems including chest pain, coughing, throat
irritation, and congestion.
It can worsen bronchitis, emphysema, and
asthma. “Bad” ozone also can reduce lung
function and inflame the linings of the lungs.
• Ground level ozone leads to damage of
agricultural fields, commercial forest yields,
reduced growth and survivability of tree
seedlings and also damage of the foliage of
trees and other plants.
• Tropospheric ozone absorbs infrared rays emanating
from the earth and works as a powerful green house
gas.
• Ozone has its strongest green house effect in upper
troposphere.

10. It is a known cause of photochemical smog, which disturbs human
respiratory functions and plant photosynthesis.
Due to its green house effect and toxicity, tropospheric ozone is
known as “bad ozone”

11. Formation of Smog

12. Formation of Smog
 Smog is composed of many different compounds but the main
composition of smog is ozone, PAN and volatile organic compounds
(VOC).
 PAN (peroxy acetyl nitrate) is the product of VOC’s oxygen and nitrogen
oxide.
 VOCs are uncombusted fuels, often the product of engines.

13. Air quality index (AQI) for ozone
• The AQI is an index for reporting daily air quality. It tells
you how clean or unhealthy your air is, and what
associated health effects might be a concern.

14. Cont… NationalAmbientAir Quality Standards (NAAQS)
AQI Ozone Concentration in ambient
concentration air
(ppb)
Time Industrial, Ecologically
0-50 0-59 Pollutant weighted residential, sensitive
51-100 60-75 average rural and area
other areas (notified by
101-150 76-95 central govt.)
151-200 96-115
Ozone 8 hours 100 100
201-300 116-374 (µg/m3)
1 hour 180 180
>300 >374
Source: EPA
Note: An AQI of 100 for ozone corresponds to an ozone level of 75 parts per
billion (averaged over 8 hours).

15. Conclusion
• Ozone in the lower troposphere is considered to be a secondary
pollutant and is called ‘bad ozone’.
• Any concentration of O3 240µg/m3 and above is considered to be
unhealthy for animals and human beings. 1 hour exposure is
sufficient to lead to illness.
• Plants have an injury threshold value of 59µg/m3 of O3 exposure for
4 hours.
• Older leaves are most sensitive to O3 pollution
• Both NOx and VOC emission which lead to ozone formation have
been increasing in Asia over the past quarter century. Such a
scenario leads to the elevation of tropospheric ozone concentration
with serious consequences.
• Urgent action is needed to reduce emissions through international
cooperation in the Asian region.

16. References
• Manahan E. Stanley (2005); Environmental Chemistry
• Roy Sovan (2003); Environmental Science, A Comprehensive
Treatise on Ecology and Environment
• EPA; AQI, a guide to air quality and your health
• Ashmore M.R; Assessing the future global impacts of ozone on
vegetation
• Acid deposition and oxidation research center Tropospheric Ozone
A Growing Threat
• Upadhyay S. N.; ozone the protective shield of life
• Madronich, S. 1993. Tropospheric photochemistry and its response
to UV changes.
• http://www.airnow.gov /index .cfm
• www.ucar.edu/learn/1_7_1.htm
• http://www.epa.gov/oaqps001/gooduphigh/bad.html#6

17. Thank