Tropospheric ozone

Seasonal average vertical columns of tropospheric ozone in Dobson units over the period 1979 to 2000. In June to August photochemical ozone production causes very high concentrations over the East Coast of the USA and China.

Ozone (O3) is a constituent of the troposphere (it is also an important constituent of some regions of the stratosphere commonly known as the ozone layer). The troposphere extends from a certain place of the Earth to between 12 and 20 kilometers above the surface of the Earth and consists of many layers. Ozone is more concentrated above the mixing layer, or ground layer. Ground-level ozone, though less concentrated than ozone aloft, is more of a problem because of its health effects.

Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night. At abnormally high concentrations brought about by human activities (largely incomplete combustion of fossil fuels, such as gasoline, diesel, etc.), it is a pollutant, and a constituent of smog. Many highly energetic reactions produce it, ranging from combustion to photocopying. Often laser printers will have a smell of ozone, which in high concentrations is toxic. Ozone is a powerful oxidizing agent readily reacting with other chemical compounds to make many possibly toxic oxides.

Tropospheric ozone is a greenhouse gas and initiates the chemical removal of methane and other hydrocarbons from the atmosphere. Thus, its concentration affects how long these compounds remain in the air.

Measurement

Satellites are able to measure tropospheric ozone. Measurements specifically of ground-level ozone require in situ monitoring technology.

Formation

The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), such as xylene, react in the atmosphere in the presence of sunlight. NOx, CO, and VOCs are called ozone precursors. Motor vehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources of these chemicals. Another source is windshield washer fluid. Although these precursors often originate in urban areas, winds can carry NOx hundreds of kilometers, causing ozone formation to occur in less populated regions as well. Methane, a VOC whose atmospheric concentration has increased tremendously during the last century, contributes to ozone formation but on a global scale rather than in local or regional photochemical smog episodes. In situations where this exclusion of methane from the VOC group of substances is not obvious, the term Non-Methane VOC (NMVOC) is often used.

The chemical reactions involved in tropospheric ozone formation are a series of complex cycles in which carbon monoxide and VOCs are oxidised to water vapour and carbon dioxide. The reactions involved in this process are illustrated here with CO but similar reactions occur for VOC as well. The oxidation begins with the reaction of CO with the hydroxyl radical (•OH).[1] The radical intermediate formed by this reacts rapidly with oxygen to give a peroxy radical HO2•

•OH + CO → •HOCO
•HOCO + O2 → HO2• + CO2

Peroxy radicals then go on to react with NO to give NO2 which is photolysed to give atomic oxygen and through reaction with oxygen a molecule of ozone:

HO2• + NO → •OH + NO2
NO2 + hν → NO + O(3P)
O(3P) + O2 → O3

The balance of this sequence of chemical reactions is:

CO + 2O2 + hν → CO2 + O3

The amount of ozone produced through these reactions can be calculated using the Leighton relationship.

This cycle involving HOx and NOx is terminated by the reaction of OH with NO2 to form nitric acid or by the reaction of peroxy radicals with each other to form peroxides. The chemistry involving VOCs is much more complex but the same reaction of peroxy radicals oxidizing NO to NO2 is the critical step leading to ozone formation.

Health effects

Health effects depend on ozone precursors, which is a group of pollutants, primarily generated during the combustion of fossil fuels. Reaction with daylight ultraviolet (UV) rays and these precursors create ground-level ozone pollution (Tropospheric Ozone). Ozone is known to have the following health effects at concentrations common in urban air:

A statistical study of 95 large urban communities in the United States found significant association between ozone levels and premature death. The study estimated that a one-third reduction in urban ozone concentrations would save roughly 4000 lives per year (Bell et al., 2004). Tropospheric Ozone causes approximately 22,000 premature deaths per year in 25 countries in the European Union. (WHO, 2008)

Problem areas

The United States Environmental Protection Agency has developed an Air Quality index to help explain air pollution levels to the general public. 8-hour average ozone mole fractions of 76 to 95 nmol/mol are described as "Unhealthy for Sensitive Groups", 96 nmol/mol to 115 nmol/mol as "unhealthy" and 116 nmol/mol to 404 nmol/mol as "very unhealthy" . The EPA has designated over 300 counties of the United States, clustered around the most heavily populated areas (especially in California and the Northeast), as failing to comply with the National Ambient Air Quality Standards.

Climate change

Melting of sea ice releases molecular chlorine, which reacts with UV radiation to produce chlorine radicals. Because chlorine radicals are highly reactive, they can expedite the degradation of methane and tropospheric ozone and the oxidation of mercury to more toxic forms.[2] Ozone production rises during heat waves, because plants absorb less ozone. It is estimated that curtailed ozone absorption by plants is responsible for the loss of 460 lives in the UK in the hot summer of 2006.[3] A similar investigation to assess the joint effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive.[4]

See also

References

  1. ↑ Reeves, Claire .E.; Penkett, Stuart A.; Bauguitte, Stephane; Law, Kathy S.; Evans, Mathew J.; Bandy, Brian J.; Monks, Paul S.; Edwards, Gavin D.; Phillips, Gavin; Barjat, Hannah; Kent, Joss; Dewey, Ken; Schmitgen, Sandra; Kley, Dieter (2002). "Potential for photochemical ozone formation in the troposphere over the North Atlantic as derived from aircraft observationsduring ACSOE". Journal of Geophysical Research 107 (D23): 4707. Bibcode:2002JGRD..107.4707R. doi:10.1029/2002JD002415.
  2. ↑ Jin Liao et al.(2013) (January 2014). "High levels of molecular chlorine in the Arctic atmosphere". Nature Geoscience Letter. doi:10.1038/ngeo2046. Retrieved January 14, 2014.
  3. ↑ "It's not just the heat – it's the ozone: Study highlights hidden dangers". University of York. Retrieved January 14, 2014.
  4. ↑ Kosatsky T. (July 2005). "The 2003 European heat waves". Eurosurveillance 10 (7). Retrieved January 14, 2014.

Further Reading

External links