Ground-level ozone
Ground-level or tropospheric ozone is created by chemical reactions between NOx gases (oxides of nitrogen produced by combustion) and volatile organic compounds (VOCs).
At abnormally high concentrations (the largest source being emissions from combustion of fossil fuels), it is a pollutant, and a constituent of smog.
The majority of ground-level ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs), react in the atmosphere in the presence of sunlight, specifically the UV spectrum.
[9][6] Motor vehicle exhaust, industrial emissions, and chemical solvents are the major anthropogenic sources of these ozone precursors.
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 volatile organic compound (NMVOC) is often used.
The limit on these interrelated cycles producing ozone is the reaction of •OH with NO2 to form nitric acid at high NOx levels.
[1] Health effects depend on ozone precursors, which is a group of pollutants, primarily generated during the combustion of fossil fuels.
[26] 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).
[27] 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.
[31] In monitoring air quality, Boulder County, Colorado is classified by the EPA as part of a nine-county group that includes the Denver metro area and North Front Range region.
However, since 2004 ozone pollution in Boulder County has regularly failed to meet federal standards set by the Environmental Protection Agency.
It is the primary constituent of urban smog, forming naturally as a secondary pollutant through photochemical reactions involving nitrogen oxides and volatile organic compounds in the presence of bright sunshine with high temperatures.
A rising environment, on the other hand, favours ozone synthesis and accumulation in the atmosphere, owing to two physicochemical mechanisms.
First, a warming climate alters humidity and wind conditions in some parts of the world, resulting in a reduction in the frequency of surface cyclones.
Climate change projections show that rising temperatures and water vapour in the atmosphere will likely increase surface ozone in polluted areas like the eastern United States.
[37] In particular, the degradation of the pollutant peroxyacetyl nitrate (PAN), which is a significant reservoir species for long-range transport of ozone precursors, is accelerated by rising temperatures.
As a result, as the temperature rises, the lifetime of PAN reduces, changing the long-range transport of ozone pollution.
The IPCC believes that "measured stratospheric O3 losses over the past two decades have generated a negative forcing of the surface-troposphere system" of around 0.15 0.10 watts per square metre (W/m2).
It is estimated that curtailed ozone absorption by plants could be responsible for the loss of 460 lives in the UK in the hot summer of 2006.
