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Nature of Ozone


Ozone (O3) is a different form of the familiar oxygen (O2).  Ozone levels broadly increase with height to reach a maximum at approximately 25 km above the Earth’s surface in what is known as the ozone layer, located in the layer of the atmosphere known as the stratosphere.  Because all ozone absorbs the harmful ultraviolet rays emitted by the sun, stratospheric ozone is essential.  At ground level, however, ozone is considered a pollutant because it also causes adverse health and environmental effects.
Ozone forms in the air during daylight hours.  In the layer of the atmosphere which is in contact with the Earth’s surface (i.e. the troposphere), it forms from complex reactions involving precursor pollutants with the most important being NOx and VOC.
Ambient ozone levels vary considerable on an hourly, daily and monthly basis, depending on the prevailing meteorological conditions and where the air comes from.  Stratospheric ozone can also at times be brought down to the surface and contribute to the ambient ozone levels.
In many parts of Canada, the short-term (1-to 8-hour averages) peak ozone levels produced from NOx and VOC are typically the highest in the summer months because ozone formation is favoured by strong sunlight and high air temperatures.  Monthly average levels, however, are typically the highest in spring months.  Like PM 2.5, ozone can be transported by the winds over large distances and affect areas hundreds to thousands of kilometres away from the sources of the precursors.     
Ground-level Ozone
Ground-level ozone forms in the air following the dissociation of nitrogen dioxide (NO2).  As NO2 absorbs sunlight, it splits into nitric oxide (NO) and an unstable form of oxygen (O), which immediately merges with the familiar oxygen (O2) to form ozone (O3).
NO2 and NO (known as nitrogen oxides, or NOx) are emitted by the same sources. However, most of the ambient NO2 is actually formed in the air from the conversion of the emitted NO.
The conversion of NO to NO2 occurs when NO reacts with other substances, such as ozone.  In addition to the generation of NO2, the reaction of ozone and NO is also a process (known as ozone scavenging) through which ozone is removed from the air, since during the reaction ozone converts to oxygen (O2).
NO, NO2 and ozone are interrelated.  If the air contained only these three substances, a cycle of ozone formation and scavenging would form, leading to equilibrium between the three substances, and resulting in ozone levels which would be relatively low.
The presence of VOC, however, disrupts this equilibrium since VOC provide a pathway for NO to convert to NO2 without scavenging ozone.  With NO2 now also being formed from reactions involving NO and VOC, the formed ozone can accumulate in the air, thereby leading to significantly higher ozone levels than would occur from the NOx-ozone equilibrium alone. 





                                                                 Ground-level Ozone



Effects of Reductions in Ambient NO
Reductions in NOx emissions in urban areas that cause a decrease in the local ambient NO levels can cause an increase in local ozone levels because of the resulting decrease in the amount of ozone scavenged.  This effect may be more pronounced in urban areas which are affected by ozone that is transported into the area.  Downwind from the urban area, however, the reductions in emissions could lead to less ozone formation and contribute to decreasing ozone levels.  
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