Ozone in the atmosphere
Ozone, O3, is a gas composed of three oxygen atoms that naturally occurs in the atmosphere of the Earth. Most of the ozone (~90%) is found in the stratosphere, which extends from about 10-15 km height, depending on the latitude, up to 50 km above the surface. The remaining 10% of the ozone resides in the troposphere, from the surface to 10-15 km height. The formation and destruction processes of ozone in those two distinct layers are very different, as well as its impact on life on Earth.
Stratospheric ozone is formed when solar energetic ultraviolet (UV) radiation dissociates molecules of oxygen, O2, into separate oxygen atoms. Free oxygen atoms can recombine to form oxygen molecules but if a free oxygen atom collides with an oxygen molecule, it joins up, forming ozone. Ozone molecules can also be decomposed by UV radiation into a free atom and an oxygen molecule. Ozone is thus continuously created and destroyed in the stratosphere by UV radiation coming from the sun.
The stratospheric region with the highest concentration of ozone, between about 15 and 35 km altitude, is known as the ozone layer.
Stratospheric ozone ("the good ozone"), is beneficial for life on Earth because it absorbs the harmful UV radiation from the Sun. In the absence of the ozone layer, the higher UV radiation from the Sun would impact many living organisms on Earth, causing sun burn, skin cancer, eye diseases like cataract, but also plankton damage in the oceans, smaller crop yields, biodiversity disappearance, etc.
The ozone creation/destruction processes in the stratosphere are normally (naturally) balanced, so that the total amount of ozone in the atmosphere should remain more or less constant. In the 1970s, it was discovered that gases containing chlorine, nitrogen and bromine atoms released by human activities could cause stratospheric ozone depletion. In these reactions, those atoms acts as catalysts, i.e. they facilitate ozone loss reactions without being consumed themselves. As such, for example, one individual chlorine atom in the stratosphere can destroy during its lifetime about 100.000 ozone molecules. In addition to the gases emitted by human activities, factors such as changes in solar radiation and the formation of stratospheric aerosol particles after explosive volcanic eruptions also influence the ozone layer.
The majority of tropospheric ozone formation occurs when nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs), react in the atmosphere in the presence of UV radiation. Tropospheric ozone in excess of the natural amounts of ozone, is considered as "the bad ozone". High ozone levels in the boundary layer (from the surface to 100-3000m) can have adverse impacts on human and animal health (e.g. respiratory problems) and on the vegetation. In situ photochemistry and transport from other regions can lead to very high ozone concentrations, referred to as SMOG ozone. The surface ozone concentrations for Belgium are continuously measured and are hourly updated by the Belgian Interregional Environment Agency (IRCEL - CELINE).
Finally, it should also be noted that ozone in the upper troposphere is an important absorber of infrared (terrestrial) radiation, and therefore acts as a greenhouse gas. Tropospheric ozone is also the main source of the OH free radical, the primary oxidant in the atmosphere, responsible for removing many compounds (including atmospheric pollutants) from tropospheric air. OH is therefore called the “detergent of the atmosphere”.