Ozone is the primary source (through photolysis by solar UV radiation) of hydroxyl radicals (OH) which, in turn, provide the mechanism by which pollutants such as carbon monoxide (CO) and methane (CH4) are removed from the lower atmosphere. Through its reactions with hydrocarbons, ozone is the source of PAN (peroxyacetylnitrate), a major ingredient of urban photochemical smog. Ozone itself is hazardous to the health of both plants and animals, and ozone in association with particulate matter causes respiratory problems in humans. Ozone is also a significant greenhouse gas, especially in the upper troposphere.
Sources of Tropospheric Ozone
Some ozone is transported down from the stratosphere (possibly as much as 50%) and some is created in situ through the photolysis of nitrogen dioxide (NO2) to nitric oxide (NO) which, in turn, reacts with carbon monoxide (CO) and hydrocarbons to form O3. Photolysis of O3 by sunlight (particularly ultraviolet light) is one of the main influential processes in the troposphere. There is a reversible reaction in which nitrogen dioxide (NO2) in the presence of sunlight leads to nitrogen monoxide (NO) plus ozone (O3). The sum of NO + NO2 is termed NOx and its presence is essential for O3 formation. In the presence of nitrogen oxides (NOx) and water vapor, the photochemical oxidation of CO produces tropospheric O3.
Within the lower and mid-troposphere, O3 photolysis in the presence of water vapor is the primary source for the important hydroxyl radical (OH), which is responsible for the removal of many pollutants like methane and CO (this is sometimes referred to as the troposphere’s “oxidizing” or “self-cleaning” capacity). Should OH become reduced, the lifetime of many damaging compounds will increase, and ultimately their atmospheric concentrations will rise to higher levels.
When water vapor is limited, and both CO and NOx are abundant, then NOx can compete with O3 for OH, and OH may be reduced. Water vapor is a catalyst that speeds up the conversion of NO to NO2. Carbon monoxide (CO) and hydroxide (OH) react to make water vapor (H2O), and are thus entangled.
Note that the NOx is not consumed in this process (i.e., it is catalytic) and becomes available for further ozone production. Urban areas, where combustion and automobile exhausts provide prolific sources of NOx, are particularly prone to atmospheric pollution (especially in areas of abundant sunlight). However, many rural areas (for example, the southeastern US) occasionally fail the EPA NOx standard for reasons that are somewhat mysterious. The sink of NOx is nitric acid (HNO3) which is rained out to the surface (and is, incidentally, a component of acid rain).
Ozone’s impact is all about location. High in the stratosphere, ozone shields us from the sun’s harmful UV rays. But beneath that, at the top of the troposphere, it acts as a greenhouse gas and contributes to global warming. In the middle of the troposphere, it plays a key role in a chemical process that cleans the air of certain pollutants. But at the bottom of the troposphere, where we live and breathe, it contributes to smog and is toxic to plants and animals. TES measurements allow scientists to track the abundance, creation, destruction, and movement of this critical chemical at various altitudes throughout the atmosphere.