Ground-level ozone (O3), also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere (the lowest level of the Earth's atmosphere), with an average concentration of 20–30 parts per billion by volume (ppbv), with close to 100 ppbv in polluted areas. Ozone is also an important constituent of the stratosphere, where the ozone layer (2 to 8 parts per million ozone) exists which is located between 10 and 50 kilometers above the Earth's surface. The troposphere extends from the ground up to a variable height of approximately 14 kilometers above sea level. Ozone is least concentrated in the ground layer (or planetary boundary layer) of the troposphere. Ground-level or tropospheric ozone is created by chemical reactions between NOx gases (oxides of nitrogen produced by combustion) and volatile organic compounds (VOCs). The combination of these chemicals in the presence of sunlight form ozone. Its concentration increases as height above sea level increases, with a maximum concentration at the tropopause. About 90% of total ozone in the atmosphere is in the stratosphere, and 10% is in the troposphere. Although ground-level ozone is less concentrated than stratospheric ozone, it is of concern because of its health effects. Ozone in the troposphere is a greenhouse gas, and as such contribute to global warming. It is the third most important greenhouse gas after CO2 and CH4, as indicated by estimates of its radiative forcing. Photochemical and chemical reactions involving ozone drive many of the chemical processes that occur in the troposphere by day and by night. At abnormally high concentrations (the largest source being emissions from combustion of fossil fuels), it is a pollutant, and a constituent of smog. Its levels have increased significantly since the industrial revolution, as NOx gasses and VOCs are some of the byproducts of combustion. With more heat and sunlight in the summer months, more ozone is formed which is why regions often experience higher levels of pollution in the summer months. Although the same molecule, ground-level ozone can be harmful to human health, unlike stratospheric ozone that protects the earth from excess UV radiation. Photolysis of ozone occurs at wavelengths below approximately 310–320 nanometres. This reaction initiates a chain of chemical reactions that remove carbon monoxide, methane, and other hydrocarbons from the atmosphere via oxidation. Therefore, the concentration of tropospheric ozone affects how long these compounds remain in the air. If the oxidation of carbon monoxide or methane occur in the presence of nitrogen monoxide (NO), this chain of reactions has a net product of ozone added to the system.
Measurement Ozone in the atmosphere can be measured by remote sensing technology, or by in-situ monitoring technology. Because ozone absorbs light in the UV spectrum, the most common way to measure ozone is to measure how much of this light spectrum is absorbed in the atmosphere. Because the stratosphere has higher ozone concentration than the troposphere, it is important for remote sensing instruments to be able to determine altitude along with the concentration measurements. A total ozone mapping spectrometer-earth probe (TOMS-EP) aboard a satellite from NASA is an example of an ozone layer measuring satellite, and the tropospheric emission spectrometer (TES) is an example of an ozone measuring satellite that is specifically for the troposphere. LIDAR is a common ground-based remote sensing technique that uses laser to measure ozone. The Tropospheric Ozone Lidar Network (TOLNet) is the network of ozone observing lidars across the United States. Ozonesondes are a form of in situ, or local ozone measuring instruments. An ozonesonde is attached to a meteorological balloon, so that the instrument can directly measure ozone concentration at the varying altitudes along the balloon's upward path. The information collected from the instrument attached to the balloon is transmitted back using radiosonde technology. NOAA has worked to create a global network of tropospheric ozone measurements using ozonesondes. Ozone is also measured in air quality environmental monitoring networks. In these networks, in-situ ozone monitors based on ozone's UV-absorption properties are used to measure ppb-levels in ambient air. Total atmospheric ozone (sometimes seen in weather reports) is measured in a column from the surface to the top of the atmosphere, and is dominated by high concentrations of stratospheric ozone. Typical units of measure for this purpose include the Dobson unit and millimoles per square meter (mmol/m2).
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