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Constituent gas of the troposphere

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 tropospheric ozone is less concentrated than stratospheric ozone, it is of concern because of its health effects. Ozone in the troposphere is considered a greenhouse gas, and may contribute to global warming. 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. 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. 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. 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).

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 sunlight, specifically the UV spectrum. NOx, CO, and VOCs are considered ozone precursors. There are many man made sources of these organic compounds including vehicle and industrial emissions, along with several other sources. Reduced lung function, making it more difficult to breathe deeply and vigorously. Breathing may become more rapid and more shallow than normal, and a person's ability to engage in vigorous activities may be limited. Ozone causes the muscles in the airways to constrict which traps air in the alveoli leading to wheezing and shortness of breath. Inflammation and damage to the lining of the lungs. Within a few days, the damaged cells are shed and replaced much like the skin peels after a sunburn. Animal studies suggest that if this type of inflammation happens repeatedly over a long time period (months, years, a lifetime), lung tissue may become permanently scarred, resulting in permanent loss of lung function and a lower quality of life. More recent data suggests that ozone can also have harmful effects via the inflammatory pathway leading to heart disease, type 2 diabetes, and other metabolic disorders. It was observed in the 1990s that ground level ozone can advance death by a few days in predisposed and vulnerable populations. 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). Tropospheric ozone causes approximately 22,000 premature deaths per year in 25 countries in the European Union. (WHO, 2008)

The United States Environmental Protection Agency has developed an Air Quality index to help explain air pollution levels to the general public. 8 hour average ozone mole fractions of 76 to 95 nmol/mol are described as "Unhealthy for Sensitive Groups", 96 nmol/mol to 115 nmol/mol as unhealthy and 116 nmol/mol to 404 nmol/mol as very unhealthy. 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. In 2000, the Ozone Annex was added to the U. S.–Canada Air Quality Agreement. The Ozone Annex addresses transboundary air pollution that contributes to ground level ozone, which contributes to smog. The main goal was to attain proper ozone air quality standards in both countries. The North Front Range of Colorado has been out of compliance with the Federal Air Quality standards. The U. S. EPA designated Fort Collins as part of the ozone non attainment area in November 2007. This means that the U. S.’s environmental law considers the air quality to be worse than the National Ambient Air Quality Standards, which are defined in the Clean Air Act Amendments. In 2018, the Lung Association ranked Larimer county 19th in the nation for high ozone days. Fort Collins was also ranked 24 for high ozone days out of 228 metropolitan areas, 52 for 24 hour particle pollution out of 217 metropolitan areas, and 156 for annual particle pollution out of 203 metropolitan areas. Attempts have been made under the Early Action Compact to bring the area's air quality up to the EPA's standards. However, since 2004 ozone pollution in Boulder County has regularly failed to meet federal standards set by the Environmental Protection Agency. The County of Boulder continues trying to alleviate some of the ozone pollution through programming that encourages people to drive less, and stop ozone polluting activities during the heat of the day.

Changes in air temperature and water content affect the air's chemistry and the rates of chemical reactions that create and remove ozone. Many chemical reaction rates increase with temperature and lead to increased ozone production. 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. Ozone depletion, on the other hand, is a radiative forcing of the climate system. Two opposite effects exist: Reduced ozone causes the stratosphere to absorb less solar radiation, cooling it while warming the troposphere; as a result, the stratosphere emits less long wave radiation downward, cooling the troposphere. 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). Furthermore, rising air temperatures often improve ozone forming processes, which has a repercussion on climate, as well. Also, since climate change is causing sea ice to melt, what occurs is the sea ice releases molecular chlorine, which reacts with UV radiation to produce chlorine radicals. Because chlorine radicals are highly reactive, they can expedite the degradation of methane and tropospheric ozone and the oxidation of mercury to more toxic forms. Ozone production rises during heat waves, because plants absorb less ozone. 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. A similar investigation to assess the joint effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive.