National Ambient Air Quality Objectives For Ground-Level Ozone- Summary - Science Assessment Document

2. Atmospheric Chemistry

Ozone is a natural constituent of both the upper atmosphere (the stratosphere) and the lower atmosphere (the troposphere). In the stratosphere, ozone (O₃) and oxygen (O₂) are part of a natural cycle of formation and destruction that is driven by energy from the sun. Most of the ozone in the stratosphere is concentrated in the middle stratosphere in a band commonly referred to as "the ozone layer". The ozone layer plays a critical function in protecting life on earth from damaging UV radiation from the sun.

At ground level, however, ozone is a pollutant. The sources of ground level ozone are:

1. direct transport from the stratosphere (a minor source), and
2. formation in the troposphere (a major source). The chemical reactions that lead to the production of ozone in the troposphere are also driven by energy from the sun; therefore ozone is a photochemical pollutant. In the troposphere, ozone is the product of a complex series of chemical reactions involving nitrogen oxides (NO_x = NO + NO₂) and volatile organic compounds (VOC). These primary pollutants, known as precursor gases, are produced during the combustion of fossil fuels and are thus associated with industry and the transportation sector. Some NO_x and VOC may be produced by biogenic sources, particularly in the summer when emissions from vegetation and soils are highest. Virtually none of the ozone in the troposphere is emitted directly from natural or human sources, therefore ozone is considered a secondary pollutant, formed in the atmosphere from chemical precursors.

While the chemistry of ozone production is complex, the main features are well established and can be summarized as follows. Nitric oxide (NO) introduced into the atmosphere reacts rapidly with O₃ to form nitrogen dioxide (NO₂, reaction R1 Box 1). NO₂ can efficiently absorb sunlight and photo-dissociate to yield oxygen atoms (O) and NO (R2). These oxygen atoms in turn will react rapidly with molecular oxygen (O₂) to reproduce O₃ (R3; M represents a third molecule such as molecular oxygen or nitrogen (N₂) that absorbs the excess energy released in this reaction, thereby stabilizing the newly formed O₃ molecule).

Reactions R1 to R3 (Box 1) describe the photostationary state between O₃, NO and NO₂. That is, in the absence of other gases in the atmosphere, an equilibrium would be established in which the amount of ozone would be controlled by the ratio of NO₂ to NO in the atmosphere and the intensity of sunlight. However, measurements of ozone in the troposphere have revealed clearly that ozone concentrations are significantly higher than would be expected under steady-state conditions. This indicates that more complex chemical reactions are occurring and indeed, the atmosphere is never free of chemical species that can interfere with the pathway outlined in R1 - R3.

In polluted atmospheres, the presence of gaseous hydrocarbons (denoted in Box 2 as RH but also known as VOC) and NO_x are instrumental in ozone formation. The key to understanding ozone formation is to recognize that if there were reactions other than R1 that could produce NO₂ without destroying an ozone molecule, and if this were followed by reactions R2 and R3 that result in the production of another ozone molecule, then a mechanism would be established that would lead to increasing ozone levels. This is in essence what is occurring in polluted atmospheres. Under certain conditions where hydroxyl radicals (HO^•) are formed photochemically, hydrocarbons (RH) are degraded to produce peroxy radicals (HO₂^• and RO₂^•) that react with NO to produce NO₂. (R4 - R8 Box 2). The net result of this series of reactions is that two ozone molecules are formed for each hydrocarbon molecule degraded. In actual fact, even more complex reactions are involved than those represented by R2 - R8, and some of these also generate ozone molecules. Considerable work has been done to try to estimate the overall yield of ozone per molecule of hydrocarbon consumed but this is a complex endeavor and it varies with the type of hydrocarbon.

Ozone is removed from the atmosphere through several processes, including both gaseous and aqueous chemical reactions, and deposition to the ground. In polluted atmospheres, during the nighttime, R2 ceases to occur since it is driven by sunlight. Consequently, R1 can dominate nighttime reactions leading to a complete removal of ozone when sufficient NO is present, as it often is in urban areas. This process is dubbed NO_x scavenging. In rural areas, NO concentrations are generally too low to scavenge ozone appreciably. NO_x scavenging can also occur during the daytime where NO concentrations are high and VOC levels are relatively low (e.g. in the early morning rush hour traffic (high NO emissions) when temperatures are still quite low (low volatility of VOC)). Therefore, ozone levels experienced during the day depend on the relative amounts of different pollutants in the atmosphere, which in turn determines which of the myriad of chemical reactions are driving ozone chemistry at a particular time.