The components of the global tropospheric ozone budget can be broken into four general categories: transport from the stratosphere, destruction at Earth's surface, photochemical destruction, and in situ photochemical production. The primary mechanism by which ozone is transported from the stratosphere into the troposphere is through meteorological events referred to as stratospheric intrusions. These events occur in conjunction with the movement of air associated with rapid changes in the intensity and position of the jet stream, the fast-moving westerly river of air that often delineates the position of strong frontal boundaries at middle latitudes. Under these conditions, the tropopause (i.e., the boundary between the troposphere and the stratosphere) often becomes contorted and its position becomes difficult to define and often takes on a "folded" depiction (see Chapter 1, "Overview: Atmospheric Chemistry). Because of this, stratospheric intrusions are also synonymous with tropopause folding events.
The topic of stratosphere-troposphere exchange was an intense research area in the 1960s and early 1970s because of the concern of transport of radioactive debris created by atmospheric nuclear bomb testing from the stratosphere into the lower atmosphere and eventually its deposition to plants, animals and human populations. During this time, the North American Ozonesonde Network was established for the primary purpose of understanding how stratospheric air was transported into the troposphere. From these data, it is generally thought that — 10% of the stratosphere is exchanged annually with the troposphere. From these estimates, the global source of tropospheric ozone from the stratosphere, which was assumed the primary natural source of tropospheric ozone could be computed (e.g., Danielsen and Mohnen, 1977).
The other primary component of the global budget of tropospheric ozone is its sink, or how it is destroyed once it is in the troposphere. The early measurements of ozone's vertical distribution always showed that lowest concentrations were near Earth's surface, implying a sink for ozone as it came in contact with the ground. These measurements generally showed much sharper vertical gradients over land and vegetated surfaces than over water and ice surfaces. Thus, one way to determine this deposition sink globally was to make a series of field measurements over a representative sample of surfaces and extrapolate these measurements to the rest of the world. Using this methodology, the globally averaged destruction rate of tropo-spheric ozone generally converged to a value near 8 to 10 x 1010 molecules 03/cm2 s. The accuracy of these estimates was claimed to be ~30%. These calculations were consistent with the few attempts to extrapolate the global input from the stratosphere resulting from stratosphere-troposphere exchange studies, which indicated that a global average of ~ 8 x 1010 molecules 03/cm2 s came from the stratosphere. Thus, up until the early 1970s, it was generally believed that the tropospheric ozone budget was balanced by the natural input from the stratosphere and the destruction at Earth's surface (Fabian and Junge, 1970). The potential impact of local-scale photochemical generation (as was known at the time for areas such as southern California) was believed to be insignificant.
A series of studies published shortly thereafter challenged this assumption and proposed that a natural source of tropospheric ozone of comparable magnitude to that of input from the stratosphere existed in the background atmosphere as a result of methane oxidation. For the first time, the paradigm of the tropospheric ozone budget was challenged resulting in a lively debate in the scientific literature in the middle and late 1970s (Chameides and Walker, 1973; Fabian, 1973; Fishman and Crutzen, 1978). These theoretical studies primarily concentrated on the generation of ozone from the oxidation of methane and carbon monoxide, the two most abundant trace gases that could lead to the photochemical formation of tropospheric ozone.
Another important component of the tropospheric ozone budget is its photochemical destruction. As ozone enters from the stratosphere, for example, it is photolyzed at shorter wavelengths to produce an excited state of atomic oxygen, O('Z)), rather than its ground state, 0(3P):
In turn, OH can react with ozone to form the hydroperoxy radical, which can set up a catalytic cycle of ozone destruction, analogous to what happens in the stratosphere:
03 + hv-> 0('Z)) + 02 (H < 320 nm) Once 0('D) is formed, it can react with water vapor to generate OH:
OH + 03 -> H02 + 02 followed by HQ2 + 03 -> OH + 2Q2
23 03 -> 02 (net reaction sequence)
6 CURRENT UNDERSTANDING OF TROPOSPHERIC OZONE BUDGET 57
Reaction (10) is the primary source of OH in the troposphere, and subsequent reactions with OH are the primary means by which most chemicals released to the atmosphere are oxidized and eventually removed. Whereas photochemistry was first proposed as an important photochemical source of tropospheric ozone in the studies written in the early 1970s, it is important to also note that photochemistry is also the dominant sink and is the primary reason that ozone concentrations are generally very low in the tropical troposphere where both water vapor and incoming solar flux are highest. The key to whether photochemistry is a net source or a net sink for tropospheric ozone is most dependent on how much NO is present.
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