Tropospheric Ozone Research Paper

1. Atmospheric Ozone

Ozone is a 3-atom allotrope of oxygen and thus has the chemical formula: O₃. It is gas with a pungent odor and hence its name, which is derived from the Greek word ozien meaning smell. Christian Fredrick Schobein generally is credited with first identifying O₃as a compound in the laboratory in 1840, and John Hartley is credited with demonstrating its ubiquitous presence in the atmosphere in 1881.

O₃ is one of a myriad of atmospheric trace species, so called because their atmospheric abundances are of the order of one part per million (ppm) or less, where 1 ppm of a gas signifies an atmospheric abundance of 1 molecule for every million atmospheric molecules. In fact, in the case of O₃ there are on average about 3.5 O₃ molecules for every 100 million atmospheric molecules. If the entire atmosphere were compressed to a pressure of 1 atmosphere (i.e., the pressure at the earth’s surface), it would have a height of about 8.5 km. If the same were done to atmospheric ozone (O₃ ), it would have a height of only 0.3 cm.

Despite its seemingly minute concentration, atmospheric O₃ has an enormous environmental impact. As a constituent of the stratosphere, where about 90 percent of the atmosphere’s O₃ resides, it protects life from harmful uv radiation from the sun. In the troposphere, O₃ plays a key role in determining the oxidizing capacity of the atmosphere, facilitating the process by which the atmosphere is flushed of a wide range of pollutants. O₃ at ground level represents a small but important component of the overall tropospheric O₃ burden. Because of its properties as a powerful oxidant, ground-level O₃ can have adverse effects on human health, agricultural crops, and forests

As illustrated in Fig. 1, O₃ concentrations generally increase with increasing altitude throughout the troposphere (the lowest 10–15 km of the atmosphere) and much of the stratosphere. O₃ concentrations typically range from about 30 ppb near the ground to about 100 ppb or more near the top of the troposphere, where 1 ppb O₃ signifies an abundance of 1 molecule of O₃ for every billion molecules of air. However, ground level O₃ concentrations can exceed 100 ppb during air pollution episodes and especially in urban areas. The highest hourly-averaged O₃ concentration on record in North America is apparently 680 ppb measured in downtown Los Angeles in 1955. When ground-level O₃ concentrations reach these levels, they threaten rather than sustain life. In particular, they can damage agricultural crops, ultimately causing a reduction in the crop yields grown in affected regions.

2. The Generation Of Tropospheric O₃

The basic chemical mechanism responsible for the generation of O₃ in the stratosphere was first worked out by Sidney Chapman in the 1930s. However, it would take a few more decades before scientists began to understand the chemical reactions that produce tropospheric ozone.

Stratospheric O₃ production involves two reactions (see Fig. 1). The first is the photolysis and dissociation of molecular oxygen (O₂ ) by uv radiation from the sun. The second reaction involves the combination of an oxygen atom produced from the first reaction with another O₂ molecule.

Because ultraviolet radiation from the sun capable of photolyzing O₂ does not penetrate into the troposphere, it was believed for a number of decades following Chapman’s work that O₃ is an inert constituent of the troposphere. It was argued that the downward transport of O -rich air from the stratosphere is the source of tropospheric O₃ and reaction with material on the earth’s surface (a process referred to as dry deposition) removes O₃ . However, the appearance of photochemical smog in Los Angeles in the 1950s, and the realization that O₃ was a major product of this pollution forced a gradual reappraisal of this simple picture. Today, it is recognized widely that chemical production of O₃ occurs throughout the troposphere, at relatively modest rates within the more pristine regions of the atmosphere and at significantly accelerated rates in polluted regions.

2.1 The Chemical Reactions That Produce Tropospheric Ozone

A scientist by the name of A. J. Haagen-Smit first unraveled the chemical mechanism that produces O₃ in the lower atmosphere. Working in Los Angeles in the early 1950s, Haagen-Smit became interested in understanding how photochemical smog was formed and what was in the smog that made it noxious. He irradiated mixtures of volatile organic compounds (VOC) and nitrogen oxides (NO_x) in a reaction chamber. (Both VOC and NO_x are emitted into the atmosphere by automobiles and other industrial processes and NO_x represents the sum of nitric oxide or NO and nitrogen dioxide or NO₂.) After a few hours, Haagen-Smit observed the appearance of cracks in rubber bands stretched across the chamber. Knowing that ozone (O₃) can harden and crack rubber, HaagenSmit correctly reasoned that photochemical smog was caused by photochemical reactions involving VOC and NO_x, and that one of the major noxious constituents in this smog was O (see Fig. 1).

Since Hagen-Smit’s work, more sophisticated lab- oratory experiments, along with advanced chemical monitors and computer simulation models have made it possible to further elucidate the chemical reactions that form O in the troposphere. A typical reaction sequence might include:

(a) RH + OH → R + H₂O

(b) R + O₂ + M → RO₂ + M

(c) RO₂ +NO → RO + NO₂

(d) RO + O₂ → HO₂ + carbonyl

(e) HO NO OH NO

(f) 2 × (NO₂ + hν → NO + O)

(g) 2 × (O + O₂ + M → O₂ + M)

—————————————-

NET:RH + 4O₂ + 2 hν → 2O + carbonyl + H₂O

In the above, RH represents a hydrocarbon (i.e., a type of VOC), R is a hydrocarbon radical or compound fragment (e.g., CH₂ CH₂ for RH = ethane), hν represents a photon from the sun and M is a nonreactive, energy absorbing molecule referred to by chemists as a ‘third body.’ In the atmosphere, this third body is most often molecular nitrogen (N₂ ) or molecular oxygen (O₃ ). Carbon monoxide (CO) can replace the VOC. In this case the reaction sequence is initiated by

(h) CO + OH → CO + H

(i) H + O₂ + M → HO₂ + M

and then followed by Reactions (e), (f), and (g).

In the above reaction sequences, VOC and CO are oxidized and consumed, while NO_x acts as a catalyst. Collectively these three compounds are often referred to as O₃ precursors. The by-product labeled ‘carbonyl’ is itself a VOC and can react and produce more O₃. The OH radical, which triggers the sequence by reacting with VOC or CO, is generated initially in the atmosphere from O₃:

(j) O₃ + hν → O* + O₃

(k) O* + H₂ O → 2OH

where O* is an electronically excited atom of oxy- gen. Because the OH consumed by Reaction (a) or (h) is regenerated by Reaction (e), and, because more OH can be generated during the breakdown of the carbonyl produced by Reaction (d), the reaction sequences listed above are net sources of O₃ .

In clean regions of the troposphere, O₃ is produced primarily by the oxidation of isoprene (C₅ H₈ ) and/or methane (CH₄ ), hydrocarbons that are emitted into the atmosphere by biogenic processes, as well as CO. The catalyst NO_x largely is supplied by lightning as well as soil emissions. In polluted regions, VOC from automobiles, solvents, and various industrial processes become important, and the NO_x arises largely from automobiles and power-plant emissions.

2.2 Ozone Pollution Events

Ground-level O₃ concentrations tend to be largest during specific periods or episodes when meteorological conditions favor the production and accumulation of pollutants within the atmospheric boundary layer. (The atmospheric boundary layer denotes the first 1–2 km of the atmosphere where winds are slowed by frictional resistance with the earth’s surface. The frictional resistance, in turn, creates turbulence within the layer that tends to keep pollutants and other chemical constituents uniformly mixed. For this reason the atmospheric boundary layer is also referred to as the mixed layer.) These conditions are often associated with summertime, slow-moving high-pressure systems that can extend over thousands of kilometers. Ground-level O₃ concentrations during

these pollution episodes tend to be highest in and around urban areas. However, suburban sprawl, growing numbers of automobiles and expanding roadway networks coupled with the large spatial scale of the meteorological conditions that foster the pollution, can lead to enhanced O₃ concentrations over thousands of kilometers, encompassing agricultural as well as urban areas. To distinguish this type of pollution from more localized or urban-scale pollution, it is often referred to as regional air pollution. The repetition of regional air pollution episodes in agricultural areas over a growing season can produce a pattern of chronic exposure that can, in principle, reduce crop yields. The extent of crop-yield reduction depends upon both the degree of exposure and the sensitivity of the crop to O₃ , as discussed below.

3. Ozone Effects On Crops

The first documentation of O₃ damage to crops arose out of studies of the effects of photochemical smog in Los Angeles in the 1940s and 1950s. Subsequent research by scientists throughout the world have provided a rudimentary understanding of the mechanisms by which O₃ can affect vegetation and the development of semiquantitative relationships between exposure to O₃ and crop yield.

O₃ damage to vegetation most often occurs as a result of penetration into the leaf interior through the stomata and then reaction with the cell’s plasmalemma (i.e., cell membrane) and/or mesophyl (i.e., cell interior). Field studies using open-top chambers to control the O₃ concentrations in contact with crops growing under otherwise ambient conditions have established that crop sensitivities to O₃ vary from crop to crop and, within a given crop, from variety to variety. In general: (a) O₃ damage to crops becomes observable when concentrations exceed a threshold of about 50–70 ppb; (b) the effects of repeated intermittent exposures above this threshold tend to accumulate over a growing season; and (c) the higher the O₃ concentration above the threshold the greater the damage.

Because O₃ damage to crops accumulates over a growing season, any metric used to quantify crop exposure to O₃ must integrate over all pollution episodes occurring in that season. One such metric is the so-called ‘SUM06’ that represents the sum of all daytime, 1-hour-averaged O₃ concentrations in excess of 60 ppb over a 3-month period. Analysis of data from the US National Crop Loss Assessment Network suggests that the average yields of crops grown in North America are reduced by about 10 percent when SUM06 exposures are about 15–25 ppm-hours, where 1 ppm-hour is equivalent to 1,000 ppb-hours. Exposures of this magnitude are not uncommon in rural areas of many industrialized and developing nations.

3.1 Implication For World Food Production

O₃ damage to crops is likely to be most severe in those areas where there is an intermingling of the urban and industrial activities that lead to the emissions of VOC, CO, and NO_x and agricultural activities on spatial scales of thousands of kilometers. It turns out that such an intermingling of the urban and industrial with the agricultural is quite common in the developed world and is most pronounced in three regions of the northern mid-latitudes: (a) Eastern North America (25–50°N 105–60°W); (b) Europe (36–70°N, 10°W– 90°E); and (c) eastern China and Japan (25–45°N, 100–146°E). Within each of these regions, urban– industrial and agricultural activities tend to cluster together into a single large network or plexus of human-impacted land-use categories. The term Metro-Agro-Plex (or MAP) is sometimes used as a shorthand to describe this intermingling of agricultural and urban-industrial activities within a developed or developing geographical area. The term Continental-Scale Metro-Agro-Plexes (or CS-MAPs) is in turn used to specifically refer to the three large MAPs of the northern mid-latitudes.

Collectively, the CS-MAPs represent a major force in the global economy and also in environmental change. Although they comprise only 23 percent of the earth’s land surface, the three CS-MAPs account for about 75 percent of world’s consumption of commercial energy and fertilizers and about 60 percent of the food crop production and food exports. They are also major source regions for atmospheric pollutants including the O₃ precursors. Estimates using a global atmospheric chemistry model indicate that, because of these emissions, about 10–35 percent of the world’s cereal crops currently are exposed to potentially damaging O₃ pollution within the CS-MAPs. The net effect of this exposure to the world’s total production of cereals and other food staples probably amounts to few percent decrease in total production. However, the presence of other pollutants that accompany O₃ in photochemical smog most likely exacerbate the crop losses. For example, the haze associated with photochemical smog and O₃ pollution reduces the amount of sunlight that reaches the earth’s surface and this likely also reduces crop yields. Moreover, as pollutant emissions increase in developing nations the impacts on agriculture within these areas will become more severe. The economic implications of such crop losses have yet to be assessed. However, in countries like China, where growing populations and rising standards of living may soon begin to strain food production capacities, the effect of O₃ and related pollutants on agriculture could conceivably have adverse or even dire consequences.

4. Tropospheric Ozone And Global Change

The spread of O₃ pollution may extend beyond the regional scale to the continental, and even hemispheric scale. Analyses of late nineteenth and early twentieth century O₃ datasets indicate that tropospheric O₃ in the northern hemisphere has undergone significant variations over the past century and continues to vary in some regions of the globe today. The evidence for long-term variations in free tropospheric O₃ is strongest in Europe where measurements of O₃ date back to the 1800s. For example, German scientists Volz and Kley (1988) used current techniques for measuring O₃ to recalibrate O₃ measurements made in Paris in the late nineteenth century. Their analysis indicates that tropospheric O₃ concentrations at the end of the 1800s were about a factor of 2 less than current concentrations over Europe. Subsequent analyses have reached similar conclusions. The implications of such a long-term O₃ trend for understanding O₃ pollution effects on humans and ecosystems have yet to be fully explored but could be considerable.

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