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Environmental Chemistry Volume 3
A Review of the Literature Published Up to End 1982
By H. J. M. Bowen
The Royal Society of ChemistryCopyright © 1984 The Royal Society of Chemistry
All rights reserved.
BY I. COLBECK AND R. M. HARRISON
There has been an enormous growth in our understanding of the chemistry of atmospheric ozone since the early 1970's. It was Schoenbein' who first suggested that there existed in the atmosphere a constituent which had a particular odour and named it ozone. In 1858 Houzeau chemically proved that ozone existed at ground level and in 1880 Chappuis made the first spectroscopic detection of atmospheric ozone. Since the measurements reported by Dobson in 1926, routine observations of total column ozone have been conducted at a large number of places all over the world. Penndorf and Dütsch summarize the measurements of the vertical ozone distributions and of the distribution of total column ozone with latitude and season.
Figure 1 shows the variation of ozone concentration and atmospheric temperature with height. Ozone absorbs solar ultraviolet radiation in the Hartley band (200-300 nm) and at mid-latitudes, the heating of the atmosphere due to ozone absorption peaks at an altitude of approximately 50 km. When integrated over a day, it would amount to an increase of temperature of 8 K, which is balanced mainly by emission of infrared radiation by carbon dioxide.
Owing to strong mixing, the ozone mixing ratio is approximately constant in the troposphere and then increases to a maximum of about 10 p.p.m. (V/V) at a height of 30 to 35 km before decreasing with altitude above this. Above 35 km the ozone is in photochemical equilibrium. However, below this altitude the photochemical relaxation time for ozone is of the order of a few days, increasing to several years at the tropopause. Hence transport processes determine the ozone distribution below 35 km and photochemistry above that height.
Since ozone plays an important role in determining the temperature structure of the stratosphere, the most important photochemical processes in the atmosphere are those leading to the formation and destruction of ozone. Chapman established the basic photochemistry of an oxygen atmosphere [equations (1)-(4)J. In the stratosphere ozone absorbs sunlight much more strongly than does oxygen. Any depletion of stratospheric ozone would lead to an increase in the amount of harmful ultraviolet light, of wavelength between 290 and 320 nm, reaching the Earth's surface. Light of such a wavelength may cause an increase in skin cancer in humans and have adverse influences on plant systems. Over the past few years there has been considerable concern over possible changes in concentrations of stratospheric ozone due to a variety of human influences including oxides of nitrogen from supersonic aircraft, and chlorofluorocarbon aerosol propellants. The resultant research has led to a great improvement in our knowledge of the causes and effects of a stratospheric ozone reduction, although accurate quantitative information is still lacking. Ozone is a major component of the stratospheric trace gas system whose concentration is controlled by the very complex series of homogeneous chemical processes in which it takes part.
2 Ozone Sources in the Unpolluted Troposphere
The origin of the natural background level of tropospheric ozone has become a controversial subject. The major possibilities are stratosphere-troposphere exchange and photochemical processes: much work has been published in favour of each mechanism. According to the classical view ozone was inserted through the tropopause, mixed downwards and destroyed at the surface. Since molecular oxygen is not photodissociated in the troposphere, the mixing ratio of ozone thus decreases towards the surface. This mechanism has been studied by several workers; the possibility of local synthesis of ozone making the major contribution to tropospheric ozone is favoured by other.
Stratosphere-Troposphere Exchange. — The large amounts of ozone produced in the stratosphere are inhibited from entering the troposphere by a sharp increase in atmospheric stability at the tropopause. Small quantities of ozone-rich stratospheric air are transported to the troposphere by one of several stratosphere-troposphere exchange processes, often operating simultaneously, but of differing magnitude.
Reiter lists these processes as:
(i) seasonal adjustments in tropopause levels,
(ii) mean meridional circulation transport,
(iii) mesoscale and small scale eddy transport across the tropopause, and
(iv) large scale eddy transports (tropopause folding).
The height of the tropopause varies with latitude and time of year. In the Northern hemisphere it is lowest in winter, highest in summer and exhibits a poleward decrease in height. The height variation is most pronounced North of latitude 30° and results in about 10% of the mass of the Northern hemisphere's stratosphere being exchanged annually with the troposphere. Some 4 to 8 X l010 kg of ozone can be added to the Northern hemisphere troposphere by this process.
The meridional circulation consists mainly of the Hadley cell circulation. This circulation pattern can be described as two cells on either side of the equator with converging equator-ward surface currents which merge at the equator into one upward current which splits in the low stratosphere into two diverging poleward currents. The upward flow of the Hadley cell transports large amounts of tropical tropospheric air into the stratosphere. This upward flux is largest in the winter and annually is equal to 38% of the mass of the stratosphere in the Northern hemisphere. The same amount of stratospheric air returns to the troposphere in the middle and high latitudes at a rate dependent upon season. However, this process is slow with a mean residence time for air of 1-4 yr.
The hemisphere asymmetries in the Hadley cells and tropical tropopause suggest that the flux is not evenly divided and more than half of it returns to the Northern hemisphere troposphere.
Mesoscale and small-scale eddy transport across the tropopause contributes about 1 to 5% of the total flux from the stratosphere. Included in this process is the penetration of the tropopause by thunderstorms. This not only injects tropospheric air into the stratosphere, but causes downward mixing of stratospheric air into the troposphere.
The dominant process for injecting stratospheric ozone into the troposphere is believed to be large scale eddy transport. Reiter attributes 20% of the total flux from the stratosphere to this process, while Danielsen assumes that it accounts for 100% of the total flux.
Reed proposed a mechanism whereby, during high-level cyclogenesis, stratospheric air may be injected into the troposphere and further studies have been made. A net outflow of stratospheric air occurs through the 'tropopause gaps' during the deformation of the tropopause in the jet stream. The process is shown schematically in Figure 2. Since the stratospheric air is moving faster than the frontal zone it is carried into the upper portion of the frontal zone, the region where the tropopause is steeply inclined on the cyclonic side of the jet maximum. This process where the tropopause deforms, becomes vertical in the core of the jet stream, and then folds beneath the jet core is known as tropopause folding. These -intrusions have been pbserved on occasions down to ground level. Danielsenm first postulated this mechanism as a possible source of ozone in the troposphere. He found a correlation between ozone, radioactivity and potential vorticity in the upper troposphere and lower stratosphere; it had already been observed that relatively high concentrations of radioactive debris at ground-level appeared to be of stratospheric origin. This finding was later confirmed by many observational studies from aircraft, which found elevated levels of ozone in the free troposphere. On occasion high ground-level concentrations have been measured which, by isentropic analysis could be traced back to the stratosphere.
Danielsen et al. presented one of the first cases of ozone of stratospheric origin to be observed in the troposphere. They found a positive correlation between ozone and radioactivity and three mesoscale folds in the boundary between the troposphere and stratosphere over the Rocky Mountains on 25 April 1969. Mohnen et al. reported that measurements of ozone concentration at Whiteface Mountain (4860 ft) exceeded that of surface stations and that the source level lay above 850 mb. Flight experiments found a significant downward transport of ozone from the stratosphere; Danielsen and Mohnen analysed three intrusion cases (18, 26, and 27 April 1975) over Colorado, Oklahoma, and Texas. Estimates of the mass transfer were made based on radioisotope (strontium-90) concentrations and observations of the rate of deposition of 90Sr. They predicted an annual outflow of 4.95 x 1014 g ozone in the Northern hemisphere. Concentrations of 100 to 210 p.p.b. near major atmospheric lows were measured from aircraft at 60°N over Canada in 1977. During project Gametag, Danielsen found that the ozone concentration increased from 40 p.p.b. up to 100 p.p.b. at 5.4 km on a flight from San Francisco to Hawaii. This air was traced back by isentropic analysis to the stratosphere, from where it had originated 3 days previously. This rate of transport is over one hundred times faster than motion due to zonal seasonal mean circulations. Singh et al. and Johnson and Viezee report on a programme of aircraft measurements to observe tropopause folding in the Spring and Autumn of 1978. Measurements in the Spring were made over Colorado, Kansas, Oklahoma, Minnesota, and Mississippi, while those in Autumn were made over Minnesota, Iowa, Illinois, and Tennessee. Ten dates are given when ozone concentrations ranged from 177 p.p.b. to 362 p.p.b. over an altitude range from 5.2 km to 7.5 km. As predicted by Danie1sen, all the ozone intrusions were found on the North side of the jet stream and were associated with upper-level low pressure troughs.
High ground-level concentrations of ozone due to isentropic transport of ozone from the stratosphere have been measured by a few workers. Attmannspacher and Hartmanngruber reported seven such occasions in the winter of 1971 with concentrations between 250 and 500 p.p.b. lasting 10 min or longer. Each occurrence was during a snow storm accompanying a passing cold front. Five consecutive hours of ozone levels in excess of 80 p.p.b. with a maximum hourly average concentration of 230 p.p.b., during the early hours of a cold night in November at Santa Rosa, California, are reported by Lamb. He concluded the stratospheric ozone was transported to the ground in precipitation-driven downdrafts. Derwent et al. observed ozone levels in excess of 100 p.p.b. at two rural sites in Britain in Spring 1976 and 1977. In both cases, frontal troughs crossed the area a day before the elevated levels were measured and cross-sections of the atmosphere showed that a stratospheric intrusion in the trailing edge of the jet stream occurred. A maximum concentration of 223 p.p.b. was reported by Haagenson et al. on 4 March 1978 in Denver, Colorado. Isentropic analysis indicated that the air had originated from a stratospheric intrusion 3 days earlier. This air was also subject to enhanced photochemistry due to local pollution.
Since there is a positive correlation between radioisotope concentrations and ozone in the region of the tropopause it is possible to identify stratospheric ozone at ground-level by measuring radioactive debris. Husain et al. used 7Be to indicate the presence of stratospheric air on the summit of Whiteface Mountain. They found two days in July 1975 with peaks in 7Be concentrations which corresponded with ozone peaks. They deduced an upper limit of 37 p.p.b. was due to stratospheric ozone. Simultaneous measurements of 7Be and ozone in the lower stratosphere have shown that their ratio is nearly constant at ca. 11f Ci m-3 p.p.b.-1. Hence ground-level 7Be can be used to estimate the associated stratospheric ozone. Dutkiewicz et al. used this method and found that the greatest impact of stratospheric ozone at Whiteface Mountain occurs during late Spring and early Summer. They estimated that 24 h average ground-level ozone was increased by 12 p.p.b. due to tropopause folding events. Reiter correlated 90Sr with ozone-sonde measurements made between December 1962 and December 1965. He found that stratospheric intrusions giving rise to ozone concentrations over 80 p.p.b. occur on 0.2% of the days of the year. Other instances of high ozone concentrations at ground-level due to stratospheric intrusions have been reported by Shapiro and Singh et al. Singh and his co-workers measured an ozone concentration of 196 p.p.b. at Zugspitze, W. Germany, in January 1976. The 7'Be concentration showed a 600% increase during this episode.
The overall contribution of stratospheric ozone to the tropospheric ozone budget will be discussed further later.
3 Photochemistry of the Clean Troposphere
It was originally thought that ozone in the troposphere was inert; it was transported from the stratosphere and destroyed at the ground. However, recently it has been suggested that ozone is not inert, even in the unpolluted troposphere, and that photochemical processes are important in the overal ozone budget. The photochemistry of the troposphere has been studied and modelled by many workers in attempts to calculate the relative importance of downward transport from the stratosphere and gas-phase photochemical production and destruction of ozone.
The hydroxyl radical plays a dominant role in the chemistry of the troposphere, The primary source of tropospheric OH results from the reaction in which ozone is photolysed to yield O('D) by ultraviolet radiation in the 300-320 nm range, reaction (3). The hydroxyl radicals are then formed from the reaction of O('D) with water
[FORMULA NOT REPRODUCIBLE IN ASCII] (5)
[FORMULA NOT REPRODUCIBLE IN ASCII] (6)
It is in this way that the OH production results in a net cycle of ozone destruction and that ozone acts as the precursor to most unpolluted troposphere photochemistry. The OH formed by reactions (3) and (5) may react with methane and carbon monoxide to initiate catalytic cycles which produce ozone in the troposphere.
[FORMULA NOT REPRODUCIBLE IN ASCII] (7)
[FORMULA NOT REPRODUCIBLE IN ASCII] (8)
[FORMULA NOT REPRODUCIBLE IN ASCII] (9)
[FORMULA NOT REPRODUCIBLE IN ASCII] (10)
If NO exists in sufficient amounts then
[FORMULA NOT REPRODUCIBLE IN ASCII] (11)
[FORMULA NOT REPRODUCIBLE IN ASCII] (12)
If formaldehyde is photolysed, then
[FORMULA NOT REPRODUCIBLE IN ASCII] (13)
[FORMULA NOT REPRODUCIBLE IN ASCII] (14)
[FORMULA NOT REPRODUCIBLE IN ASCII] (15)
[FORMULA NOT REPRODUCIBLE IN ASCII] (16)
and ozone is produced via reaction (2). The net cycle for methane oxidation is then
[FORMULA NOT REPRODUCIBLE IN ASCII] (17)
For CO, oxidation reactions (9) and (10) can be followed by reactions (15), (16), and (2) resulting in the net cycle
[FORMULA NOT REPRODUCIBLE IN ASCII] (18)
At low concentrations of NO ozone may be destroyed by
[FORMULA NOT REPRODUCIBLE IN ASCII] (19)
competing with (15).
Ozone is also destroyed by its reaction with nitric oxide
[FORMULA NOT REPRODUCIBLE IN ASCII] (20)
Thus the operation of a tropospheric photochemical source for ozone depends upon the background levels of NOx. The amount of ozone which can be formed in the troposphere by this mechanism is limited by the concentrations of CO and CH4. It is possible that methane oxidation could yield 3.5 molecules of ozone per molecule of methane. Methane is released from the earth's surface to the atmosphere at an average rate of 1.5 x 10l1 molecules cm-2s-1. Carbon monoxide is produced in the troposphere from the formaldehyde formed during methane oxidation. Other sources of carbon monoxide provide 2.5 x 1011 molecules CO cm-2 s-l, Hence, the possible yield of ozone from CH4 and CO oxidation could be 8 x 1011 molecules cm-2 s-l if NO existed in sufficient quantities.
The equilibrium distribution of O3, in sunlight, is governed by reactions (16), (2), and (20), so in a photostationary state
[FORMULA NOT REPRODUCIBLE IN ASCII]
Ozone is destroyed via reactions (3), (5), (9), (10), and (19) and the net ozone destruction reaction is
[FORMULA NOT REPRODUCIBLE IN ASCII] (21)
A schematic representation of the odd oxygen photochemistry in the troposphere is shown in Figure 3.
It is possible, by using numerical models, to calculate the effect of the various reactions in the troposphere and compare the results with in situ measurements. The results of such calculations lead to the proposition that significant amounts of ozone are produced in the troposphere from CH4/COO/NOx, photochemistry. Fishman and Crutzen based their argument on the fact that the hemispheric asymmetry in the tropospheric ozone distribution could not be explained satisfactorily by differences in the stratosphere-troposphere exchange rates in the two hemispheres. They postulated that the difference between the fluxes into and out of each hemisphere could be caused by photochemical processes in the troposphere.
Excerpted from Environmental Chemistry Volume 3 by H. J. M. Bowen. Copyright © 1984 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
ContentsChapter 1 Tropospheric Ozone By I. Colbeck and R. M. Harrison, 1,
Chapter 2 The Environmental Chemistry of Organotin Compounds By S. J. Blunden, L.A. Hobbs, and P. J. Smith, 49,
Chapter 3 Determination of Heavy Metals in Sewage Sludge By S. A. Katz, 78,
Chapter 4 Inorganic Deposits in Invertebrate Tissues By M. G. Taylor and K. Simkiss, 102,