630 Environ. Sci. Technol.. Vol. 25, No. 4, 1991
CHANGING
O Z O N E Evidence for a perturbed atmosphere Stuart A. Penkett University of East An& Norwich NR4 7T1,England
I
The composition of our atmosphere is dominated by nitrogen and oxygen, which are both products of fundamental biological processes. The presence of ozone, however, shows that chemistry also has a major role to play in determining atmospheric composition and long-term stability, especially at the trace gas level. The concentration of ozone in the Earth's atmosphere is on average ahout 3 parts in 10 million by volume and does not exceed 10 parts in a million, yet the mass flux through ozone is equal to that through oxygen. When looked at this way, which takes chemical reactivity as well as molecular abundance into account, ozone becomes a primary indicator of the overall stability of the atmosphere. There is much legitimate concern therefore about the dramatic loss of ozone that is observed in the stratosphere over Antarctica in the Austral spring (1). There should he even more concern ahout the increasing losses of ozone observed recently in polar and midlatitude regions of the stratosphere in the northern hemisphere (21. In addition to the ozone losses occurring in the stratosphere, there is now compelling evidence that the ozone concentration in large parts of the northern hemisphere troposphere is increasing steadily (3-5). This is almost certainly caused by emissions of hydrocarhons, carhon monoxide, and nitrogen oxides from combustion processes of many kinds, but principally transport and biomass burning. The increase in ozone concentration in the troposphere will ameliorate the loss in the stratosphere to
0013-936W91/0925-630$02.50/0 0 1991 American Chemical Society
some extent with respect to the levels of UV light reaching the ground (6). However, more damage will be caused to vegetation by higher ozone levels close to the ground, and changes in ozone throughout the atmosphere will influence its vertical temperature profile. Ozone formation in the troposphere All ozone in the atmosphere is formed photochemically. In the stratosphere it is formed from oxygen photolysis by short wavelength UV light (< 242 nm), and some of this is transferred to the troposphere. Ozone is formed in the troposphere from the photolysis of nitrogen dioxide by light of longer wavelength. hv NO, LN O + O (1) h < 400 nm O+O,+M+O,+M
(2)
The ozone formed in Equation 2 reacts rapidly with the NO formed in Equation 1: 0, + NO +NO,
+ 0,
(3)
In the absence of other gases, a simple photochemical equilibrium results:
For ozone to build up, other processes are required that oxidize NO to NO, without consuming ozone. These involve peroxy radicals, such as HO, and RO,, formed in the course of oxidation of many hydrocarbons (RH),and carhon monoxide by hydroxyl radicals. Environ. Sci. Technol., Vol. 25. NO.4, 1991 631
It is interesting to compare detailed ozone records from sites in Europe H + 0, + HO, (5) and the United States. Two published data sets can be used; alHO, + N O + NO, + OH ( 6 ) though they represent only part of the overall picture, they are not atypand ical. The ozone record for Bottesford in central England is shown in FigRH+OH+R+H,O (7) ure 1, where the data are presented as maximum hourly mean ozone R + 0, + RO, ( 8 ) concentrations for each day throughout 1984. When shown this way, the RO, + NO + NO, + RO (9) high ozone episodes in summer are seen to sit on top of an annual averRO, + 0, + HO, + R'CHO (10) age ozone concentration, which peaks in spring. For sites in the Equations 6 and 9 adjust the balance southeastern United States (Figof NO and NO2 in Expression A and ure 21, this spring maximum is much cause ozone to build up. Equation 6 harder to pick out because of the also leads to the reformation of hy- large amount of ozone production droxyl radicals, which can then fur- occurring from April through Nother oxidize CO and hydrocarbons in vember. The spring maximum probaa reaction chain. bly is still present in Figure 2 and can be seen in ozone records from the Regional ozone formation western United States (11). Photochemical ozone formation The reasons for these differences was first observed in Los Angeles in are complex, and meteorology has a the 1940s (7)and subsequently in large influence. The English site is other cities in the United States (8). probably a more polluted site with In the years between the 1940s and respect to nitrogen oxides than is the present, research has shown the U.S. site in Tennessee. This is that extensive ozone formation can shown by the low values of ozone occur in the summer months; that that sometimes occur in winter at is, elevated-ozone episodes can si- the English site (Figure 31, caused multaneously cover large parts of almost certainly by the reaction of western Europe or the southeastern ozone with NO to form NO, (EquaUnited States. The realization that tion 3). Low ozone concentrations ozone was involved in long-range occur less frequently at the U.S. site. In spite of being less polluted, the transport was first made in Europe (91,and European studies have fo- atmospheric regime in the United cused on ozone as a regional rather States appears to be more conduthan an urban pollutant (10). This cive to ozone formation than it does regional issue has also received at- in England. This may be caused in tention in the United States over the part by large emissions in the southern United States of natural hydropast decade ( I I). CO+OH+CO,+H
(4)
632 Envimn. Sci. Technol., Vol. 25,No. 4, 1991
carbons, such as isoprene and various terpenes, from vegetation stimulated by higher temperatures and a greater degree of forest cover. In this respect it has been calculated that 5 ppbv (parts in 10') of isoprene is as effective in generating ozone as 50 ppbv of a typical mixture of anthropogenically emitted hydrocarbons (12, 13). The differences between Europe and the United States are intriguing, and a common research program that pools data on measurements of ozone and its precursors, particularl y hydrocarbons and nitrogen oxides, would increase our understanding of this important regional problem. Background ozone formation It used to be argued that the cause of the spring maximum observed in many ozone records (see Figure 11 was enhanced transfer of ozone from the stratosphere during this period (14). This may be partly true, but improved theories accounting for ozone in the troposphere now suggest that in situ production is the dominant mechanism (4). Also, there are data from old ozone records to suggest that ozone levels were much lower a century ago and that the seasonal variation was much smaller. Instead of the average concentration of 20 to 40 ppbv typically observed in Europe now (15). the levels were then much closer to 10 ppbv (3). The data in F i g u r e 3 c o n t r a s t two o z o n e records, a recent one from Cape Arkona on the Baltic coast of eastern Germany and one from an observatory southwest of Paris, where
separation of HO, from NO by differential solubility in cloud droplets can also act as an additional ozone sink (181. The key to increased ozone production in the background atmosphere is therefore larger and more widespread sources of NO,. This can occur from industrialization of underdeveloped nations, whose emissions are expected to increase considerably over the next few decades (19), and from extensive biomass burning (20). There is no doubt now that activities such as biomass burning are emitting large amounts of NO, in South America, Africa, and parts of Asia. Experiments in these regions have shown elevated ozone concentrations over areas remote from large urban sources (20). Another important source of NO, in remote areas could be exhaust from subsonic commercial aircraft, which, although they emit only a small percentage of the total NO,, do so in a way that it reaches parts of the atmosphere that are relatively low in NO, and that are isolated from ground-based sources (21-231. Large fractions of the NO, emitted at the ground will be removed by chemical reaction Equations such as 15, 16, and 1 7 before they can pollute the background atmosphere. NO, + OH -+ " 0 ,
NO,
+ 0, +NO, + 0,
NO, +NO,
+ N,O,
(15) (16) (17)
The nitric acid formed in reaction Equation 15 and from the reaction of N,O, with water droplets is mostly removed either in rain or by absorption at the ground surface. over a century ago ozone was measured using a well-calibrated technique. The pronounced spring maxi m u m i s definitely a m o d e r n phenomenon, possibly associated with photochemical destruction of a reservoir of ozone precursor molecules that builds up at mid to high latitudes in the winter (16, 17). In the background atmosphere, photochemistry can be both a source and a sink for ozone.
as Equations 4 and 5, can react with ozone, or they can combine with themselves when NO concentrations are low.
+ 0, + HO + 2 0 ,
(13)
+ HO, + H,O, + 0,
(14)
HO, HO,
The combination of the reaction Equations 11-14 represents a substantial loss of ozone. Comparing the rates of reaction Equations 6 hv and 13 shows a ratio of about 5000, 0, = ___) 0,+ O('D1 (11) which means that ozone will be destroyed photolytically at NO conh < 310 nm centrations < IO pptv (parts in IO" O('D) + H,O -+ OH + OH (12) by volume). This is a common situation i n the more remote atmoThe HO, radicals produced in sub- sphere over the ocean, especially in sequent reactions of hydroxyl, such tropical regions. It is possible that
Regional ozone exposure levels There is evidence that ozone concentrations are increasing throughout substantial parts of the troposphere of the northern hemisphere. This is shown in various sets of data from long-running ozone measurement experiments, including the National Oceanic and Atmospheric Administration network of sites (24). Two spectacular data sets are those from Hohenpeissenberg in Bavaria (25) and Payerne in Switzerland, where ozone sondes have shown increases in ozone, from the ground to the tropopause, from 1968 to the present (26). Figure 4 shows the data record for Payerne with losses in ozone in the low stratosphere and gains in ozone Environ. Sci. Technol., Vol. 25, No. 4, 1991 633
I l l
throughout the troposphere. Calculations confirm that these increases in the troposphere would be expected from the increase in precursor emissions known to have taken place over the same period (27). The overall effect of these changes is to increase 'the baseline level on which ozone concentrations in regional pollution episodes are superimposed. Control measures introduced to prevent high summer ozone concentrations may therefore be ineffective if they do not address the problem of the changing background ozone. This can be seen quite readily in the ozone data records shown in Figures 1 and 2. If the background ozone levels are now between 25 and 40 ppbv, rather than 10 to 15 ppbv, ozone concentrations will more frequently exceed 80 ppbv, the level at which damage occurs for many plant species (1 5). International ozone studies Internationally coordinated studies are now being carried out to assess the potential for perturbed ozone chemistry over large areas of the northern hemisphere and are planned for parts of the southern hemisphere. EUROTRAC is an environmental project of the EUREKA initiative, which seeks to stimulate cooperation of European countries on matters, mostly technical, of mutual interest. The EUROTRAC project was
I
I
I
I
set up in 1986 to study oxidant formation, acidity production, and interaction of pollutants with the biosphere. Within EUROTRAC is the subproject TOR, which focuses on tropospheric ozone research in the atmosphere over and surrounding Europe (28). At present TOR has two main components. One is the continuous collection of high-quality measurements of ozone and other oxidants (e.g., hydrogen peroxide and peroxyacetyl nitrate), along with many of their precursors (e.& NO,, NOy, and hydrocarbons, etc.) at a network of ground-based sites. These sites stretch from northern Norway down through Britain, Holland, and Germany, and across to Austria and Yugoslavia. Several sites are coastal, others are low-level continental, and a few are high-level (mountain) continental. The object is to develop a data base on the concentrations of these many chemical species throughout the year, which can then be analyzed by comprehensive models of chemistry and transport covering the region where the measurements are collected. In addition, vertical soundings covering the troposphere and the lower stratosphere are made of ozone and, in the future, of H,O, and NO,. These data will be helpful in determining the capacity of the atmosphere t o generate ozone above, as well as within, the planetary boundary layer.
634 Environ. Sci. Technol., Vol. 25, NO. 4, 1991
There is an analogous program in the southeastern United States. The Southern Oxidant Study (SOSl is based on principles similar to those of TOR but focuses its experimental effort on high-quality ground-based measurement sites and more numerous, but less well equipped, auxiliary sites, measuring ozone and a few other compounds. Highquality modeling efforts are more advanced in this study; this should create a much better understanding of ozone formation from both anthropogenic and natural emissions of precursor gases. Both TOR and SOS are concerned primarily with ozone production over continents, which are the predominant source of the precursor gases. For the northern hemisphere background concentration of ozone to be perturbed, however, it is necessary that ozone, or its precursors, be transported over thousands of kilometers and that ozone is produced in areas that are truly remote from sources (29). There is no difficulty for longlived molecules such as methane and carbon monoxide to reach most parts of the northern hemisphere within their atmospheric lifetimes (ca. 10 years and 2 months, respectively). Also, the lifetimes of many other hydrocarbons become sufficiently long in the winter for them to become well mixed in the free troposphere at mid to high latitudes (17). Measurements confirm this, but as yet there are not enough data on the levels of NOJNO compounds in the backgroundatmosphere. This gap in chemical information needs to be addressed, and the best way to do this is through the International Geosphere-Biosphere Programme (IGBP). The IGBP was created in response to concern about changes in the global environment (30). The program has many components and incorporates most scientific disciplines from biology through chemistry to physics and meteorology. The atmospheric components deal with changes known to be taking place in the stratosphere (STIBl and the troposphere [International Global Atmospheric Chemistry Program or IGAC). IGAC is further subdivided into scientific foci dealing with the atmospheric chemistry of discrete regions of the earth. One of the activities within IGAC is the North Atlantic Regional Experiment [NAREI, which has scientists involved in regional programs such as TOR and SOS on both sides of the Atlantic.
IGAC has the following objectives: to understand the transport, transformation, and deposition of continental emissions-especially hydrocarbons, CO, NO,, SO,, and resulting reaction products such as 0,, H,O,, HNO,, and sulfate aerosol-in the marine troposphere: and to study the delivery of these compounds to the ocean and their impact on surface sea water chemistry and marine biological . productiiity Essentiallv NARG seeks to understand the i d h e n c e of continental emissions from North and Central America, Europe, and North Africa on atmospheric chemistry occurring over the North Atlantic. In particular, it aims to examine the capability for ozone formation and destruction throughout the year by carefully measuring NO, and NOY species and studying their influence on the fate of free radicals, such as HO,. The nitrogen compounds are present in air over the Atlantic from emissions from the surrounding continents, from natural sources (mostly lightning), and from subsonic commercial aircraft, which may play a critical role. Other IGAC activities concerned with ozone production in the global atmosphere are the Global Tropospheric Ozone Network and the
.
Global Atmospheric Chemistry Survey. Also, the Impact of Tropical Biomass Burning and the Polar Atmospheric Chemistry programs will have ozone production studies as a primary objective. It is hoped that these extensive studies of ozone in the background troposphere materialize and allow us to develop a clear quantitative link between emissions of pollutants and their effects. The changes we have already observed in the ozone concentrations throughout the atmosphere clearly demonstrate the capacity for human activity to influence the fundamental workings of our atmosphere. We should, at the least, try to understand what we are doing, even if we find it difficult to stop. References (1) Farman. (2)
(3) (4)
(5)
(6)
(7) (8)
(9)
J, C.; Gardiner. B. G.; Shanklin. 1. D. Nature 1986.315.207-10. Stratospheric Ozone 1990: United Kingdom Stratospheric Ozone Review Group. HMSO London, 1990. Volz. A.; Kley, D. Nature 1988. 332, 240-42. Penkett, S. A. In The Changing Atmosphere; Rowland. F. S.: Isaksen. I.S.A.. Eds.; Wiley: Chichester, England, 1988; pp. 91-103. Crutzen. P. J. In Tropospheric Ozone. Regional and Global Scale Intemctions: Isaksen. I.S.A.. Ed.: D. Reidel: Dordrecht. Holland, 1988: pp. 3-32. Bruhl, C.; Crutzen. P. J. Geophys. Res. Letts. 1989. 16. 703-706. Haagen-Smit. A. J.: ind. Eng. Chem. 1952,44.1362. Air Quality Criterio for Photochemic a l Oxidants: U.S. Department of Health. Education. and Welfare, Public Health Service. US. Government Printing Office: Washington. DC. 1 9 6 6 NAPCA-PUB-AP-63. p. 202. Cox, R. A. et al. Nature 1975, 225.
118-21.
(lo) Grennfelt. P.: Salthones. J.; ScholdagStuart A. Penkeit is professor of atmoof the University of East Anglio in Norwich. Englond. He hos been studying chemicol phenomena i n t h e atmosphere for more t h a n 20 years andfirst made the suggestion that sulfur dioxide was oxidized by ozone, and particulorly by hydrogen peroxide, to form sulfuric ocid in clouds. This is now recognized as the principal route to the acidification of rainwater. He is a member of the Max Planck Society and of the Notional Academy of Sciences Committee on Atmospheric Chemistry and vice-chairman of t h e Scientific Steering Committee of EUROTRAC. His research interests include studies of cloud chemistry, ozone chemistry in the troposphere, and the tmce gas composition of the troposphere o n d the stmto-
spheric chemistry
sphere.
er. J. In Rep. NlLU OV22l87, Norway Institute for Air Resources: Lillestrom, Norway. 1987. (11) Logan, I. A. /. Geophys. Res. 1989, 94lD6). 8511-32. (12) Trainer, M. el al. Nature 1987, 329, 7n5 . (13) Chameides. W. L. et al. Science 1988. 241,1473-75. (14) Singh. H. E.: Ludwig. F. L.; Iohnson. W. B. Atmos. Environ. 1978. 12. 2185-96. (15) Ozone in the UnitedKingdom. United Kingdom Photochemical Oxidants Review Group Interim Report: Department of the Environment: London, 1987. (16) Penkett. S. A.: Brice. K. A. Nature 1986.319.655-57. (17) Lightman. P. et al. Tellus, in press. (18)Leleiveld, J.: Crutzen. P.1. Nature 1990,343,227-33. (19) Galloway, J.N. Ambio 1989. I S . 161-
J.: Andreae, M. 0. Science, in press. (21) Hidalgo, H.; Crutzen. P.J. /. Geophys. Res. 1977,82.5833-66. (22) Isaksen, I.S.A. et al. In Proceedings of the 4th Quadrennial International Ozone Symposium: Bojkov. R. D: Fabian, P.,Eds.; Deepak: Hampton. VA. 1989. (23) Beck, J.P. et al. Atmos. Environ.. in press. (24) Oltmans. S. J.; Komhyr, W. D.I. Geophys. Res. 1986,91.5229-36. (25) Attmannspacher. W. In Gesellschofl fur Stmhlen- und Umweltforschung mbH; BPT Bericht: Munich, 1982; 5/82, pp. 12-17. (26) Staehelin, J.: Schmid, W. Atmos. Environ.. in press. (27) Hough. A.: Derwent. R.G. Nature 1990,344,64548. (20) Crutzen. P.
(28)
Penkett.S.A.;Isaksen,I.S.A.;Kley.D.In
Tropospheric Ozone. Regional and GI& bo1 Scale Intemctions: Isahen. I.S.A., Ed.: D. Reidel: Dordrecht. Holland, 1988: pp. 345-63. (29) Liu, S. C. et al. /. Geophys. Res. 1987. 92.4191-4207. (30) The International Geosphere-Biosphere Programme: A Study of Global Change. Report No. 12, Graphic Systems A B Stockholm, Sweden, 1990.
PUBLISHED MONTHLY Editor: Donald R. Paul Unrv. ol Texas. Austin
Quality information that gives you the leading edge Each month, IBEC RESEARCH delivers peermv ewed. primary research on the fundamental ana theoretical aspects 01 chemical engineering...process design and development...and product RBD. Ba PrsparebSubscribsTodql allToll Fras (U.S. and C.Mda):1.80D227-5558 In D.C. and OYlside the U S . and Canada: ZU2.872-4363
Far: 202.872.4615 T e k : 440159ACSPUlor892582ACSPUBS Csbb Addmrs: JEICHEM
or write:
Amrlcsn C h m l u l SoClMy MarXellnp Communlu(lons apl 1155 Slmmnlh Slrnl. N.W. Washlnglon. D.C. 200%
66.
Environ. Sci. Technol.. Vol. 25. No. 4. 1991 635
