Historical data of surface ozone at tropical latitudes

the Science of the Total Environment The Science of the Total Environment 148 (1994) 23-29

ELSEVIER

Historical data of surface ozone at tropical latitudes S. S a n d r o n i *a, D. A n f o s s i b aEnvironment Institute, Joint Research Centre of the CEC, 21020 lspra, Varese, Italy bIstituto di Cosmogeofisica del C.N.R, Corso Fiurne 4, 10133 Torino, Italy

(Received 9 January 1993; accepted 23 March 1993)

Abstract

During the last decades of the 19th century day-time and night-time data of surface ozone were collected by means of the Sch6nbein method at the coastal observatories of Rio de Janeiro, Brazil (23°S) and Manila, Philippines (14°N) and at the mountain observatory of Oaxaca de Juarez, Mexico (17°N, 1574 m a.s.l.). Taking into account the uncertainties of the method, daily readings have been converted to ppbv concentrations by a procedure already applied to other series of historical data. The calculated time-averaged values show that l century ago the ozone levels at tropical latitudes were considerably lower than at mid-latitudes of the northern and southern hemispheres, similar to the present-day measurements. Key words: Ozone; Tropics; Historical data; Ozone trend

1. Introduction The global distribution of tropospheric ozone is one of the important issues in atmospheric science: it is related to global air quality and photochemistry and has an impact on the atmospheric thermal radiation field. Tropospheric ozone is generally on the increase, in contrast to the stratospheric ozone layer, for which there are much publicised fears of a trend towards lower concentrations. Although many surface ozone measurements have been made in and around urban areas, particularly in the northern hemisphere (NH), there are relatively * Corresponding author.

few measurements which can be defined as clean air (Oltmans, 1981; Crutzen, 1988). Observations and theoretical studies have demonstrated that the transport and transformation of ozone and its precursors from emission sources greatly influence the ozone distribution, even at rural locations. The photochemical source of tropospheric ozone is associated with emissions of CO, hydrocarbons and NOx from fossil fuel combustion, a dominant process at northern mid-latitudes, where the highest concentrations occur in spring and summer. Ozone injections from the stratosphere are effective mainly at mid-latitudes during late winter and spring, however, they play a minor role. At Cape Arkona on the Baltic Sea, a 'remote' station at

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mid-latitudes of the NH. Since the 1950s surface ozone has increased at a mean rate of - 1-2% per year, with large fluctuations from year to year, mainly in relation to the meteorological course (Feister and Warmbt, 1987; Low et al., 1992). At 'remote' stations of the southern hemisphere (SH), levels are lower than at comparable latitudes of the NH and no clear evidence of a positive trend is observed (Scheel et al., 1990). In Antarctica a negative trend of the tropospheric level has been observed in the warm season by Schnell et al. (1991). Continuous surface ozone data are particularly scarce in the tropics (Fabian and Pruchniewicz, 1977; Logan and Kirchhoff, 1986; Oltmans and Komhyr, 1986), which constitute a transition region between NH and SH in the atmospheric circulation pattern. In these regions ozone is mainly supplied by photochemical reactions related to biomass burning. The few measurements available in the tropics show that tropospheric ozone is less abundant than at midlatitudes and relatively more abundant in the northern than in the southern tropics (Fabian and Pruchniewicz, 1977; Routhier et al., 1980; Seiler and Fishman, 1981; Fishman et al., 1990; Piotrowicz et al., 1991). Furthermore, a significant longitudinal variability in concentration, due to the presence of continents, is observed.

2. The Sch6nbein method and the conversion procedure Important issues are both the trend of tropospheric ozone at different sites and latitudes and its original level in the pre-industrial era, when anthropogenic emission of precursors played a much smaller role than today. A comparison between historical and present-day levels can provide information on the order of magnitude of the total increase. Industrialisation started in the last decades of the 19th century in the NH, therefore, data of interest should refer to a century ago. At that time systematic ozone observations were made at many observatories in the world mainly by the Sch6nbein method, applied in different versions (Fox, 1873). A test paper-strip impregnated with potassium iodide and starch was exposed for a constant time (usually 12 h) to open air under an

S. Sandroni et al. / Sci. Total Environ. 148 (1994) 23-29

inverted funnel device to protect it from direct sunlight and rain. Ozone converts iodide to iodine, the violet colour being intensified by starch. After exposure the paper strip was dipped into distilled water to develop the full colour and immediately compared with an ozone colour scale. The Sch6nbein reference scale had 10 units between the extremes 0 (white paper) and 10 (dark colour), but alternative scales (e.g. Lender, 0-14; Salleron 0-21) were used. This method is not strictly quantitative, since it is sensitive to other factors, in particular relative humidity and exposure time and to chemical interferences, positive (oxidants) or negative (SO2). Only Levy (1878a) at the Montsouris observatory, near Paris, after 5 years of daily observations by the Sch6nbein technique, moved to a wet chemistry method, based on arsenite oxidation in aqueous solution. The conclusion after one year of parallel observations by both techniques, was that 'the (mean) Sch6nbein numbers agree fairly well with the quantitative measurements; one should not hesitate to use the paperstrip method, if the other one cannot be used' (Levy, 1878b). Levy's quantitative data were reconsidered by Bojkov (1986) and later by Volz and Kley (1988), who deduced that the present-day ozone levels at mid-latitudes of the NH are approximately twice as high as a century ago. As the arsenite method was also sensitive to SO 2 emission from Paris, a correction was introduced for particular wind directions. The observations made by the Sch6nbein method require an appropriate conversion before they are considered. As the relative humidity was recognized to be the main parameter influencing these readings, Linvill et al. (1980) experimentally constructed some conversion curves to relate the Schfnbein numbers to parts-per-billion in volume (ppbv) as a function of humidity; other parameters such as manual operation, apparatus and exposure time could be standardized. Based on some parallel observations by the Sch6nbein and the arsenite methods at Montsouris, Bojkov (1986) used a linear regression to convert time-averaged data directly, without considering the influence of humidity. The Sch6nbein data are no doubt less reliable than data obtained with contemporary instrumentation, but their patterns are reliable (Logan, 1985; Bojkov, 1986).

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S. Sandroni et al. / Sci. Total Environ. 148 (1994) 23-29

Recently, Anfossi et al. (1991) have combined both the Linvill and the Bojkov methods to establish a conversion procedure which has been applied to the analysis of daily readings of a 26-year data series from Moncalieri, northern Italy. A Montsouris data series of simultaneous readings by the Sch6nbein and the arsenite methods accompanied by meteorological data were used for the calibration. First the original Sch6nbein numbers are corrected for the humidity by an algorithm derived from the Linvill curves, then a linear regression is applied. The ozone levels at Moncalieri for the years 1868-1893 have been demonstrated to be comparable with the contemporary data of Montsouris, i.e. 8-10 ppbv. This agreement does not automatically mean that the processes for surface ozone production, removal and transport at the two locations were necessarily the same; each region is characterized by its orography, emission sources and meteorological conditions. From the Moncalieri data series Anfossi et al. (1991) could state that in the free troposphere the present-day ozone level is approximately twice as high as a century ago. More recently, historical data series recorded at mid-latitudes of the SH at Montevideo, Uruguay and Cordoba, Argentina by scientists trained in Europe and analysed by the same procedure, have shown that the surface ozone levels at those latitudes of the SH were comparable with the contemporary values of the NH (Sandroni et al., 1992). It is worthy of note that the possibility of drawing valuable information on ozone concentrations from historical data in general and the Sch6nbein method in particular, is controversial; scientists have conflicting opinions. Some appreciate all efforts to reconsider the historical observations and to convert them by appropriate procedures to concentration values comparable in some way with present-day measurements; some claim that the uncertainties are so large that no valid conclusion can be reached. On the other hand, other scientists (Linvill et al., 1980; Lauscher, 1983; Bojkov, 1986; Khrgian, 1988; Lisac and Grubisich, 1991; Anfossi et al., 1991; Varotsos and Cartalis, 1991; Lauscher, 1991; Sandroni et al., 1992) maintain that historical data, if properly converted and taken with care, can provide reliable indicators of the relative ozone levels. Furthermore, we emphasize that our

conversion procedure is based on a regression curve constructed by a series of parallel ozone measurements by the Sch6nbein and the arsenite methods carried out by Levy (1878a) at the same place and under the same conditions. A further confirmation of the validity of the procedure is given by the experimental reconstruction of the Sch6nbein technique made by Kley et al. (1988). Within the limitations of a laboratory simulation, they were able to show, among other things, that at 80% relative humidity the Sch6nbein numbers up to 7 units are proportional to ppbv concentration. The individual historical readings we have considered are within such a linearity range.

3. Ozone observations in the tropics As previously mentioned, historical ozone data in the tropics are an interesting complement to the information available for NH and SH. The irregular spatial and temporal distribution of incoming solar radiation, outgoing longwave radiation, sea surface temperature, water vapour, convection and precipitation coupled with the uneven presence of continents with mountain chains, characterize the air circulation in the region and consequently the ozone distribution. Although understanding of tropical circulation has greatly improved during the last decades (Rasmusson, 1991), yielding a satisfactory picture of many aspects such as Hadley cells, monsoons, trade winds, Walker circulation, El Nifio southern oscillation (ENSO), La Nifia some aspects such as coupling between ocean and atmosphere and the hydrological cycle on intra-seasonal to inter-annual time scales are not yet fully understood. We have, analysed historical ozone observations made by the same procedure and comparable as far as possible with contemporary observations made at other sites of the NH and SH. The historical data series we have analysed were collected at national observatories equipped with conventional instrumentation, mainly imported from Europe, and associated with the world meteorological network of that time. These data series refer to: (a) the Observatory of Manila, Philippines

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S. Sandroni et aL / Sci. Total Environ. 148 (1994) 23-29

Table 1 Examples of mean original Sch6nbein readings and humidity at the three stations Manila 1882

January February March April May June July August September October November December

Rio de Janeiro 1882

Oaxaca 1892-1899

Sch. no.

r. hum.(%)

Sch. no.

r. hum.(%)

Sch. no.

r. hum.("/,,)

2.20 2.75 2.90 1.05 1.65 1.90 3.90 2.90 1.90 1.55 1.80 1.75

71 73 69 67 71 76 83 79 82 82 78 78

2.4 2.7 2.3 2.2 1.8 2.1 2.0 2.4 3.5 3.6 3.2 2.6

84 87 80 81 81 82 82 80 82 80 76 80

2.05 2.2 2.9 3.4 2.85 2.4 2.2 2.0 1.7 1.8 2.1 1.0

59 55 51 55 54 70 72 79 80 66 62 57

(14°N) managed by the Jesuits; the data series extends between 1879-1882 (Faura, 1883); (b) the Imperial Observatory of Rio de Janeiro, Brazil (23°S); the data series extends between 1882-1889 (Cruls, 1882-1885); (c) the National Observatory of Oaxaca de Juarez, Mexico, at 1574 m a.s.1. (17°N); the data series extends between 1892-1899 (Dominguez and Dominguez, 1899). Routine observations were periodically published in a bulletin distributed to most of the observatories in the world. Faura (1883) presented a detailed report at the Amsterdam meteorological conference in 1883; Cruls (1892) had continuous contacts with Montevideo. Ozone observations were made using the same experimental method with a 12-h exposure time, from 600 (or 900) LT to 1800 (or 2100) LT and from 1800 (or 2100) LT to 600 LT (or 900 LT). Some examples of monthly average Sch6nbein readings and corresponding humidity for the three stations are given in Table I. During the years under consideration, the climate of Manila (Philippines) was characterized by two pronounced seasons, i.e. dry during January - - April and rainy the rest of the year

with a maximum of precipitation in July and August. The monthly mean temperature ranged between 23°C and 31 °C, while the relative humidity was between 66 and 80% (Saderro Maso, 1915). At Oaxaca de Juarez (Southern Mexico), sited in a fertile valley on a plateau surrounded by the mountain chain of Sierra Madre del Sur, the climate was characterized by a rainy season from June - - September and dry the rest of the year. The monthly mean temperature ranged between 17°C in December and 24°C in May, with a relative humidity between 50 and 80%. The mean wind speed was 1-2 m s -l (Moreno y Anda, 1905). The climate of Rio, on the Atlantic coast of Brazil, was warm and humid; the monthly mean temperature ranged between 26°C in January - - February and 20°C in July, with a mean relative humidity of 80%, approximately constant throughout the year (Cruls, 1892). The original daily readings at Manila and Oaxaca and the monthly mean values at Rio have been converted by the two-step procedure described by Anfossi et al. (1991). No correction has been introduced for positive or negative interfering agents: the absence of industrialisation, the mild climate and the isolated position of each observatory in respect to the urban areas encouraged

S. Sandroni et al./Sei. Total Environ. 148 (1994) 23-29

27

Pl by Oj

January

.

Seplember

day

Fig. 1. Comparison of daily mean ozone level at Manila, Philippines in a dry (January) and in a wet (September) m o n t h of 1882.

30

ppbv 0 3

20

10

~x - . ~

~

.

Moncalieri Oaxaca

e--

×- L ?-~-m---'ll~'-~---~,--_

Rio Manila

.

±

J

.

F

i

l

M

i

A

[

_

M

_

J

_

l

J

l

A

l

l

S

l

O

l

J

N

D

Fig. 2. Mean seasonal course of historical ozone levels in the tropics (Manila, 14°N, 1879-1882; Oaxaca de Juarez, 1574 m, 17°N, 1892-1899; Rio de Janeiro, 22°54'S, 1882-1885). The course of Moncalieri (44°59'N, 1869-1893; Anfossi et al., 1991) is added for comparison.

this respect. Fig. 1 shows the daily mean values for 2 months in 1882 at Manila, with large fluctuations from day to day, in relation to meteorological conditions. The small difference between day and night observations (not shown here) shows the weak photochemical activity at that time, as already observed for other sites. The mean yearly courses at the three tropical stations are compared in Fig. 2; the mean level is nearly the same ( ~ 5 ppbv) and approximately half the levels recorded at mid-latitudes of the N H in those years. Only a weak seasonal change has been observed at Oaxaca, sited on a plateau. Recent continuous measurements of surface ozone in the tropics are very scarce and most of them are at coastal stations (Fabian and Prucniewicz, 1977; Oltmans and Komhyr, 1986; Logan and Kirchhoff, 1986). Measurements from aircraft and ships over and along the oceans (Routhier et al., 1980; Seiler and Fishman, 1981; Winkler, 1988; Smit et al., 1989; Piotrowicz et al., 1991) have shown a minimum of the tropospheric ozone level (_< 10 ppbv) at tropical latitudes. A lower level in comparison with mid-latitudes fits well with the calculated distribution of ozone on the global scale (Penkett, 1988). At the present time, over the latitude range of 10°N - - 10°S the measured ozone levels are - 2 0 ppbv in the free troposphere (3-6 km) and of the order of 10 ppbv in the oceanic boundary layer (0-2 km) (Routhier et al., 1980; Oltmans, 1981). During a cruise over the Atlantic ocean from 45°N - - 4 0 ° S Smit et al. (1989) recorded the lowest surface mixing ratio (8 ppbv) at - 10°S. There are also substantial variations with longitude. Continuous measurements available at present show that the mean concentrations in the continental boundary layer are higher than in oceanic zones. As regards the seasonal cycle, the atmospheric circulation in the tropics is influenced by largescale variations such as the Hadley cells, which are centred north of the Equator in December - February and south of it in July - - August. Zonal circulation patterns such as the Walker cells are superimposed on large-scale circulation. Anthropogenic and natural emission of hydrocarbons and NOx over continental areas may be transported up to the oceans. In the southern

28 tropics m e a s u r e m e n t s carried o u t at the coastal station o f N a t a l , Brazil (6°S; L o g a n a n d K i r chhoff, 1986; Kirchhoff, 1989) exhibit a p r o n o u n c ed m a x i m u m in the austral spring ( S e p t e m b e r - October), which is m a i n l y a t t r i b u t e d to p h o t o chemical processes related to b i o m a s s b u r n i n g occurring inland especially d u r i n g the d r y season. A similar yearly cycle is o b s e r v e d at the c o n t i n e n t a l station o f Brazzaville, C o n g o (4°S; C r o s et al., 1988). A t L u a n d a , A n g o l a (9°S), a coastal station at a latitude c o m p a r a b l e with R i o a n d N a t a l a n d influenced by sea breeze circulation, m a x i m a are observed in July, while the a n n u a l m e a n is 14.4 ppbv. A seasonal cycle has also been observed at Samoa, (14°S) in the e q u a t o r i a l Pacific ( O l t m a n s and K o m h y r , 1986). In the n o r t h e r n tropics, P a n a m a (9°N) shows a little seasonal v a r i a t i o n (Logan, 1985), while a p r o n o u n c e d yearly cycle is observed at M a u n a Loa, H a w a i i (20°N), a station sited at 3500 m. A t N d j a m e i n a , C h a d (12°N), a continental station, the a n n u a l m e a n is 16.7 ppbv, c o m p a r a b l e with Sa de Bandeira, A n g o l a (15°S; 17.0 ppbv). All these d a t a are higher than historical levels, but still lower t h a n levels recently observed at m i d latitudes o f the N H .

4. Conclusion W i t h i n the intrinsic limitations o f the Sch6nbein m e t h o d a n d the accuracy o f the p r o c e d u r e used to convert historical data, it is possible to state that a century ago the o z o n e levels at tropical latitudes were distinctly lower t h a n at m i d - l a t i t u d e s b o t h in the N H a n d the SH. Similar differences are observed at present in surface o z o n e levels at those latitudes.

5. References Anfossi, D., S. Sandroni and S. Viarengo, 1991. Tropospheric ozone in the nineteenth century: the Moncalieri series. J. Geophys. Res., 96: 17349-17352. Bojkov, R.D., 1986. Surface ozone during the second half of the nineteenth century. J. Climat. Appl. Met., 25: 343-352. Cros, B., R. Delmas, D. Nganga, B. Clairac and J. Fontan, 1988. Seasonal trends of ozone in equatorial Africa: experimental evidence of photochemical formation. J. Geophys. Res., 93: 8355-8366. Cruls, L., 1883-1889. Bull. astron, m6t. Observ. Imperial de Rio de Janeiro, H. Lombaerts & C., Rio de Janeiro.

s. Sandroni et al./ Sci. Total Environ. 148 (1994) 23-29

Cruls, L., 1892. O clima do Rio de Janeiro, H. Lombaerts and C., Rio de Janeiro. Crutzen, P.J., 1988. Tropospheric ozone: an overview. In: I.S.A. lsaksen (Ed), Tropospheric Ozone. Reidei Publ. Co, Dordrecht, pp. 3-32. Dominguez, J.A. and A.M. Dominguez, 1898-1899. Boletin Mensual del Observatorio Meteoroiogico del Estado de Oaxaca, Oaxaca de Juarez, Mexico. Fabian, P. and P.G. Pruchniewicz, 1977. Meridional distribution of ozone in the troposphere and its seasonal variations. J. Geophys. Res., 82: 2063-2073. Faura, F., 1883. Observaciones verificadas desde 1879 a 1882 inclusive. Observ. Met. Ateneo Municipal de Manila, Binondo, Manila. Feister, U. and W. Warmbt, 1987. Long-term measurements of surface ozone in the German Democratic Republic. J. Atmos. Chem., 5: 1-21. Fishman, J., C.E. Watson, J.C. Larsen and J.A. Logan, 1990. Distribution of tropospheric ozone determined frqm satellite data. J. Geophys. Res., 95: 3599-3617. Fox, C.B., 1873. Ozone and Antozone, Their History and Nature. Churchill, London. Khrgian, A. Kh., 1988. Long-term variations of tropospheric ozone. Meteorologiya i Gidrologiya (Soviet Meteorology and Geology), 2: 34-39. Kley, D., A. Volz and F. Miilbeims, 1988. Ozone measurements in historical perspective. In: I.S.A. lsaksen (Ed), Tropospheric Ozone. Reidel Publ. Co., Dordrecht, pp. 63-72. Kirchhoff, V.W.J.H., 1989. A comparative study of tropospheric ozone in the Amazonian rainforest, the cerrado, and a coastal site. In: R.D. Bojkov and P. Fabian (Eds), Ozone in the Atmosphere. Deepack Publ., Hampton, Va., pp. 426-429. Lauscher, F., 1983. Aus der Friihzeit atmosph/irischer Ozonforschung. Wetter und Leben, 35: 69-80. Lauscher, F., 1991. Neubearbeitung der Messungen des bodennahen Ozons in Wien zwischen 1853 und 1990. Eigenverlag, X: 1-30. Levy, A., 1878a. Analyse de l'air-ozone. Ann. Observ. Montsouris: pp. 398-404. Levy, A., 1878b. Sur la recherche de l'ozone dans l'air atmosph6rique. C.R. Acad. Sciences, Paris, LXXXVI: 1263-1265. Linvill, D.E., W.J. Hooker and B. Olson, 1980. Ozone in Michigan's environment 1876-1880. Monthly Weather Rev., 108: 1883-1891. Lisac, I. and V. Grubisich, 1991. An analysis of surface ozone data measured at the end of the 19th century in Zagreb, Yugoslavia. Atmos. Environ., 25A: 481-486. Logan, J.A., 1985. Tropospheric ozone: seasonal behaviour, trends and anthropogenic influence. J. Geophys. Res., 90: 10 463-10 482. Logan, J.A. and V.W.J.H. Kirchhoff, 1986. Seasonal variations of tropospheric ozone at Natal, Brazil. J. Geophys. Res., 91: 7875-7881. Low, P.S., P.M. Kelly and T.D. Davies, 1992. Variations in surface ozone trends over Europe. Geophys. Res. Lett., 19: 1117-1120.

S. Sandroni et al./ Sci. Total Environ. 148 (1994) 23-29 Moreno y Anda, M., 1905. Observaciones Meteor61ogicas practicadas en el Observatorio Astron6mico Nacional de Tacubaya y alcunas otras estaciones mexicanas. Secretaria de Formento, Mexico City. Oltmans, S.J., 1981. Surface ozone measurements in clean air. J. Geophys. Res., 86: 1174-1180. Oltmans, S.J. and W.D. Komhyr, 1986. Surface ozone distributions and variations from 1973-1984 measurements at the NOAA geophysical monitoring for climatic change baseline observatories. J. Geophys. Res., 91: 5229-5236. Piotrowicz, S.R., H.F. Bezdek, G.R. Harvey and M. SpringerYoung, 1991. On the ozone minimum over the equatorial Pacific Ocean. J. Geophys. Res., 96:18 679-18 687. Penkett, S.A., 1988. Indications and causes of ozone increase in the troposphere. In: F.S. Rowland and I.S.A. lsaksen (Eds), The Changing Atmosphere. John Wiley & Son, New York, pp. 91-103. Rasmusson, E.M., 1991. Development of our knowledge of the general circulation of the tropics. In: ECMWF (Eds), Tropical Extra-tropical Interactions, Reading, UK, pp. 3-40. Routhier, F., R. Dennett, D.D. Davis, A. Warburg, P. Haagenson and A.C. Delany, 1980. Free tropospheric and boundary-layer airborne measurements of ozone over the latitude range of 58°S to 70°N. J. Geophys. Res., 85: 7307-7321. Saderra Maso, M., 1915. Historia del Observatorio de Manila. McCullough & Co., Manila. Sandroni, S., D. Anfossi and S. Viarengo, 1992. Surface ozone

29 levels at the end of the 19th century in South America. J. Geophys. Res., 97: 2535-2539. Scheel, H.E., E.G. Brunke and W. Seiler, 1990. Trace gas measurements at the monitoring station Cape Point, South Africa, between 1978 and 1988. J. Atmos. Chem., ll: 197-210. SchneU, R.C., S.C. Liu, S.J. Oltmans, R.S. Stone, D.J. Hofmann, E.G. Dutton, T. Deshler, W.T. Sturges, J.W. Harder, S.D. Sewell, M. Trainer and J.M. Harris, 1991. Decrease of summer tropospheric ozone concentration in Antarctica. Nature, 351: 726-729. Seiler, W. and J. Fishman, 1981. The distribution of carbon monoxide and ozone in the free troposphere. J. Geophys. Res., 86: 7255-7265. Smit, H.G.J., D. Kley, S. McKeen, A. Volz and S. Gilge, 1989. The latitudinal and vertical distribution of tropospheric ozone over the Atlantic ocean in the southern and northern hemispheres. In: R.D. Bojkov and P. Fabian (Eds), Ozone in the Atmosphere. Deepack Pub., Hampton, Va., pp. 419-422. Varotsos, C. and C. Cartalis, 1991. Re-evaluation of surface ozone over Athens, Greece, for the period 1901-1940. Atmos. Res., 26: 303-310. Volz, A. and D. Kley, 1988. Evaluation of the Montsouris series of ozone measurements made in the nineteenth century. Nature, 332: 240-242. Winkler, P., 1988. Surface ozone over the Atlantic ocean. J. Atmos. Chem., 7: 73-91.