A 3D model study of the global sulphur cycle: Contributions of anthropogenic and biogenic sources

Pergamo~

Atmospheric Environment Vol. 30, Nos 10/11, pp. 1815-1822, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352 2310/96 $15.00 + 0.00

1352-2310 (95)00390-8

A 3D MODEL STUDY OF THE GLOBAL SULPHUR CYCLE: CONTRIBUTIONS OF ANTHROPOGENIC A N D BIOGENIC SOURCES M. P H A M , * J.-F. M U L L E R , t G. P. BRASSEUR,~: C. G R A N I E R : ~ a n d G. M I ~ G I E * *Service d'Adronomie, 4 Place Jussieu, 75 252 Paris, Cedex 05, France; ~-Institut d'Adronomie Spatiale, 3 Avenue Circulaire, B-1180 Brussels, Belgium; and ~National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, U.S.A. (First received 3 January 1995 and in final form I September 1995)

Abstract--The impact of anthropogenic emissions on the global distributions and budgets of the main atmospheric sulphur species [namely dimethylsulphide (DMS), sulphur dioxide (SOz), and non-sea-salt (nss-) sulphate3 (SO]-)] is investigated using the IMAGES three-dimensional tropospheric chemistrytransport model. A previous study showed a broad consistency between modelled and observed concentrations of sulphur species for the present-day (c. 1985) atmosphere. Here, in order to assess the relative contributions of biogenic and man-made sources, we compare distributions calculated for the preindustrial atmosphere with the present-day results. The calculations show a large increase in the concentrations of sulphur dioxide and nss-sulphates since preindustrial times, amounting to a factor of 2-3 on global average, and reaching more than two orders of magnitude at the surface in some parts of the Northern Hemisphere. Biogenic species such as DMS are also shown to be influenced by industrialization through changes in the oxidizing capacity of the atmosphere. Over the most polluted areas, the increase in sulphates deposition is found to have reached a factor of 30. Key word index: Sulphur emissions, acidification, modelling, dimethylsulphide, sulphur dioxide, nss-

sulphates.

1. INTRODUCTION Volatile sulphur compounds are released to the atmosphere by various natural and anthropogenic sources. Following their emissions, these gases are transported and oxidized in the atmosphere, and are eventually removed by wet scawmging and dry deposition. These processes supply sulphur as a nutrient to the biosphere but may also affect the Earth's ecosystems by increasing rain acidity. Since the beginning of our century, human population growth and industrialization have led to a substantial increase of the sulphur emissions, amounting to about a factor of three on the global scale. In the Northern Hemisphere mid-latitudes, these sources play a major role in the acidification of lakes and forests (OECD, 1977). Furthermore, it has been pointed out recently that anthropogenic aerosols produced by the conversion of sulphur dioxide into non-sea-salt (nss-) sulphates may increase the albedo of the Earth and, at certain locations, reduce greenhouse warming (Charlson et al., 1992). In this work, we ,evaluate the respective roles of natural and anthropogenic sources in the sulphur cycle, by using a three-dimensional chemistry/transport model of the troposphere, called the IMAGES

model (MOiler and Brasseur, 1994 ; Pham et al., 1995). Using this model, we estimate the distributions and budgets of the major sulphur species for present-day and preindustrial periods. As anthropogenic emissions of gases (biomass burning included) were almost negligible at preindustrial times about two centuries ago, the sulphur cycle was controlled by natural emissions only. Comparing the model results for present-day and preindustrial periods thus provides a quantitative assessment of the role played by anthropogenic emissions in the distributions of the sulphur species. The main features of the model are briefly described in Section 2. Seven sulphur compounds, namely dimethylsulphide (DMS), hydrogen sulphide (H2S), carbon disulphide (CS2), sulphur dioxide (SO2), dimethylsulphoxide (DMSO), methanesulphonic acid (MSA) and non-sea-salt sulphates (nss SO 2-) are considered. Comparison with measurements shows a broad agreement between model and observations: model results are within a factor of 2-3 or less of long-term observations at pristine sites and in polluted areas of Europe and North America. The typical ratios of calculated concentrations to campaign measurements concentrations range from 0.2 to 3 depending on the location and the compound, reflecting the

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M. PHAM et al.

climatological character of the model. The main un- ' meterized according to Rodhe and GrandeU (1972), certainties in the model are related to the estimates of assuming that SO2 is completely oxidized in the presbiogenic emissions and the parameterizations of cloud ence of clouds. This parameterization might be not processes: DMS emissions and concentrations are es- entirely appropriate in polluted areas, where SO2 may timated within a factor of 2-3, assumptions on the exceed H202 and where the oxidation by ozone is less volcanic source tend to overestimate upper tropo- efficient because of the high acidity of the cloud dropspheric concentrations of SO2 and nss-sulfates by lets. Moreover, in these areas, catalytic reactions can about 30% and 24%, respectively. This is also be important. Nevertheless, according to the box the case for the decoupling of wet scavenging and model calculations of Lelieveld (1990), the assumption convective transport in the model. Sensitivity studies that SO2 is completely oxidized in each cloud event to discuss these uncertainties, as well as sulphur distri- appears to be correct even for SO2/H202 concentrabutions for the present-day atmosphere and compari- tion ratios as large as 25, which is higher than the sons with measurements are presented in Pham et al. modelled values of this ratio in the regions where (1995). Thus, the validation of the sulphur model will clouds are present. Further details regarding the paranot be considered here. The focus of this study is on meterizations of wet scavenging, dry deposition and the changes in sulphur species distributions since in-cloud conversion of SO2 are given in Pham et al. preindustrial times (Section 3). (1995). The IMAGES model was run to steady-state for two different cases. The "present-day" (1980s) run includes the sulphur cycle as described above (see also 2. GENERAL DESCRIPTION OF THE IMAGES MODEL Pham et al., 1995). In the preindustrial case (c. 1850), The IMAGES model calculates the three-dimen- all technological sources were neglected, including sional distributions and the global budgets of 47 SOx, NOx, CO and hydrocarbon technological emischemically active gases. Their chemical sources and sions. Biomass burning emissions were reduced by sinks are described by about 125 chemical reactions a factor of 3, in order to account for the lower populaand 26 photochemical reactions. The model domain tion density in the tropical areas. Biogenic emissions, extends from the Earth's surface to the lower strato- climate and transport were assumed to be similar to sphere and includes 25 vertical ("sigma") levels. The those of the present-day atmosphere. horizontal resolution is 5° in longitude and in latitude. Surface emission and deposition of chemical compounds, as well as wet removal of soluble species are 3. RESULTS taken into account. Advective and diffusive transport is based on monthly winds data (Trenberth and Figure 1(a) and (b) illustrate the total sulphur emisOlson, 1988), while convective transport is para- sions in January for the preindustrial and present-day meterized according to Costen et al. (1988) and based periods, respectively. Preindustrial emissions exceed on the distribution of clouds from the International 50 mg S m - 2 y r - 1 in equatorial Africa and in the subSatellite Cloud Climatology Program (ISCCP) (Ros- tropical oceanic band. These values are due to biosow et al., 1987). The performance of the transport mass burning and DMS emissions over the oceans. scheme has been tested against observations of simple Present-day emission rates, however, commonly tracers, like 85Kr and 222Rn, as described by Miiller reach 1000 mg S m - 2 y r - 1 in heavily industrialized and Brasseur (1994). Further details concerning the areas like Europe, northeastern America, and the Far validation of the model for non-sulphur compounds East, where fossil-fuel combustion plays a major role. can also be found in the latter study. They exceed 100 m g S m -2 yr -~ in South Africa and The emissions considered for sulphur gases include South America. As shown on these figures, anthropotechnological sources (SO2, CS2), biomass burning genic emissions, which have increased substantially (SO2), volcanoes (SO2), emissions by soils and plants over the past 150 years, now considerably exceed (DMS, H2S, CS2) and emissions by the oceans (DMS, natural emissions. CS2). Globally, anthropogenic sources (biomass burnThis increase has a direct impact on compounds ing included) contribute to 76% of total emission, whose sources are mainly anthropogenic, like SO2, with SO2 being the most abundant compound emit- CS2 and their oxidation products. As shown in Fig. ted. Oceans represent the most important biogenic 2(a), preindustrial concentrations of SO2 in January source, with 16% of total emission in the form of reach maximum values of approximately 130 pptv DMS. Once in the atmosphere, these compounds are over the oceans in the tropical belt, where DMS oxidized either in the gas phase (by reaction with emission fluxes are highest, and over continental areas OH and NO3 radicals), or in the liquid phase, before influenced by biomass burning emissions. At the presbeing removed by wet and dry deposition. Removal ent time, fossil fuel combustion and industrial activby precipitation is computed using climatological ities lead to SO2 concentrations of around 1-10 ppbv precipitation rates (Shea, 1986), cloud cover data in Europe, northeastern America, and eastern Asia, (Rossow et al., 1987), and scavenging efficiencies. and values exceeding 100-250 pptv in South Africa, Aqueous conversion of SO2 into sulphates is para- South America and Australia [see Fig. 2(b)]. On

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(a) 45-

O_

Above

5000

1000 - 5000 500 - 1000 100 -

-45-

75

-

50

-

500 100

lO Below

-180

-90

0

90

75

50 10

mgS/m2/yr

(b) 45

0 m ~1

-45

Above 5000 1000 - 5000 500 - 1000 100 500 75 100 50

~1

- 180

-90

0

90

-

10 B~ow

75

50 10

mgS/m2/yr

Fig. 1. Total annual sulfur emission fluxes in mgS m -2 yr-l: (a) preindustrial (total source strength: 31.1 TgS yr-~-, partitioned in the ratio: biomass burning 3.2%, volcanoes 29.6%, biosphere 67.2%) and (b) present-day I-totalsource strength: 125.5 Tg S yr- 1,partitioned in the ratio: anthropogenic sources 76% (biomass burning included), volcanoes 7.3%, biosphere 16.7%].

a yearly zonal average basis, SO2 concentrations have increased from preindustrial times by a factor of 2-250 in the Northern Hemisphere. This factor is between 1 and 10 i~n the Southern Hemisphere. As illustrated in Fig. 3, the vertically integrated amount of nss-sulphates shews a similar pattern: the ratio of present-day to preindustrial nss-sulphate integrated columns reaches approximately 25-40 over Europe and Asia, and 2-10 over South America, South Africa and Australia. The broad features of these changes are very similar to the results presented by Langner et aL (1992). On an hemispheric average, the total burden of SO2 increases by a factor of 3 in the Northern Hemisphere, and by 35% in the Southern Hemisphere. The increase for nss-sulphate is a factor of 4 in the Northern Hemisphere and +45% in the Southern Hemisphere. Langner et al. (1992) found a similar increase in the Southern Hemisphere but a lower one in the Northern Hemisphere (2.7), probably due to our different assumptions on emissions (the factor of increase in the emission source strengths is 4 in our work, 3 in the latter study).

/tie 30:10/11-$

The changes in the atmospheric sulphur concentrations induced by human activities are accompanied by increases in the sulphuric acid deposition. The ratio of present-day to preindustrial annual nss-sulphate total deposition exceeds about 30 above central Europe, showing the impact of anthropogenic sources on the acidic deposition in polluted areas (Fig. 4). In Greenland, the modelled ratio of 2-5 is in good agreement with values derived from measurements in ice cores (Neftel et al., 1985; Mayewski et al., 1986; Whung et al., 1994) (see Fig. 4) and with the calculated values by Langner et al. (1992). Wet deposition accounts for 83% of total nss-sulphates preindustrial deposition and 72% in the present-day case: while nss-sulphates wet deposition has increased by a factor of 2.5, nsssulphates dry deposition has been multiplied by five. As a matter of fact, SO2 increases take place near the surface, where the compound is emitted, so that sulphate production through SO2 oxidation is increased near the surface, where the aerosols are more easily deposited. For the same reason, SO2 deposition has increased by a factor of 10 since preindustrial times.

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M. PHAM et al. a / Preindustrial

45

0~

Above 120.0-100.085.070,0 50,0 35.0 15.0 -

-45.

135.0 155,0 120.0 100.0 85.0 70.0 50.0 35.0

-

-

/::~ ~o ::::7 B ~ -180

-90

0 b / Industrial

90

15.o 1o

I~:,tv

Above 5000,01000,0 750.0 500.0 250.0 100.0 50.0 1.0 Below -

-

180

-90

0

90

]0000.0 10000.0 5000.0 1000.0 750.0 500.0 250.0 100.0 50.0 1.0

ppt,~

Fig. 2. Calculated SO2 distribution at the surface in January: (a) preindustrial, and (b) present-day. This explains the lower conversion of SO2 into sulphates in the present-day atmosphere (about 50% of the global sink of SO2) compared to the preindustrial case (65%). However, due to the large increase in SO2 emissions, total sulphate production has increased by a factor of 3 from preindustrial to present times. The relative contribution of the gas-phase oxidation of SO2 to the total sulphate production has slightly decreased, amounting to 11.5 % (or 2.4 TgS yr-1) and 9% (5.7 TgSyr-1) in the preindustrial and presentday cases, respectively. This decrease is due to the fact that anthropogenic emissions take place in the midlatitudes where average OH concentrations are relatively low. It should be noted that though the changes in the amount of SO2 oxidized in the gas-phase, depend both on simulated changes in the emission source strengths and the oxidizing capacity of the atmosphere, the relative changes are similar in this study (factor of 1.9) and the work of Langner et al. (1992) (factor of 2.3).

Although DMS is purely biogenic, and biogenic sources were assumed to remain unchanged, significant changes are predicted for the DMS concentrations since preindustrial times, as a result of the calculated changes in hydroxyl (OH) and nitrate (NO3) radical concentrations (see Fig. 5). OH and N O 3 concentrations have generally increased at midlatitudes, where the NO~ technological emissions have the largest impact. The increase in the nitrate concentration at the surface exceeds a factor of 5 in most of the Northern Hemisphere, and a factor of 2 in the Southern Hemisphere. As NO3 is produced by the O3 + NO2 reaction, this change is a straightforward consequence of the substantial increase in the mixing ratios of nitrogen oxides and to some extent of the larger ozone concentration resulting from photochemical production in polluted area. The changes in the hydroxyl distribution are more complex. The larger CO and CH4 mixing ratios of the present-day atmosphere tend to reduce OH levels, whereas the

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(o) 45" Above

O.

0.8 0.7 0.6 0.5 0.4 0.3 0.2 O. 1

-45"

- 180

-90

0

(b)

90

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1.0 1,0

-

0.8

-

0,7

-

0.6

-

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-

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f ~ Below mgS/m2

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O.

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:':~:; -180

-90

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90

Above 6,0 4,0 2.0 1.0 0,8 0.6 0,4 0.2 Below

-

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-

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0.6

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mgS/m2

Fig. 3. Annual nss-sulphates integrated column: (a) preindustrial and (b) present-day. Using a simple relationship giving the change in the incident solar flux AF due to sulphate aerosols as a function of the integrated sulfate column (Charlson et al., 1992), we evaluate a global forcing of -0.48 Wm -2 and - 1.28 W m-2 for the preindustrial and present-day scenarios, respectively. This rough estimate corresponds to a radiative forcing due to anthropogenic aerosols of -0.8 W m- 2, which is somewhat higher than the forcing d,:rived by Charlson et al. (1991) (-0.6 Wm -2) and Kiehl and Briegleb (1993) (-0.28 W m- 2).

increased NO levels and 03 photochemical production lead to an enhancement of OH concentrations. The largest OH increases are predicted to occur over the polluted areas of the Northern Hemisphere in summer. During winter, ozone and OH changes are found to be highest at lower latitudes, where photochemistry is more intense. Over remote areas, the influence of anthropogenic NOx is weak, so that OH decreases, as a cortsequence of the higher CH4 and CO mixing ratios. The importance of the Tropics in the global atmosphere is stressed by the fact that the globally averaged OH decreased from preindustrial times to present, de:~pite the large percentage increases of OH at mid-latitudes. The average photochemical lifetime of CH4 increased by about 10% as a result of industrialization. The increase of DMS oxidants at mid-latitudes leads to a decrease in DMS concentrations. On an annual basis, the ~verage mixing ratio of DMS has decreased by about 30% in the Northern Hemisphere,

while it increased in the Southern Hemisphere. We find that the DMS reaction with NO3 becomes an important sink of DMS in the present-day atmosphere [see Fig. 6(a) and (b)]. The contribution of this reaction to the total sink of DMS in the Northern Hemisphere has more than tripled, accounting for 9% and 28% of DMS sinks in the preindustrial and present-day cases, respectively. On the global scale, the contribution of this reaction increased from 7% to 14%. In the Southern Hemisphere and over the oceans in the Tropics, however, OH remains the main oxidant of DMS [see Fig. 6(a) and (b)]. In these remote regions, the decrease in the OH concentrations of about 0-25% leads to an increase in DMS concentrations of approximately 0-10% in the Southern Hemisphere (Fig. 5). The average concentration of DMS in the Southern Hemisphere has increased by 3.1% since preindustrial times. These results are strongly dependent on the calculated changes in OH and NO3 distributions.

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45Above

O.

-45-

30.0

25,0

-

30.0

20.0

-

25.0

15.0 -

20.0

10.0-

15.0

5.0

-

2.5

-

5.0

1.0 -

2.5

0.5 Below

-180

-90

0

10.0

1.0 0.5

90

Fig. 4. Calculated ratio of present-day to preindustrial annual deposition of nss-sulphates.

45,

0" Above 25.0 10.0 0.0 10.0

50.0 50.0 25.0 10.0 0,0 -25,0 - -10.0 -50.0 - -25.0 -75.0 -50.0 -95.0 - -75.0 Below -95.0

-45"

-

-180

-90

0

90

Fig. 5. Percentage differences between DMS mixing ratios during the present-day and preindustrial periods (January). Although this evolution is difficult to verify, it should be noted that the globally averaged OH fields calculated by the model for present-day conditions are within the ranges of the latest estimates based on methylchloroform budget analyses (Miiller and Brasseur, 1994). The estimated change in average OH concentrations from preindustrial to present time ( - 1 0 % ) agrees with previous model estimates (e.g. Valentin, 1990; Thompson, 1992; Hauglustaine et al., 1994). Table 1 gives a comparison of the calculated NO3 surface concentrations with available observations. The model generally overestimates NO3 at these sites. It should be noted, however, that the model provides climatological results at a rather coarse horizontal resolution, and hence does not reproduce the high variability in nitrogen oxides concentrations. This is particularly the case for JiJlich, which is strongly influenced by local source of pollution. NOa also strongly depends on the heterogeneous conversion of N2Os on the surface of aerosol partides. This reaction is not taken into account in the

calculations presented here. We find from sensitivity tests that including the heterogeneous reactions of NO3 and N205 on tropospheric aerosols as described in Miiller and Brasseur (1994), cause the model to underestimate nitrate concentrations at the sites of the observations. Further work is needed to better understand the mechanisms driving NO3 tropospheric concentrations.

4. CONCLUSIONS

Anthropogenic emissions of trace gases have more than doubled since the beginning of the century. In the case of sulphur compounds, they now exceed by far the natural sources. This increase has a direct effect on sulphur compounds, whose main sources or precursors are anthropogenic. Our results for the preindustrial and present-day atmospheres show that the concentrations of SO2 and nss-sulphates have increased all over the world at the surface. The

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a/January preindustrial

45,

0, Above

-45

95,0

85.0 -

95.0

70.0 60,0 -

85.0 70.0

50.0 -

60,0

35.0 25.0 -

50.0 35.0

10.0 -

25.0

1.0Below

- 180

-90

0 b/January industrial

10.0 t .0

90

45

Above 85,0 70.0 60.0 50.0 35.0 25.0

-45

10.o

1.0Below

-180

-90

0

95.0 - 95.0 - 85.0 - 70,0 - 60.0 - 50.0 - 35.0 -

25.0

10.0 1.0

x

90

Fig. 6. Column integrated contribution ofDMS + NO3 to the total DMS photochemical sink in January: (a) preindustrial and (b) present-day.

Table 1. Measurements of NO3 at several locations at the surface and comparison with calculated concentrations for the same areas and time periods (pptv) Location

Time period

Mean

Range

Model

Reference

California

April May October November December

17 37 33 58 83

4-42 17-93 <2-67 4-110 55-110

14 13 15 15 16

Plattet Platt et Platt et Platt et Platt et

JiJlich (Germany)

May July August

10 27 31

9-11 8-78 23-39

47 44 42

Platt et al. (1981) Platt et al. (1981) Platt et al. (1981)

Schauinsland (Germany) Penmarch' (Brittany, France)

August

7

< 3-9.5

43

Mihelcic et al. (1993)

July

0.7

<0.3-2.6

2.4

al. al. al. al. al.

(1984) (1984) (1984) (1984) (1984)

Brauers et aL (1990)

Note: Observations have been averaged, when above the detection limit. Model results are nighttime averages.

1822

M. PHAM et al.

increase is most pronounced at Northern Hemisphere mid-latitudes, where most anthropogenic sources are concentrated. Above central Europe, man-made emissions have induced an increase of 25-30 in the rates of wet sulphate deposition in polluted areas. Our results also show that biogenic compounds like D M S have also been affected by the increase of anthropogenic sources, through the changes in O H and NOa radicals concentrations. In the Northern Hemisphere, where O H and NO3 have both increased, D M S decreased by about 30%. In the Southern Hemisphere, however, where O H decreased on average and is the D M S main oxidant, the D M S mean concentration has slightly increased ( + 3%). Acknowledoements--The National Center for Atmospheric Research is sponsored by the National Science Foundation. One of us (J. F. Miiller) is a Research Assistant at the Belgian National Fund for Scientific Research (F.N.R.S.). This work is partly supported by the Belgian Federal Office for Scientific, Technical and Cultural Affairs (O.S.T.C.).

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Lelieveld J. (1990) The role of clouds in tropospheric photochemistry, Doctoral dissertation, University of Utrecht, Netherlands, pp. 74-80. Mayewski P. A., Lyons W. B., Spencer M. J., Twickler M., Dansgaard W., Koci B., Davidson C. I., Honrath R. E. (1986) Sulfate and nitrate concentrations from a south Greenland ice core. Science 232, 975-977. Mihelcic D., Klemp D., Mfisgen P., P~itz H. W. and VolzThomas A. (1993) Simultaneous measurements of peroxy and nitrate radicals at Schauinsland. J. atmos. Chem. 16, 313-335. Miiller J. F. and Brasseur G. P. (1994) IMAGES: A threedimensional chemical transport model of the global troposphere. J. oeophys. Res. 100, 16,445-16,490. Neftel A., Beer J., Oeschger H., Ziircher F. and Finker R. C. (1985) Sulphate and nitrate concentrations in snow from South Greenland 1895 1978. Nature 314, 611-613. OECD (1977) The OECD Program on the long-range transport of air pollutants, measurements and findings. Organisation for Economic Cooperation and Development, Paris. Pham M., Miiller J.-F., Brasseur G. P., Granier C. and M6gie G. (1995) A 3D model of the global sulfur cycle. J. 9eophys. Res. (in press). Platt U., Perner D., Schr6der J., Kessler C. and Toennissen A. (1981) The diurnal variation of NO3. J. 9eophys. Res. 86, 11,965-11,970. Platt U. F., Winer A. M., Biermann H. W., Atkinson R. and Pitts J. N. Jr (1984) Measurement of nitrate radical concentrations in continental air. Environ. Sci. Technol. 18, 365-369. Rodhe H. and Grandell J. (1972) On the removing time of aerosol particles from the atmosphere by precipitation scavenging. Tellus 24, 442-454. Rossow W. B., Garder L. C., Lu P. J. and Walker A. W. (1987) International Satellite Cloud Climatology Project (ISCCP) Documentation on cloud data WMO/TDNo266. World Meteorological Organization, Geneva. Shea D. J. (1986) Climatological atlas: 1950-1979. NCAR Technical Note, NCAR/TN-269 + STR, NCAR, Boulder, U.S.A. Thompson A. M. (1992) The oxidizing capacity of the Earth's atmosphere: probable past and future changes. Science 256, 1157-1165. Trenberth K. E. and Olson J. G. (1988) ECMWF Global Analyses 1979-1986: Circulation statistics and data evaluation. NCAR Technical Note NCAR-/TN-300 + STR, NCAR, Boulder, U.S.A. Valentin K. M. (1990) Numerical modeling of the climatological and anthropogenic influences on the chemical composition of the troposphere since the last glacial maximum. Ph.D. Thesis, Mainz University, Germany, pp. 141-146. Whung P.-Y., Saltzman E. S., Spencer M. J., Mayewski P. A., Gundestrup N. (1994) Two-hundred-year record of biogenic sulfur in a south Greenland ice core (20D). J. geophys. Res. 99, 1147-1156.