Clouds are a tropospheric environment for multiphase chemical transformations. Chemical constituents in the liquid phase of clouds derive from the incorporation of soluble species contained in the aerosol particles on which cloud droplets grow (cloud condensation nuclei, CCN) and from the dissolution of trace gases in cloud water. The different species introduced in cloud droplets can eventually react in the liquid phase to form other products. The processing of chemical compounds in liquid water droplets influences the chemical composition of the tropospheric gas phase as well as the properties of the aerosol after cloud processing. A proper description of liquid phase chemical processes is therefore necessary to assess the role of clouds in a changing atmosphere. Understanding the chemistry of clouds requires a detailed knowledge of the factors controlling chemical composition and concentration of chemical species in droplets.

Sulfur(IV) oxidation reactions occur at a much faster rate in clouds than in the clear air. Model calculations [e.g., Langner and Rodhe, 1991] have shown that, on a global scale, tropospheric in-cloud S(IV) oxidation is 2-5 times more important than out-of-cloud oxidation. Many details of the processes involved, however, are still the subject of a wide variety of laboratory studies in multiphase modeling and field measurements.

Chemical systems

S(IV) oxidation by non-radical oxidants

Appreciable progress has been made over the last decade by laboratory studies in identifying the reactions responsible for S(IV) oxidation in the atmospheric liquid phase, and in determining the associated rate coefficients [Warneck, 1991]. Reviews are available on S(IV)-oxidation by H₂O₂ and other peroxides [Warneck et al., 1996a] or by ozone [Hoffmann, 1986]. It should be noted that organic peroxides can also potentially contribute to sulfur oxidation. Because the hydrogen peroxide concentration will generally exceed the concentration of organic peroxides, oxidation by H₂O₂ will dominate.

As the oxidation of S(IV) by hydrogen peroxide appears to be the most effective pathway in cloudwater, special attention needs to be paid to assess all potential sinks and sources which may not only be incorporated from the gas phase but also formed by (1) reaction of HO₂/O₂^– with Fe²⁺ and Cu⁺, or (2) peroxyl radical recombination in solution. Because of this, a coupling to organic cloud chemistry exists (see below).

A data and literature compilation with regards to oxidation of S(IV) by non-radical oxidants is found in the HALIPP final report [Warneck et al., 1996a].

S(IV) oxidation by radical oxidants

Aqueous chemical conversions may not only be driven by the above-mentioned classical oxidants but also by free radicals. Radicals may originate from the gas phase as OH, HO₂, NO₃, CH₃O₂ and others [Davidovits et al., 1995] or from photolysis within solution [Zellner et al., 1990]. These species are referred to as primary radicals, as opposed to secondary radicals which are solely formed within aqueous atmospheric particles such as SO_X^– (x = 3,4,5), Cl/Cl₂^–, Br/Br₂^– and CO₃^–. Radicals exist in solution in very small concentrations which can currently only be assessed from box models, however, they exhibit a very high reactivity towards inorganic and organic constituents of cloud droplets.

Radical reactions in diluted aqueous solutions can only be of importance in the troposphere. A classic compilation is the review of Graedel and Weschler [1981], which still gives a good overview on the topic of this contribution. In addition, two more recent treatments by Huie [1995] and Zellner and Herrmann [1995] summarize the state of knowledge as of 1995. The radical oxidation may be initiated by thermal decomposition of transition metal complexes or by direct reaction of radicals X with S(IV), i.e., for cloud conditions mainly bisulfite:

X + HSO₃^– Æ X^– + H⁺ + SO₃^– (R-1)

Table 1. Comparison of sulfite radical anion generation processes initiated by free radicals and radical anions. S denotes the source strength of each process, tX is the half–life of the radical X with regards to its reaction with HSO3– (T = 298 K, applying [HSO3–] = 5x10–7 mol l–1 after Warneck [1992]).

References: 1Lelieveld and Crutzen, 1991; 2Jacob et al., 1989; 3Chameides, 1986; 4Chameides and Davis, 1982; 5Huie and Neta, 1987; 6Exner et al., 1992; 7Buxton et al., 1993; 8Herrmann et al., 1991.

Table 1 gives a comparison of fluxes for the most important tropospheric aqueous phase radical oxidants.

The hydroxyl radical appears to be most efficient in initiating S(IV) oxidation. It should be noted, however, that radical concentrations for the calculation of fluxes in Table 1 have been estimated based on available considerations from literature. More details are found in Zellner and Herrmann [1995].

Under the conditions of the atmospheric aqueous phase sulfite radical anions (SO₃^–) are converted to peroxo-monosulfate radical anions (SO₅^–) by reaction (R-2):

SO₃^– + O₂ Æ SO₅^– (R-2)

The peroxomonosulfate radical anion itself may react with HSO₃^–:

SO₅^– + HSO₃^– Æ SO₃^– + HSO₅^– (R-3a)

Æ SO₄^– + HSO₄^– (R-3b)

Table 2. Kinetic data for the reaction of peroxomonosulfate radical anions with bisulfite.

Remarks: (a) Branching ratio k3b/(k3a + k3b), (b) Estimated, based on pulse radiolysis experiments, (c) Measured upper limit for the overall reaction (3), branching ratio estimated, (d) Value derived from steady-state radiolysis experiment, (e) Value determined from scavenger experiments, absolute rate constant not determined.

This branching reaction (R-3 a,b) apparently is a key process in the conversion of S(IV) whenever sulfur-oxy radicals and no transition metal ions are involved. However, the currently available kinetic data base shows considerable scatter (Table 2). In reaction (3a) the peroxomono-sulfate anion will undergo a non-radical oxidation of HSO₃^–. The sulfate radical anion on the other hand is produced by reaction (3b). It will subsequently react with HSO₃^– according to reaction (R-1) for X = SO₄^–.

Termination of the chain reaction will occur in radical-radical recombination reactions. Under laboratory conditions, these may involve reactions of the sulfur-oxy radical anions formed in the system. Under natural conditions, however, recombination with HO₂ or O₂^– will become much more important due to their high solution phase concentrations. See Huie [1995] and Zellner and Herrmann [1995] for a discussion of processes, and Herrmann et al. [2000] for a more recent modeling study.

Interactions with transition metal ions (TMI's)

Table 3. Mechanism of the iron catalyzed oxidation of S(IV) at pH = 3 after Ziajka et al. [1993, 1994] and Warneck et al. [1996b].

Remarks: (a) In units of l mol–1s–1 unless otherwise stated, data from Ziajka et al. [1993]; (b) see Warneck et al. [1996b] and references therein; (c) Rate constant for the overall reaction (3a)+(3b).

Transition metals are common constituents of atmospheric water [see Warneck, 1992; Graedel et al., 1986; Weschler et al., 1986; Pehkonen et al., 1992, 1993]. The most important of these species apparently is Fe with concentrations in the aqueous atmospheric phase of about 1x10^–6 mol 1^–1 [Warneck, 1992]. However, field measurements indicate that the concentration of Fe may vary by several orders of magnitude, i.e., between nanomolar and micromolar in rain and between micromolar and millimolar in fogwater [Hoigné et al., 1993]. The Fe content of tropospheric droplets originates from Fe₂O₃ which dissolves in acidic solutions (pH < 4). A method is available for the simultaneous determination of Fe(II) and Fe(III) from Pehkonen et al. [1992]. Recent field experiments showed a large fraction of Fe in tropospheric droplets to exist in the form of Fe(II) [Behra and Sigg, 1990; Erel et al., 1993]. For the Fe-catalysed oxidation of sulfur (IV) at pH=3 the reaction scheme shown in Table 3 is suggested by Ziajka et al. [1993] and Warneck et al. [1996].

In general the mechanism of Table 3 represents a combination of the thermal generation of sulfite radical anions followed by the reaction of sulfur-oxy radical anions. During the course of the reaction chain Fe(III) is recycled by the reactions of Fe(II) with HO₂, H₂O₂ (reactions (R-9) and (R-10)) as well as with peroxomonosulfate and sulfate radical anions (reactions (R-6) and (R-11)). The role of HO₂ radicals in this mechanism is both that of an oxidant (reaction (R-9)) and also that of a reductant (reaction (R-8)). Due to this effect, the HO₂/O₂^– system is expected to maintain Fe(II) at steady state [Warneck, 1992]. For completeness, the reaction scheme also contains several non-radical reactions in which S(IV) is converted to S(VI), i.e., reactions (R-16) and (R-17).

It has been suggested that manganese (II) [Berglund and Elding, 1993] and cobalt (II) [Coichev and van Eldik, 1991] may react with S(IV) in a manner similar to Fe(II). The addition of Mn(II) to systems as shown in Table 3 has been shown to have a synergistic effect [van Eldik et al., 1992; Coichev et al., 1992], i.e., the efficiency of catalytic cycling is strongly enhanced.

Copper is believed to be present in the atmospheric aqueous phase in concentrations of about one tenth the concentration of dissolved Fe [Sedlak and Hoignè, 1993]. It has been suggested that Cu might significantly influence the interactions of dissolved S(IV) with other transition metal ions and also free radicals because it is expected to react efficiently with HO₂/O₂^– according to Hoignè et al. [1993] and Sedlak and Hoignè [1993]:

Cu²⁺ + HO₂ Æ Cu⁺ + H⁺ + O₂ (R-18)

The rate constant for (R-18) has been determined by pulse radiolysis as k₁₈ = 1x10⁸ l mol^–1s^–1 [Rabani et al., 1973]. Copper may not only be reduced by HO₂ but it may also be oxidized from the Cu (I) to the Cu (II) oxidation state by HO₂/O₂^–, i.e.:

Cu⁺ + O₂^– + 2 H⁺ Æ Cu²⁺ + H₂O₂ (R-19)

with k₁₉ = 1x10¹⁰ l mol^–1s^–1 [Rabani et al., 1973]. It has hence been concluded that due to the high rate coefficients of these reactions the effect of the considerably smaller concentration of Cu in comparison to Fe is compensated and HO₂/O₂^– is expected to be efficiently converted to either molecular oxygen or hydrogen peroxide by the Cu⁺/Cu²⁺ redox pair. The interaction of Cu⁺/Cu²⁺ with hydro-peroxyl radicals has to be considered also in the context of the production of HO₂ formed following the photolysis of iron-oxalato complexes. HO₂ radicals produced in the catalytic cycle will be converted to O₂ and H₂O₂. The H₂O₂ produced will subsequently react with S(IV) in a non-radical process. Alternately, H₂O₂ may be subject to photolysis [Zellner et al., 1990] and produce OH radicals which in turn initiate the radical induced oxidation of S(IV) in polluted air masses. As a consequence, the catalytic decomposition of iron-oxalato complexes in the presence of Cu has to be regarded as an in situ source of hydrogen peroxide rather than only a source of hydroperoxyl radicals (see also above).

For a very detailed treatment of the transition–metal catalyzed S(IV) oxidation, the reader is referred to Brandt and van Eldik [1995].

Peroxynitric Acid (PNA) as an oxidant

Recombination of NO₂ and HO₂ in the gas phase leads to the formation of peroxynitric acid (PNA):

NO₂ + HO₂ Æ HNO₄ (R-20)

PNA can be taken up by aqueous particles very effectively and then contribute to the oxidation of S(IV):

HNO₄ + HSO₃^– Æ HNO₃ + HSO₄^– (R-21)

It has been suggested by Amels et al. [1996] based on laboratory experiments and demonstrated in a modeling study by Warneck [1999] that this sequence could on the one hand substantially contribute to the removal of NO_X from the troposphere as well as to aqueous phase sulfur oxidation.

Interaction of cloud sulfur oxidation with organic chemistry

S(IV) species may form complexes with aldehydes commonly encountered in tropospheric clouds. This subject is described in detail by Olson and Hoffmann [1989]. The complexes formed are no longer subject to effective oxidation by hydrogen peroxide or ozone. Possible losses are thermal decomposition and degradation by radicals.

Only for the most simple adduct, i.e., hydroxy-methanesulfonate, are kinetic data for the most important radical reactions available. For cloud chemistry models to be further developed to include higher organics, rate parameters are needed to better describe the possible degradation of condensation products of aldehydes and S(IV). It is interesting to note that such S(IV)/aldehyde condensation products are not only formed by aliphatic aldehydes but also by aromatic species such as benzaldehyde.

Another coupling of organic cloud chemistry to sulfur oxidation in clouds exists because oxidation of organics will lead to the formation of hydroperoxyl radicals, whose ultimate sink will be reaction with TMIs or recombination to form H₂O₂ which effectively will oxidize S(IV).

Recent box model results

Figure 1. Contributions of different oxidation pathways to sulfur (VI) formation. CAPRAM 2.4 (MODAC mech.) urban case, with emissions and deposition.

Recent model results with CAPRAM 2.4 (MODAC mechanism, an updated and enlarged version of CAPRAM 2.3 (see Herrmann et al. [2000], Ervens et al. [2001] and the CAPRAM website) show that for the used set of initial concentrations and emissions for an urban scenario, oxidation by hydrogen peroxide is most efficient. Results for the integrated production fluxes for S(VI) over 24 hours from the different S(IV) oxidation pathways for a permanent cloud and a cloud existing for 18 minutes are shown in Figure 1.

As can be seen, the oxidation by H₂O₂ is most efficient in both cases. It is very interesting to note that for the more realistic case of an 18 minute cloud duration about half of the S(IV) oxidized in total is converted by H₂O₂ in cloud water. The second most important pathway is then oxidation by the gas phase reaction of OH with SO₂ followed by the peroxynitric acid oxidation reaction. The reason for the smaller relative contribution of hydrogen peroxide for the shorter cloud period is the much smaller aqueous phase concentration of H₂O₂ compared to the continuous cloud case.

Organic oxidants such as methylhydroperoxide (CH₃OOH) constitute only a minor contribution. It has to be noted that the outcome of such reaction pathway comparisons is highly dependent on the concentration scenario established in the considered air mass. For the effect of clouds on tropospheric ozone this has been clearly demonstrated by Walcek et al. [1997].

Oxidation of reduced sulfur species

It should be noted that DMS oxidation might yield products such as DMSO, DMSO₂, methanesulfonic acid (MSA) and methanesulfinic acid (MSI), which may partition into marine aqueous particles, i.e., cloud droplets and marine aerosols [for DMSO in rainwater, see Sciare et al., 1998]. In solution, these species might undergo fast radical reactions as has been shown by Sehested and Holcman [1996]. Initial multiphase modeling studies have been undertaken [see Campolongo et al., 1999]. Many elementary rate coefficients needed for a better understanding of multiphase DMS oxidation and its coupling to tropospheric cloud and aerosol chemistry still need to be characterized. Apparently, another field of sulfur multiphase chemical conversions starting from species more reduced than S(IV) is emerging here. It combines both radical reactions with organics but then also couples to 'conventional' S(IV) conversion because this is an intermediate step in the oxidation chain from S(-II) as found in DMS to S(VI) as found in MSA or sulfate.

Summary and outlook

The radical-driven oxidation of S(IV) to S(VI), after a great deal of research in the last hundred years, still constitutes an extensive research topic of its own. Open questions exist with regards to the dependencies of elementary reaction rate constants on temperature and also to ionic strengths and ionic composition of solution. Aqueous phase process studies turn more and more to investigating the effects of organics. Organic chemistry in cloud water may influence the rate of S(IV) oxidation in droplets [Warneck, 1996a], alter the pattern of precipitation composition, and may also lead to the production of harmful substances (see, e.g., Lüttke et al. [1997] for the special case of nitrophenols as well as other contributions in this newsletter). Another potentially very interesting coupling to organic chemistry exists in the area of multiphase DMS oxidation.

For a more systematic understanding of multiphase sulfur oxidation, the broader solution phase chemistry needs to be better characterized in laboratory studies, with results implemented into tropospheric multiphase chemistry models. These, subsequently, have to be coupled to microphysical models, to be tested in field experiments with regards to their ability to describe measured gas phase trace gas and oxidant concentrations, cloud water constituent concentrations and chemical aerosol composition in the tropospheric multiphase system.

References

1. Amels, P., H. Elias, U. Götz, U. Steinges and K. J. Wannowius (1996), Kinetic investigation of the stability of peroxonitric acid and of its reaction with sulfur(IV) in aqueous solution. In: Heterogeneous and Liquid Phase Processes (P. Warneck, Ed.), Springer, Berlin, 77-88.
2. Barlow, S., J. Berglund, G. V. Buxton, S. Mc Gowan and G. A. Salmon (1993), Kinetics and mechanisms of acid generation in clouds and precipitation, in: Annual EUROTRAC Report 1992, Part 6, GCE/HALIPP, EUROTRAC ISS, Garmisch- Partenkirchen, pp.49-56.
3. Behra, P. und L. Sigg (1990), Evidence for Redox Cycling of Iron in Atmospheric Water Droplets, Nature 344, 419-421.
4. Berglund, J. and L. I. Elding (1993), Kinetics and mechanism for metal catalysed oxidation of sulfur(IV) by oxygen in aqueous solution, in Annual EUROTRAC Report 1992, Part 6, GCE/HALIPP, EUROTRAC ISS, Garmisch-Partenkirchen 1993, pp.19-22.
5. Brandt, C. and R. van Eldik (1995), Transition metal-catalysed oxidation of sulfur(IV)oxides. Atmospheric relevant processes and mechanisms, Chem. Rev. 95, 119-190.
6. Buxton, G.V., G. A. Salmon, N. D. Wood, S. Croft and S. Mc Gowan (1991), Kinetics and mechanisms of acid generation in clouds and precipitation, in: Annual EUROTRAC Report 1990, Part 6, HALIPP, EUROTRAC ISS, Garmisch- Partenkirchen, pp.38-43.
7. Buxton, G.V., S. McGowan and G. A. Salmon (1993), The free radical chain oxidation of S(IV). In P.M. Borrell, P. Borell, T. Cvitas and W. Seiler (eds.), The Proceedings of EUROTRAC Symposium `92, SPB Academic Publishing bv, The Hague, pp. 599-604.
8. Campolongo, F., A. Saltelli, N.R. Jensen, J. Wilson and J. Hjorth (1999), The role of multiphase chemistry in the oxidation of dimethylsulphide (DMS). A latitude dependent analysis, J. Atmos. Chem. 32, 327-356.
9. Chameides, W.L. and D. D. Davis (1982), The free radical chemistry of cloud droplets and its impact upon the composition of rain, J. Geophys. Res. 87, 4863-4877.
10. Chameides, W.L. (1986) Photochemistry of the atmospheric aqueous phase, In W. Jaeschke (Ed.), Chemistry of Multiphase Atmospheric Systems, , Springer, Berlin, pp. 369-414.
11. Coichev, N., K. Bal Reddy and R. van Eldik (1992), The synergistic effect of manganese (II) in the sulfite-induced autoxidation of metal ions and complexes in aqueous solution, Atmos. Environ. 26A, 2295-2300.
12. Coichev, N. and R. van Eldik (1991), Kinetics and mechanism of the sulfite-induced autoxidation of Co(II) in aqueous azide medium, Inorg. Chem. 30, 2375-2380.
13. Davidovits, M., Hu, J. H., Worsnop, D. R., Zahniser, M. S. and Kolb, C. E. (1995), Entry of Gas Molecules into Liquids, Faraday Discuss. 100, 65-82.
14. Erel, Y., S. O. Pehkonen and M.R. Hoffmann (1993), Redox Chemistry of Iron in Fog and Stratus Clouds, J. Geophys. Res. 98, 18423-18434.
15. Ervens, B., H. Herrmann, G. V. Buxton, G. A. Salmon, J. Williams, F. Dentener, C. George and P. Mirabel (2001) CAPRAM2.4 (MODAC mechanism): An extended and condensed tropospheric aqueous phase mechanism and its application, in preparation., see also CAPRAM webpage via www.tropos.de
16. Exner, M., H. Herrmann and R. Zellner (1992), Laser based Studies of Reactions of the Nitrate Radical in Aqueous Solution, Ber. Bunsenges. Phys. Chem. 96, 470-477.
17. Graedel, T. E. and C. J. Weschler (1981), Chemistry Within Aqueous Atmospheric Aerosols and Raindrops, Rev. Geophys. Space Phys. 19, 505-539.
18. Graedel, T. E., M. L. Mandich and C. J. Weschler (1986), Kinetic model Studies of Atmosheric Droplet Chemistry 2. Homogeneous Transition Metal Chemistry in Raindrops, J. Geophys. Res. 91, 5205-5221.
19. Hoffmann, M. R. (1986), On the Kinetics and Mechanism of Oxidation of Aquated Sulfur Dioxide by Ozone, Atmos. Environ. 20, 1145-1154.
20. Herrmann, H., M. Exner and R. Zellner (1991), A laser photolysis study of reactions of the carbonate radical anion (CO₃^—) in aqueous solution, in K.H.Becker (Ed), Air Pollution Research Report 33: Atmospheric Oxidation Processes, CEC, Brussels, pp.134-138.
21. Herrmann, H., B. Ervens , H.-W. Jacobi, R. Wolke, P. Nowacki and R. Zellner (2000), CAPRAM2.3: A Chemical Aqueous Phase Radical Mechanism for Tropospheric Chemistry, J. Atmos. Chem. 36, 231-284.
22. Hoigné, J., Y. Zuo, M. von Piechowski, R. Bühler and D. Sedlak (1993), The role of iron and copper species for reactions of photooxidants and photochemical reactions in atmospheric waters, In J. Peeters (ed), Air Pollution Research Report 45: Chemical Mechanisms Describing Tropospheric Processes, CEC, Brussels, 1993, p.5-16.
23. Huie, R. E. and P. Neta (1987), Rate constants for some oxidations of S(IV) by radicals in aqueous solutions, Atmos. Environ. 21, 1743-1447.
24. Huie, R.E. (1995), Free radical chemistry of the atmospheric aqueous phase, in Progress and Problems in Atmospheric Chemistry, Ed. J.R. Barker, World Scientific, Singapore, pp 374-419.
25. Jacob, D. J., E. W. Gottlieb and M. J. Prather (1989), Chemistry of a Polluted Cloud Boundary Layer, J. Geophys. Res. 94, 12975-13002.
26. Jacob, D. J. (2000), Heterogeneous chemistry and tropospheric ozone, Atmos. Environ. 34, 2131-2159.
27. Langner, J. and H. Rodhe, H. (1991) A global three-dimensional model of the tropospheric sulfur cycle. J. Atmos. Chem. 13, 255-263.
28. Lelieveld, J., and P. J. Crutzen (1991), The role of clouds in tropospheric photochemistry, J. Atmos. Chem. 12, 229-267.
29. Lüttke J., V. Scheer, K. Levsen, G. Wünsch, J. N. Cape, K. Hargreaves, R.L. Storeton-West, K. Acker, W. Wieprecht and B. Jones (1997), Occurrence and formation of nitrated phenols in and out of cloud, Atmos. Environ. 31, 2637-2648.
30. Olson, T. M. and M. R. Hoffmann (1989), Hydroxyalkylsulfonate formation: its role as a S(IV) reservoir in atmospheric water droplets, Atmos. Environ. 23, 985 - 997.
31. Pehkonen, S. O., R. Siefert, S. Webb, Y. Erel, and M. R. Hoffmann (1992), Simultaneous spectrophometric measurement of Fe(II) and Fe(III) in atmospheric water, Environ. Sci. Technol. 26, 1731-1736.
32. Pehkonen, S.O., R. Siefert, Y. Erel, S. Webb and M. R. Hoffmann (1993), Photoreduction of iron oxyhydroxides in the presence of important atmospheric organic compounds, Environ. Sci. Technol. 27, 2056-2062.
33. Rabani, J., D. Klug-Roth and J. Lilie (1973), Pulse radiiolytic investigations of the catalyste diproportionation of peroxy radicals. Aqueous cupric ions, J. Phys. Chem. 77, 1169-1173.
34. Sciare, J., E. Baboukas, R. Hancy, N. Mihalopoulos and B.C. Nguyen (1998), Seasonal variation of dimethylsulfoxide in rainwater at Amsterdam Island in the southern Indian Ocean: Implications on the biogenic sulfur cycle, J. Atmos. Chem. 30, 229-240.
35. Sedlak, D. L. und J. Hoigné (1993), The Role of Copper and Oxalate in the Redox Cycling of Iron in Atmospheric Waters, Atmos. Environ. 27A, 2173-2185.
36. Sehested, K. and J. Holcman (1996), A pulse radiolysis study of the OH radical induced autoxidation of methanesulfinic acid, Radiat. Phys. Chem. 47, 357-360.
37. van Eldik, R. N. Coichev, K. Bal Reddy and A. Gerhard (1992), Metal ion catalyzed autoxidation of sulfur(IV)-oxides: Redox cycling of metal ions induced by sulfite, Ber. Bunsenges. Phys. Chem. 96, 478-485.
38. Walcek, C. J., H. H. Yuan and W. R. Stockwell (1997), The influence of aqueous-phase chemical reactions on ozone formation in polluted and nonpolluted clouds, Atmos. Environ. 31, 1221-1237.
39. Warneck, P. (1991), Chemical reactions in clouds, Fresenius J. Anal. Chem. 340, 585-590.
40. Warneck, P. (1992), Chemistry and photochemistry in atmospheric water drops, Ber. Bunsenges. Phys. Chem. 96, 454-460.
41. Warneck, P. (1993), HALIPP annual report 1992, In Annual EUROTRAC Report 1992, Part 6, GCE/HALIPP, Garmisch- Partenkirchen, pp.9-16
42. Warneck, P., P. Mirabel, G. A. Salmon, R. van Eldik, C. Vinckier, K.J. Wannowius and C. Zetzsch (1996a), Review of the Activities and Achievements of the EUROTRAC subproject HALIPP, in Warneck, P. (Ed.), Heterogeneous and Liquid-Phase Processes, Springer, Berlin, pp 7-74.
43. Warneck, P., H.-J. Benkelberg, U. Deister, M. Fischer, A. Schäfer and J. Ziajka (1996b), Chain Oxidation of S(IV) in Aqueous Solution: Application of Radical Scavenger Techniques, in Warneck, P. (Ed.), Heterogeneous and Liquid-Phase Processes, Springer, Berlin, pp. 140-145.
44. Warneck, P. (1999), The relative importance of various pathways for the oxidation of sulfur dioxide and nitrogen dioxide in sunlit continental fair weather clouds, Phys. Chem. Chem. Phys. 1, 5471-5483.
45. Weschler, C. J., M. L. Mandich and T. E. Graedel, 1986, Speciation, Photosensitivity and Reactions of Transition Metal Ions in Atmosheric Droplets, J. Geophys. Res. 91, 5189-5204.
46. Zellner, R., M. Exner and H. Herrmann, , 1990, Absolute OH Quantum Yields in the Laser Photolysis of Nitrate, Nitrite and Dissolved H₂O₂ at 308 and 351 nm in the Temperature Range of 278-353 K, J. Atm. Chem. 10, 411-425.
47. Zellner R. and H. Herrmann (1995), Free Radical Chemistry of the Aqueous Atmospheric Phase, in Advances in Spectroscopy, Vol. 24, Spectroscopy in Environmental Science, Eds. R.J.H. Clark and R.E. Hester, pp. 381 - 451, Wiley, London.
48. Ziajka, J, F. Beer and P. Warneck (1993) Chain Oxidation of S(IV) in aqueous solution: application of radical scavenger techniques. In Annual EUROTRAC Report 1992, Part 6, GCE/HALIPP, Garmisch-Partenkirchen, pp. 64-69.
49. Ziajka, J., F. Beer, and P. Warneck, (1994) Iron-Catalysed Oxidation of Bisulphite Aqueous Solution: Evidence for a Free Radical Chain Mechanism, Atmos. Environ. 28, 2549 - ?2552.