1.- 1.1 Highlights of the Results.- 1.2 Original Aims of the EUROTRAC Subproject HALIPP.- 2 Review of the Activities and Achievements of the EUROTRAC Subproject HALIPP.- 2.1 Introduction.- 2.2 Sulfur (IV) Oxidation by Ozone, Hydroperoxides and Hypochlorous Acid.- 2.3 Transition Metal Ions and Their Role in Atmospheric Waters.- 2.4 Photochemical Sources of Radicals in Atmospheric Water Drops.- 2.5 Free Radical Reactions in the Aqueous Phase.- 2.6 Gas-Liquid Interactions.- 2.7 Aerosol Surface Reactions and Photocatalysis.- 2.8 General Conclusions.- Individual Reports from HALIPP Contributors.- 3 Sulfur(IV) Oxidation.- 3.1 Kinetic Investigation of the Stability of Peroxonitric Acid and its Reaction with Sulfur(IV) in Aqueous Solution T. Amels, R. Elias, U. Götz, U. Steingens and K. J. Wannowius (Darmstadt).- 3.2 Electrolyte Effects on Aqueous Oxidation of Sulfur Dioxide by Strong Oxidants J. Lagrange, P. Lagrange, C. Pallarès and G. Wenger (Strasbourg).- 4 Transition Metal Ions in Atmospheric Waters.- 4.1 Metal Ion Catalysed Autoxidation of Dissolved Sulfur Dioxide J. Berglund and L. I. Elding (Lund).- 4.2 Catalytic Effects of Atmospheric Aerosol Particles in the Aqueous Phase of Trace Gases I. Grgic, M. Poje, M. Novic, M. Bizjak and V. Rudnik (Ljubljana).- 4.3 Kinetics of Ozonation Processes in Cloud and Fog Water: Effect of Transition Metal Ions on Free Radical Reactions J. Hoigné and R. Böhler (DübendorflZürich).- 4.4 Kinetics of S(IV) Oxidation in Aqueous Solution: Impact of Transition Metal Ion Transformations W. Pasiuk-Bronikowska K. J. Rudzinski T. Bronikowsk J. Ziajka (Warsaw).- 4.5 The Interaction of SOX and NOy Species in Aqueous Solution: Photo and Metal Ion Catalysis R. van Eldik M. Gei?ler C. Brandt V. Lepentsiotis (Erlangen).- 5 Photochemical Sources and Reactions of Free Radicals in the Aqueous Phase.- 5.1 Kinetics and Mechanisms of Acid Generation in Clouds and Precipitation G. V. Buxton G. A. Salmon S. Barlow T. N. Malone S. McGowan S. A. Murray J. E. Williams N. D. Wood (Leeds).- 5.2 Chain Oxidation Mechanisms of S(IV) in Aqueous Solution: Application of Radical Scavenger Techniques P. Warneck H.-J. Benkelberg U. Deister M. Fischer A. Schäfer J. Ziajka (Mainz).- 5.3 Formation and Reaction of Oxidants in the Aqueous Phase R. Zellner H. Herrmann M. Exner H.-W. Jacobi G. Raabe A. Reese (Essen).- 6 Gas-Liquid Interactions.- 6.1 Photocatalysis of Tropospheric Chemistry by Sea Spray W. Behnke M. Elend C. George H.-U. Kröger V. Scheer C. Zetzsch (Hannover).- 6.2 Phase Partitioning and Redox Processes in Fog and Cloud Systems W. Jaeschke J. Dierssen S. Volkwein J. Wohlgemuth (Frankfurt/Main).- 6.3 Gas-Liquid Interactions P. Mirabel C. George L. Magi J. L. Ponche (Strasbourg).- 6.4 Laboratory Determination of Physico-chemical Rate Parameters Pertinent to Mass Transfer into Cloud and Fog Droplets U. Schuraih A. Bongartz J. Kames C. Wunderlich T. Carstens (Bonn).- 7 Aerosol Surface Reactions and Photocatalysis.- 7.1 Determination of ß-Pinene OH Radical Rate Constant - Possible Impact of Solid Particles P. Foster I. Denis V. Jacob (Grenoble).- 7.2 Calorimetric Study of Lewis Acid Centres and Adsorbed Water in the Heterogeneous Reactions Involving Tropospheric Particles J.-C. Petit J. Thlibi (Orléans).- 7.3 Laboratory Studies of the Phototransformation of Organic Compounds in the Presence of Water and/or Inorganic Particulate Solids P. Pichat C. Guillard C. Hoang-Van H. Delprat D. Mas F. Marine T. Bouvier B. Servajean (Lyon).- 7.4 Heterogeneous Reactions of Nitrogen Oxides (NO2, N25, HNO3, CIONO2) with Surfaces Representative of Atmospheric Aerosol M. J. Rossi F. F. Fenter K. Tabor F. Caloz L. Gutzwiller (Lausanne).- 7.5 Heterogeneous Reactions of Air Pollutants on Various Solid Surfaces C. Vinckier N. Van Hoof S. Ashty F. Compernolle (Leuven).- 8 HALIPP Publications 1988-1995.