Executive Summary

4.1 Introduction

4.1.1 Sources of Greenhouse Gases
4.1.2 Atmospheric Chemistry and Feedbacks
4.1.3 Trace Gas Budgets and Trends
4.1.4 Atmospheric Lifetimes and Time-Scales

4.2 Trace Gases: Current Observations, Trends and Budgets

4.2.1 Non-CO₂ Kyoto Gases
4.2.1.1 Methane (CH₄)
4.2.1.2 Nitrous oxide (N₂O)
4.2.1.3 Hydrofluorocarbons (HFCs)
4.2.1.4 Perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆)
4.2.2 Montreal Protocol Gases and Stratospheric Ozone (O₃)
4.2.3 Reactive Gases
4.2.3.1 Carbon monoxide (CO) and hydrogen (H₂)
4.2.3.2 Volatile organic compounds (VOC)
4.2.3.3 Nitrogen oxides (NO_x)
4.2.4 Tropospheric O₃
4.2.5 Stratospheric H₂O
4.2.6 Tropospheric OH and Photochemical Modelling
4.2.6.1 Laboratory data and the OH lifetime of greenhouse gases
4.2.6.2 Atmospheric measurements and modelling of photochemistry

4.3 Projections of Future Emissions

4.3.1 The Adjusted/Augmented IPCC/SRES Emission Scenarios
4.3.2 Shifting Regional Emissions of NO_x, CO and VOC in 2100
4.3.3 Projections of Natural Emissions in 2100

4.4 Projections of Atmospheric Composition for the 21st Century

4.4.1 Introduction
4.4.2 The OxComp Workshop
4.4.3 Testing CTM Simulation of the Current (Y2000) Atmosphere
4.4.4 Model Simulations of Perturbed and Y2100 Atmospheres
4.4.5 Atmospheric Composition for the IPCC Scenarios to 2100
4.4.6 Gaps in These Projections - the Need for Coupled Models
4.4.7 Sensitivity Analysis for Individual Sectors

4.5 Open Questions

4.5.1 Chemical Processes Important on the Global Scale
4.5.1.1 Missing chemistry, representation of small scales, and changing emission patterns
4.5.1.2 Aerosol interactions with tropospheric O₃ and OH
4.5.1.3 Stratosphere-troposphere coupling
4.5.1.4 Uncertainties in the tropospheric O₃ budget
4.5.2 Impacts of Physical Climate Change on Atmospheric Chemistry
4.5.3 Feedbacks through Natural Emissions

4.6 Overall Impact of Global Atmospheric Chemistry Change

References

Co-ordinating Lead Authors
D. Ehhalt, M. Prather

Lead Authors
F. Dentener, R. Derwent, E. Dlugokencky, E. Holland, I. Isaksen, J. Katima, V. Kirchhoff, P. Matson, P. Midgley, M. Wang

Contributing Authors
T. Berntsen, I. Bey, G. Brasseur, L. Buja, W.J. Collins, J. Daniel, W.B. DeMore, N. Derek, R. Dickerson, D. Etheridge, J. Feichter, P. Fraser, R. Friedl, J. Fuglestvedt, M. Gauss, L. Grenfell, A. Grübler, N. Harris, D. Hauglustaine, L. Horowitz, C. Jackman, D. Jacob, L. Jaeglé, A. Jain, M. Kanakidou, S. Karlsdottir, M. Ko, M. Kurylo, M. Lawrence, J.A. Logan, M. Manning, D. Mauzerall, J. McConnell, L. Mickley, S. Montzka, J.F. Müller, J. Olivier, K. Pickering, G. Pitari, G.J. Roelofs, H. Rogers, B. Rognerud, S. Smith, S. Solomon, J. Staehelin, P. Steele, D. Stevenson, J. Sundet, A. Thompson, M. van Weele, R. von Kuhlmann, Y. Wang, D. Weisenstein, T. Wigley, O. Wild, D. Wuebbles, R. Yantosca

Review Editors
F. Joos, M. McFarlan

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