5 Aerosols, their Direct and Indirect Effects Co-ordinating Lead Author J.E. Penner Lead Authors M. Andreae, H. Annegarn, L. Barrie, J. Feichter, D. Hegg, A. Jayaraman, R. Leaitch, D. Murphy, J. Nganga, G. Pitari Contributing Authors A. Ackerman, P. Adams, P. Austin, R. Boers, O. Boucher, M. Chin, C. Chuang, B. Collins, W. Cooke, P. DeMott, Y. Feng, H. Fischer, I. Fung, S. Ghan, P. Ginoux, S.-L. Gong, A. Guenther, M. Herzog, A. Higurashi, Y. Kaufman, A. Kettle, J. Kiehl, D. Koch, G. Lammel, C. Land, U. Lohmann, S. Madronich, E. Mancini, M. Mishchenko, T. Nakajima, P. Quinn, P. Rasch, D.L. Roberts, D. Savoie, S. Schwartz, J. Seinfeld, B. Soden, D. Tanré, K. Taylor, I. Tegen, X. Tie, G. Vali, R. Van Dingenen, M. van Weele, Y. Zhang Review Editors B. Nyenzi, J. Prospero Contents Executive Summary 291 5.4.1 Summary of Current Model Capabilities 313 5.4.1.1 Comparison of large-scale sulphate 5.1 Introduction 293 models (COSAM) 313 5.1.1 Advances since the Second Assessment 5.4.1.2 The IPCC model comparison Report 293 workshop: sulphate, organic carbon, 5.1.2 Aerosol Properties Relevant to Radiative black carbon, dust, and sea salt 314 Forcing 293 5.4.1.3 Comparison of modelled and observed aerosol concentrations 314 5.2 Sources and Production Mechanisms of 5.4.1.4 Comparison of modelled and satellite- Atmospheric Aerosols 295 derived aerosol optical depth 318 5.2.1 Introduction 295 5.4.2 Overall Uncertainty in Direct Forcing 5.2.2 Primary and Secondary Sources of Aerosols 296 Estimates 322 5.2.2.1 Soil dust 296 5.4.3 Modelling the Indirect Effect of Aerosols on 5.2.2.2 Sea salt 297 Global Climate Forcing 324 5.2.2.3 Industrial dust, primary 5.4.4 Model Validation of Indirect Effects 325 anthropogenic aerosols 299 5.4.5 Assessment of the Uncertainty in Indirect 5.2.2.4 Carbonaceous aerosols (organic and Forcing of the First Kind 328 black carbon) 299 5.2.2.5 Primary biogenic aerosols 300 5.5 Aerosol Effects in Future Scenarios 330 5.2.2.6 Sulphates 300 5.5.1 Introduction 330 5.2.2.7 Nitrates 303 5.5.2 Climate Change and Natural Aerosol 5.2.2.8 Volcanoes 303 Emissions 330 5.2.3 Summary of Main Uncertainties Associated 5.5.2.1 Projection of DMS emissions with Aerosol Sources and Properties 304 in 2100 331 5.2.4 Global Observations and Field Campaigns 304 5.5.2.2 Projection of VOC emissions 5.2.5 Trends in Aerosols 306 in 2100 331 5.5.2.3 Projection of dust emissions 5.3 Indirect Forcing Associated with Aerosols 307 in 2100 331 5.3.1 Introduction 307 5.5.2.4 Projection of sea salt emissions 5.3.2 Observational Support for Indirect Forcing 307 in 2100 332 5.3.3 Factors Controlling Cloud Condensation 5.5.3 Simulation of Future Aerosol Nuclei 308 Concentrations 332 5.3.4 Determination of Cloud Droplet Number 5.5.4 Linkage to Other Issues and Summary 334 Concentration 309 5.3.5 Aerosol Impact on Liquid-Water Content 5.6 Investigations Needed to Improve Confidence in and Cloud Amount 310 Estimates of Aerosol Forcing and the Role of 5.3.6 Ice Formation and Indirect Forcing 311 Aerosols in Climate Processes 334 5.4 Global Models and Calculation of Direct and References 336 Indirect Climate Forcing 313 Aerosols, their Direct and Indirect Effects 291 Executive Summary concentrations. Global measurements are not available for many aerosol properties, so models must be used to interpolate This chapter provides a synopsis of aerosol observations, source and extrapolate the available data. Such models now include the inventories, and the theoretical understanding required to enable types of aerosols most important for climate change. an assessment of radiative forcing from aerosols and its uncertainty. A model intercomparison was carried out as part of preparation for this assessment. All participating models simulated surface • The chemical and physical properties of aerosols are needed to mass concentrations of non-sea-salt sulphate to within 50% of estimate and predict direct and indirect climate forcing. observations at most locations. Whereas sulphate aerosol models are now commonplace and reasonably well-tested, models of Aerosols are liquid or solid particles suspended in the air. They both organic and black carbon aerosol species are in early stages have a direct radiative forcing because they scatter and absorb of development. They are not well-tested because there are few solar and infrared radiation in the atmosphere. Aerosols also alter reliable measurements of black carbon or organic aerosols. warm, ice and mixed-phase cloud formation processes by The vertical distribution of aerosol concentrations differs increasing droplet number concentrations and ice particle substantially from one model to the next, especially for concentrations. They decrease the precipitation efficiency of components other than sulphate. For summertime tropopause warm clouds and thereby cause an indirect radiative forcing conditions the range of model predictions is a factor of five for associated with these changes in cloud properties. Aerosols have sulphate. The range of predicted concentrations is even larger for most likely made a significant negative contribution to the overall some of the other aerosol species. However, there are insufficient radiative forcing. An important characteristic of aerosols is that data to evaluate this aspect of the models. It will be important to they have short atmospheric lifetimes and therefore cannot be narrow the uncertainties associated with this aspect of models in considered simply as a long-term offset to the warming influence order to improve the assessment of aircraft effects, for example. of greenhouse gases. Although there are quite large spreads between the The size distribution of aerosols is critical to all climate individual short-term observed and model-predicted concentra- influences. Sub-micrometre aerosols scatter more light per unit tions at individual surface stations (in particular for carbonaceous mass and have a longer atmospheric lifetime than larger aerosols. aerosols), the calculated global burdens for most models agree to The number of cloud condensation nuclei per mass of aerosol within a factor of 2.5 for sulphate, organics, and black carbon. also depends on the chemical composition of aerosols as a The model-calculated range increases to three and to five for dust function of size. Therefore, it is essential to understand the and sea salt with diameters less than 2 µm, respectively. The processes that determine these properties. range for sea salt increases to a factor of six when different present day surface wind data sets are used. • Since the last IPCC report, there has been a greater appreciation of aerosol species other than sulphate, including sea salt, dust, • An analysis of the contributions of the uncertainties in the different and carbonaceous material. Regionally resolved emissions have factors needed to estimate direct forcing to the overall uncertainty been estimated for these species. in the direct forcing estimates can be made. This analysis leads to an overall uncertainty estimate for fossil fuel aerosols of 89% (or For sulphate, uncertainties in the atmospheric transformation of a range from –0.1 to –1.0 Wm–2) while that for biomass aerosols –2 anthropogenic sulphur dioxide (SO2) emissions to sulphate are is 85% (or a range from –0.1 to –0.5 Wm ). larger than the 20 to 30% uncertainties in the emissions themselves. SO2 from volcanoes has a disproportionate impact on For this analysis the central value for the forcing was estimated sulphate aerosols due to the high altitude of the emissions, resulting using the two-stream radiative transfer equation for a simple box in low SO2 losses to dry deposition and a long aerosol lifetime. model. Central values for all parameters were used and error Modelled dust concentrations are systematically too high in the propagation was handled by a standard Taylor expansion. Southern Hemisphere, indicating that source strengths developed Estimates of uncertainty associated with each parameter were for the Sahara do not accurately predict dust uplift in other arid developed from a combination of literature estimates for areas. Owing to a sensitive, non-linear dependence on wind speed emissions, model results for determination of burdens, and of the flux of sea salt from ocean to atmosphere, estimates of global atmospheric measurements for determination of mass scattering sea salt emissions from two present day estimates of wind speed and absorption coefficients and water uptake effects. While such differed by 55%. The two available inventories of black carbon a simple approach has shortcomings ( e.g., it tacitly assumes both emissions agree to 25% but the uncertainty is certainly greater than horizontal and vertical homogeneity in such quantities as relative that and is subjectively estimated as a factor of two. The accuracy humidity, or at least that mean values can well represent the of source estimates for organic aerosol species has not been actual distributions), it allows a specific association of parameter assessed, but organic species are believed to contribute signifi- uncertainties with their effects on forcing. With this approach, the cantly to both direct and indirect radiative forcing. Aerosol nitrate most important uncertainties for fossil fuel aerosols are the is regionally important but its global impact is uncertain. upscatter fraction (or asymmetry parameter), the burden (which includes propagated uncertainties in emissions), and the mass • There is a great spatial and temporal variability in aerosol scattering efficiency. The most important uncertainties for 292 Aerosols, their Direct and Indirect Effects biomass aerosols are the single scattering albedo, the upscatter A major challenge is to develop and validate, through fraction, and the burden (including the propagated uncertainties observations and small-scale modelling, parametrizations for in emissions).