This chapter provides a synopsis of aerosol observations, source inventories, and the theoretical understanding required to enable an assessment of radiative forcing from aerosols and its uncertainty.

• The chemical and physical properties of aerosols are needed to estimate and predict direct and indirect climate forcing.

Aerosols are liquid or solid particles suspended in the air. They have a direct radiative forcing because they scatter and absorb solar and infrared radiation in the atmosphere. Aerosols also alter warm, ice and mixed-phase cloud formation processes by increasing droplet number concentrations and ice particle concentrations. They decrease the precipitation efficiency of warm clouds and thereby cause an indirect radiative forcing associated with these changes in cloud properties. Aerosols have most likely made a significant negative contribution to the overall radiative forcing. An important characteristic of aerosols is that they have short atmospheric lifetimes and therefore cannot be considered simply as a long-term offset to the warming influence of greenhouse gases.

The size distribution of aerosols is critical to all climate influences. Sub-micrometre aerosols scatter more light per unit mass and have a longer atmospheric lifetime than larger aerosols. The number of cloud condensation nuclei per mass of aerosol also depends on the chemical composition of aerosols as a function of size. Therefore, it is essential to understand the processes that determine these properties.

• Since the last IPCC report, there has been a greater appreciation of aerosol species other than sulphate, including sea salt, dust, and carbonaceous material. Regionally resolved emissions have been estimated for these species.

For sulphate, uncertainties in the atmospheric transformation of anthropogenic sulphur dioxide (SO₂) emissions to sulphate are larger than the 20 to 30% uncertainties in the emissions themselves. SO₂ from volcanoes has a disproportionate impact on sulphate aerosols due to the high altitude of the emissions, resulting in low SO₂ losses to dry deposition and a long aerosol lifetime. Modelled dust concentrations are systematically too high in the Southern Hemisphere, indicating that source strengths developed for the Sahara do not accurately predict dust uplift in other arid areas. Owing to a sensitive, non-linear dependence on wind speed of the flux of sea salt from ocean to atmosphere, estimates of global sea salt emissions from two present day estimates of wind speed differed by 55%. The two available inventories of black carbon emissions agree to 25% but the uncertainty is certainly greater than that and is subjectively estimated as a factor of two. The accuracy of source estimates for organic aerosol species has not been assessed, but organic species are believed to contribute significantly to both direct and indirect radiative forcing. Aerosol nitrate is regionally important but its global impact is uncertain.

• There is a great spatial and temporal variability in aerosol concentrations. Global measurements are not available for many aerosol properties, so models must be used to interpolate and extrapolate the available data. Such models now include the types of aerosols most important for climate change.

A model intercomparison was carried out as part of preparation for this assessment. All participating models simulated surface mass concentrations of non-sea-salt sulphate to within 50% of observations at most locations. Whereas sulphate aerosol models are now commonplace and reasonably well-tested, models of both organic and black carbon aerosol species are in early stages of development. They are not well-tested because there are few reliable measurements of black carbon or organic aerosols.

The vertical distribution of aerosol concentrations differs substantially from one model to the next, especially for components other than sulphate. For summertime tropopause conditions the range of model predictions is a factor of five for sulphate. The range of predicted concentrations is even larger for some of the other aerosol species. However, there are insufficient data to evaluate this aspect of the models. It will be important to narrow the uncertainties associated with this aspect of models in order to improve the assessment of aircraft effects, for example.

Although there are quite large spreads between the individual short-term observed and model-predicted concentrations at individual surface stations (in particular for carbonaceous aerosols), the calculated global burdens for most models agree to within a factor of 2.5 for sulphate, organics, and black carbon. The model-calculated range increases to three and to five for dust and sea salt with diameters less than 2 mm, respectively. The range for sea salt increases to a factor of six when different present day surface wind data sets are used.

• An analysis of the contributions of the uncertainties in the different factors needed to estimate direct forcing to the overall uncertainty in the direct forcing estimates can be made. This analysis leads to an overall uncertainty estimate for fossil fuel aerosols of 89% (or a range from –0.1 to –1.0 Wm^–2) while that for biomass aerosols is 85% (or a range from –0.1 to –0.5 Wm^–2 ).

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