= Solution
There are several distinct radiative pathways:
* \b[Direct scattering:] predominantly nonabsorbing particles, such as many sulfate or sea-salt aerosols, redirect sunlight and commonly increase reflected shortwave radiation over a dark surface. Hygroscopic growth increases their wet size and can increase scattering.
* \b[Direct absorption:] absorbing particles, such as soot or some mineral dust, heat the layer containing them while reducing sunlight at the surface. The top-of-atmosphere sign depends on particle optics, altitude, clouds and surface albedo; atmospheric heating is not the same as surface warming.
* \b[Cloud activation:] a soluble, hygroscopic particle can act as a <cloud condensation nucleus>. At fixed liquid-water path, more activated droplets lower the effective radius and increase cloud albedo through the <Twomey effect>. Precipitation and water-content adjustments can modify that response. Insoluble particles may still activate when sufficiently wettable or coated, and some serve as ice nuclei, changing cloud phase.
* \b[Cloud adjustment to aerosol heating:] absorbing particles alter temperature, relative humidity and stability. This <semi-direct aerosol effect> can evaporate cloud or, in other configurations, help sustain it; its sign is not universal.
Solubility and optical absorption are independent properties: insoluble does not mean absorbing, and soluble does not mean nonabsorbing. Coarse particles can also affect terrestrial infrared radiation, while dark-particle deposition on snow can lower surface albedo. These effects should not be folded into a single universal aerosol cooling rule. <The Earth's Energy Budget, Climate Feedbacks, and Climate Sensitivity by Piers Forster and others>[§7.3.3, aerosol–radiation and aerosol–cloud interactions and adjustments]{ref}
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