 Solution

ID: past-exam-of-the-atmospheric-physics-course-of-the-university-of-cambridge/2012/paper-1/2/d/solution

There are several distinct radiative pathways:

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.
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.
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.
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. ^[ref]

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