= Solution
\b[Direct effect:] nonabsorbing aerosol scatters sunlight, usually increasing reflected shortwave flux over dark surfaces. Absorbing aerosol also heats its atmospheric layer while reducing surface sunlight; the top-of-atmosphere sign depends on its altitude, optical properties and underlying albedo.
\b[Cloud albedo effect:] soluble, hygroscopic aerosol readily takes up water and can activate as <cloud condensation nucleus>[cloud condensation nuclei]. At fixed liquid-water path, more smaller droplets increase optical depth and cloud reflectivity: the <Twomey effect>. Suppressed collision-coalescence can delay rain and change cloud amount or lifetime, though dynamical adjustments prevent a universal sign.
\b[Semi-direct effect:] absorption can alter stability and evaporate clouds, or change entrainment and cloud cover. Insoluble dust may be a poor soluble-droplet nucleus but still scatters/absorbs radiation or nucleates ice; coatings can alter its activation. Solubility, absorptivity and ice-nucleating ability are distinct properties, not mutually equivalent categories.<The Influence of Pollution on the Shortwave Albedo of Clouds by S. Twomey>[Fixed-water cloud albedo response]{ref}
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