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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.

Cloud albedo effect: soluble, hygroscopic aerosol readily takes up water and can activate as 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.

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.^[ref]

First, weakly absorbing sulfate scatters shortwave electromagnetic radiation and tends to cool the surface/planetary energy budget, with magnitude and sign influenced by cloud background and particle optics. Second, sulfate can supply cloud condensation nuclei: at fixed cloud water, more activated droplets give smaller droplets and greater cloud albedo through the Twomey effect; atmospheric precipitation and water-content adjustments can modify that idealized cooling. Third, sulfate particles change atmospheric composition through uptake, aqueous chemistry and heterogeneous surface reactions, with their acidity and available reactants controlling what is converted. Particle surfaces alone do not guarantee polar-style stratospheric chlorine activation in an ordinary tropospheric cloud.^[ref]

For a liquid cloud at fixed water content and geometrical thickness, more cloud condensation nuclei can produce more, smaller droplets. The increased total extinction area raises optical depth and reflected sunlight: the Twomey effect gives negative shortwave radiative forcing. It is an indirect aerosol effect mediated by cloud microphysics, rather than direct aerosol scattering.^[ref]

Smaller droplets can also delay collision-coalescence and drizzle, potentially increasing liquid-water retention, cloud lifetime or coverage; that adjustment would usually strengthen cooling for low reflective clouds. It is not universal: evaporation and mixing with dry air can reduce water content or cloud cover, and dynamical changes can oppose the simple lifetime argument. Absorbing particles can heat an atmospheric layer, sometimes evaporating cloud and producing a positive semi-direct forcing, but their vertical position can instead stabilize a cloud layer and reverse the response. Aerosols can additionally alter ice nucleation and mixed-phase precipitation, with no single forcing sign independent of cloud type. The net response combines shortwave reflection and longwave thermal effects, and depends on cloud height, optical depth and adjustments. Thus the fixed-water Twomey sign is robust within its assumptions; adjustment and ice-cloud signs require context.^[ref]

Increasing cloud droplet number at fixed liquid-water content and cloud thickness reduces mean droplet size and increases total extinction area, raising shortwave optical depth and cloud albedo. This idealized response produces negative shortwave radiative forcing. Changes in cloud amount, water content and dynamics are separate adjustments.^[ref]