Growth by collision and attachment of existing particles. Coagulation reduces particle number while conserving particulate material in a closed system without other sources or sinks. Larger particles can have a different scattering-per-mass efficiency and faster sedimentation, so increasing precursor injection need not increase albedo proportionally.
Atmospheric particles containing sulfate, commonly in sulfuric-acid/water or neutralized-salt mixtures. Particle size, water uptake, acidity and accompanying material determine scattering, absorption, cloud activation and heterogeneous chemistry; sulfate mass alone does not specify radiative forcing.
A particle's tendency to take up water vapor, which depends on composition, solubility and ambient humidity. In a common parameterization, a dimensionless summarizes soluble-water uptake for use with dry diameter in Köhler theory. A single composition parameter does not remove the importance of particle size or parcel supersaturation.
A change in cloud microphysics or cloud properties caused by aerosol, including droplet activation, precipitation changes and adjustments of water content or coverage. The Twomey effect isolates a fixed-water-content optical response; it does not by itself establish the sign or magnitude of every cloud adjustment.
Cloud changes caused by aerosol absorption of radiation and the resulting heating. Heating can evaporate clouds and reduce shortwave reflection, but the effect can reverse when heating changes stability and cloud maintenance differently. The forcing sign depends on the relative positions of cloud and absorbing aerosol and on dynamical adjustment.
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]
The Influence of Pollution on the Shortwave Albedo of Clouds by S. Twomey. Fixed-water-content droplet-number and albedo response
For at fixed liquid-water content and thickness, and , the derivative is . For a fixed fractional number increase, the initial response is greatest at . The exact finite-change expression is with . Uncertain activation of additional particles and changes in water content limit direct application to real clouds.
An aerosol particle able to activate into a growing cloud droplet at the supersaturation attained by a cloud parcel. Activity depends on particle dry size, hygroscopicity and interfacial properties. The number active at a specified supersaturation differs from total aerosol number.
The equilibrium water-vapor saturation ratio above a solution droplet combines a curvature effect, which raises vapor pressure, with dissolved-solute reduction of water activity. Its maximum defines a critical supersaturation for activation. In a dilute hygroscopicity approximation using droplet diameter and dry diameter , , where . Then . These approximate forms assume the stated diameter convention and dilute solution; composition and surface tension must be consistent.[ref]
A single parameter representation of hygroscopic growth and cloud condensation nucleus activity by M. D. Petters and S. M. Kreidenweis. Equations describing hygroscopic growth and critical activation supersaturation
The maximum equilibrium supersaturation along a solution-droplet growth curve. Above this threshold, an aerosol particle can grow into an activated cloud droplet under the conditions of the parcel. In Köhler theory, larger dry diameter or greater hygroscopicity generally lowers the threshold; aerosol number alone does not specify it.
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An aerosol is a suspension of fine solid particles or liquid droplets in a gas. The gas is typically air, and the particles can be composed of various materials, including water, chemicals, or biological substances. Aerosols can be naturally occurring, such as fog, smoke, and dust, or they can be manmade, resulting from activities like spray painting, the use of aerosol cans, or industrial processes.