An aerosol is a dispersion of solid particles or liquid droplets in a gas. Fog droplets and smoke particles are examples; the dispersed phase is not itself a gas.
Particle size, composition, and number affect scattering, settling, and chemical interaction with the surroundings. A cloud of droplets can therefore have optical and reactive properties not predicted by the gas composition alone.
The dimensionless column optical depth from aerosol extinction measures attenuation by absorption and scattering at a stated wavelength. It depends on particle size, composition, humidity and vertical distribution; it is not a direct count of cloud condensation nuclei. Using it as a proxy for droplet-number forcing requires control of meteorological covariates and cloud-contaminated retrievals.
A particle count per stated volume and logarithmic size interval. Its integral over gives a number density, unlike an aerosol mass distribution. In a cloud-droplet activation estimate, integrating the dry-size distribution above the size-dependent Köhler theory threshold gives activated number for a specified peak atmospheric supersaturation and composition.
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.
Marine cloud brightening proposes increasing suitable cloud condensation nuclei over the ocean to increase reflected sunlight. At fixed cloud liquid-water content and thickness, the Twomey effect makes more, smaller droplets increase cloud albedo. Changes in cloud water, cloud fraction, entrainment and precipitation can reinforce or oppose this fixed-cloud response.
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.
An aerosol particle becomes a freely growing cloud droplet after crossing its Köhler theory growth barrier under the parcel's atmospheric supersaturation history. Counting particles whose critical supersaturation is below a specified peak gives an equilibrium-threshold estimate; calculating that peak requires cooling, water vapor depletion and condensational-growth kinetics. Neither total aerosol number nor dry size alone determines activation.
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 equilibrium saturation ratio over a chemical solution droplet as a function of wet radius for fixed solute content. In the dilute radius approximation , its maximum occurs at . The corresponding critical supersaturation is . Diameter and radius coefficients differ and must not be mixed.
The first-order combination of the Kelvin equation and Raoult's law for a chemical solution droplet with and . Its maximum is at , with . For ambient saturation ratio , the smaller equilibrium radius is stable and the larger is unstable: the sign of condensational growth is the sign of . In particular, exceeding alone need not activate continued growth at subcritical atmospheric supersaturation.
Writing the same curve using diameter changes its coefficients: and in . The critical saturation ratio is unchanged, while . Mixing a diameter curvature coefficient with a radius solute term gives a spurious activation threshold.
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.
In the dilute radius form , the curvature coefficient A has length units and B has volume units. Setting its derivative to zero gives the displayed maximum. Ambient supersaturation above it permits activation into a growing cloud droplet. Below it, a sufficiently small wet equilibrium can be stable, while the larger-radius intersection is unstable. The approximation should not be extrapolated into the dry core.
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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.