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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsIn one 2006 study at a non-urban site in Germany, aerosol particle size distribution was a stronger influence on measured cloud condensation nuclei (CCN) concentrations than particle chemistry. The result is specific to that study: it does not mean chemistry is irrelevant, or that size alone controls cloud formation.
What did the 2006 study find?
Dusek and colleagues measured size-resolved CCN spectra for different aerosol types at a non-urban German site. They found that the aerosol number size distribution was the main determinant of CCN concentrations, while chemical composition produced distinct but secondary variation in particle activation. In their analysis, size distribution alone explained 84–96% of the observed variation in CCN concentrations when temporal variation in chemical effects was neglected. That percentage describes the study’s data and analytical condition, not a universal share of cloud formation. Read the 2006 study in Science.
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The original report’s conclusion was comparative: size had the stronger influence in that setting, but composition still mattered. Katharine Sanderson’s 2006 Chemistry World report covered the finding.
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How do aerosols form cloud droplets?
Cloud condensation nuclei are aerosol particles that provide surfaces on which water vapor can condense and grow into cloud droplets. A particle activates when conditions allow it to grow past a critical size. Its size is important, but activation also depends on its chemical composition and hygroscopicity (how readily it takes up water), surface tension, how its components are mixed, and the supersaturation of water vapor in the surrounding air.
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Size and chemistry work together
Köhler theory describes how the curvature of a small droplet and the dissolved material within it together affect the conditions needed for continued growth. Particle size can strongly shape the likelihood of activation; soluble, water-attracting material can also change that likelihood. A 2019 review of aerosol mixing state and CCN activity explains why size and composition must be considered together. See the review in Reviews of Geophysics.
Updrafts affect the conditions for activation
Air rising into a cloud cools and can become supersaturated with water vapor. Updraft velocity influences the supersaturation particles experience, and therefore which particles activate. A review of aerosol-cloud interactions notes that droplet concentrations in nascent warm clouds are governed largely by the sizes of aerosols that activate and the updraft velocity carrying them to activation altitude. Modern parameterizations also account for particle size distribution and composition. Read the PNAS review.
Why surface behavior complicates a simple size-versus-chemistry answer
A separate 2016 Berkeley Lab experiment examined how organic molecules at the water interface can affect droplet formation. Using dicarboxylic acids and ammonium sulfate in custom-built equipment, researchers found that interfacial organic molecules could lower surface tension. In that experimental system, measured droplets were reported as 50–60% larger than predictions from the tested standard models based on how readily particles dissolve. This is a result for that laboratory setup, not a general correction factor for cloud droplets. Berkeley Lab’s account of the experiment quotes study senior author Kevin Wilson: “Accurately describing the connection between the chemistry of aerosol particles and the formation of cloud droplets remains difficult, and it is a key challenge for models to correctly predict climate.”
What does this mean for clouds and climate?
Droplet size can affect how brightly a cloud reflects sunlight: smaller, more numerous droplets scatter more sunlight, potentially cooling Earth’s surface. But the 2006 CCN result does not by itself quantify climate cooling or warming. Cloud lifetime, precipitation, atmospheric dynamics, and other cloud-scale properties also shape the net climate response. The findings help explain variables that climate models need to represent; they do not establish a single, universal effect of particle size on climate.
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