The influence of aerosols and clouds represent one of the largest known uncertainties in the scientific understanding of trends in our past global climate - and also hinder predictions of future climate change.

Researchers have now shown that the rate of condensation of water on organic aerosol particles in the atmosphere can be very slow, taking many hours for a particle to change in size. This could have significant consequences for understanding how clouds are formed, affecting climate.

Aerosols are small particles less than 1 micrometer in diameter and clouds are liquid droplets of 1–1,000 micrometers in diameter. 

One of the most significant 'known unknowns' is how quickly water can condense on the small aerosol particles to grow and become cloud droplets, influencing the albedo (reflectivity) of clouds and cloud lifetime (precipitation).

In a study published today, Professor Jonathan Reid of the University of Bristol and colleagues show that the rate of cloud droplet growth can be strongly dependent on the composition of the aerosol. For aerosol particles that have high viscosity, equivalent to saying they behave like treacle or even bitumen, water evaporation and condensation can be very slow, taking many hours. 

For particles that are much less viscous (more like olive oil or even water), evaporation and condensation can be very fast: less than 1 second.

Professor Reid says, "Although not providing all the answers, this work helps us better understand the 'known unknowns'. Most importantly, it demonstrates that better understanding the rate at which water condenses on particles in the atmosphere is crucial for understanding clouds."

‘Comparing the mechanism of water condensation and evaporation in glassy aerosol’ by David Bones, Jonathan Reid, Daniel Lienhard and Ulrich Krieger is published in Proceedings of the National Academy of Sciences