"We are confident now that on a global scale, biological particle concentrations are too low to play a significant role in cloud ice formation, and that instead mineral dust is the major component," Corinna Hoose from the University of Oslo, Norway and Karlsruhe Institute of Technology, Germany told environmentalresearchweb. "This is in agreement with many measurements of ice crystal residuals."

Together with colleagues from Oslo and the Max Planck Institute for Chemistry, Germany, Hoose calculated that biological particles contributed a maximum of 0.6% to the global average ice nucleation rate. The team used the CAM–Oslo aerosol climate model and newly available estimates and laboratory measurements to come up with their results.

"Recently, the first field observations of ice nucleation active biological particles in clouds and precipitation have become available, but it was still unclear how representative the reported cases were," said Hoose. "At the same time, tools were developed and published which finally allowed [us] to do the first global simulations of biological ice nucleation: bacteria, fungal spore and pollen emission parameterizations, compilations of biological particle concentration measurements at various locations, and ice nucleation parameterizations based on laboratory data."

Ice nucleation in clouds is of particular interest as it is the first step in the creation of snow and most types of rain.

Previously, high concentrations of biological ice nuclei have been found in a wave cloud over Wyoming and in the Amazon basin, and the particles have been found to be ubiquitous in precipitation on different continents.

"[Our study] means that living or dead micro-organisms are less involved in cloud ice and precipitation formation than has been speculated, and that the observed cases with high biological ice nucleus concentrations are not representative of average atmospheric conditions," said Hoose.

Now the researchers plan to use regional and cloud-resolving, i.e. smaller-scale, models to investigate under which conditions biological particles can play a role on local scales and possibly act as a trigger for cloud glaciation. "We have to put a lot of effort into improving the simulated biological particle concentrations, especially their strong temporal and spatial variability," said Hoose. "Measurements with novel instruments will help us evaluate our models."

The researchers reported their work in ERL.