Parameterizing bioaerosols and their ability for ice nucleation

Bioaerosols and cloud formation

Biological aerosols, such as fungi, bacteria, algae, and pollen, are present everywhere in the atmosphere. But in different locations and under different atmospheric conditions, the types of bioaerosols and their concentration vary significantly. In clouds, bioaerosols can act as cloud condensation nuclei as well as ice nuclei and influence the hydrological cycle and climate.

However, there has been much uncertainty about the potential of various bioaerosol groups to facilitate precipitation through ice nucleation. Each bioaerosol group differs significantly in its ice nucleation activity. Currently, most of the information about the ice nucleation activity of bioaerosols is based on laboratory studies which may not represent the real atmosphere.

This is where Sachin Patade, Vaughan Phillips, and their colleagues come in. The purpose of the mission was to resolve the ice nucleation activity of the major bioaerosol groups by collecting samples directly from the atmosphere.  A bioaerosol-rich atmosphere like the Amazon Rainforest makes the ATTO site an ideal place for their research.

Sachin Patade on the 80 meter tower collecting samples.
Sachin Patade on the 80 meter tower collecting samples. © Sachin Patade / Uni Lund

Empirical parameterization

The scientists collected bioaerosol samples at ATTO and analyzed them under Scanning Electron Microscope (SEM). From high-resolution SEM images of bioaerosols, they collected information about their types, sizes, and number concentrations. They also processed the collected bioaerosol samples in cloud chambers to estimate their ice nucleation activity. With this information, the team formulated an empirical parameterization where the ice nucleation activity of individual bioaerosol groups is a function of their total surface area present in the sampled air. Based on their analysis, the team formulated an  empirical parameterization for five bioaerosol groups:

  • Fungal spores and their fragments
  • Bacteria and their fragments
  • Pollen and their fragments
  • Detritus of plants, animals and viruses
  • Algae
SEM images of different types of bioaerosols. @Sachin Patade / Uni Lund
SEM images of different types of bioaerosols. Top left to bottom right they are: a bacteria, an insect egg, grashopper secretion, spore of penicillium fungus, plant wax, and a fungal spore.
@Sachin Patade / Uni Lund

This new parameterization now makes it possible to resolve the ice nucleation activity of each bioaerosol group listed above in the numerical simulation of clouds. By implementing the proposed parameterization in cloud models, it is possible to investigate the role of each bioaerosol group on cloud properties and precipitation.

Patade et al. published their study Empirical formulation for multiple groups of primary biological ice nucleating particles from field observations over Amazonia in the Journal of the Atmospheric Sciences.

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