There are multiple categories of aerosols that are termed biological, so let me start with that. If you are referring to microbes, then for those that are known to be related genetically in terms of having the ice nucleation gene become active as warm as -3C, but mostly in the region from -5C and colder, fully expressing their activity by around -12 to -15C. To be clear, no one has proven the role of bacterial INPs as critical for ice formation in clouds (whether alone or as triggers for secondary ice formation). The numbers of microbial INPs in the atmosphere appear to be very low except in regions where plants or land are being strongly perturbed. Some people have inferred that other bacteria become active at lower temperatures, but no one has identified such other bacterial INPs or why they would possess this activity. The other biological ice nucleators are organic byproducts of the action of microbes in soils, and these may be cell-free proteins or hyphae of ice nucleating fungi, or macromolecules from pollen surfaces that are washed into soils, or other organic products formed in the soils (least known category). These various things often leave soils or oceans attached to other particles (dust, salt), and have a range of freezing temperatures from -5 to -30C even. Is there any better explanations?