Bacteria and fungal spores enter the atmosphere from plant surfaces, soils, and other sources, and can be transported through the clouds to even greater heights. This study measured these biological particles at altitudes up to 10 km, and investigated their ability to help freeze cloud droplets.
In some locations, biological particles can contribute a significant amount to the total aerosol mass in the atmosphere, so including them in models could improve the ability of the models to accurately simulate total amount of particulates present. Additionally, while most naturally-occurring biological particles are harmless to humans, some have impacts on human health by triggering allergies, and certain biological particles may increase cloud droplet freezing at warmer temperatures, which can influence precipitation and how the cloud develops.
A research team, including Department of Energy scientists at Pacific Northwest National Laboratory, directed five aircraft flights at altitudes up to 10 km and higher in September and October, and measured airborne particle concentrations, mostly over agricultural regions in Colorado, Nebraska, South Dakota and Wyoming. Researchers analyzed the captured particles to determine if they fluoresced (glowed brightly) when exposed to a laser, a method used to reveal naturally-occurring biological particles, such as certain bacteria, fungal spores, or pollen. Researchers from Pacific Northwest National Laboratory (PNNL) helped create a custom inlet system for the aircraft, and also contributed to the design, setup, and interpretation of model simulations of naturally occurring biological particle concentrations. By comparing the model’s particle concentration simulations at different altitudes with the onboard aircraft measurements, research team performed the first test of such models’ ability to reproduce the concentrations of biological particles at different altitudes throughout the lower atmosphere, including at altitudes where cloud formation occurs. They found that while the model was generally consistent with the observations, it usually predicted lower biological particle concentrations, typically by an order of magnitude. The researchers offer this could be due to a number of factors, including errors in modeled particle emissions, transport, or removal processes; missing particle types in the model, such as leaf litter or arable soils; or the strong spatial and seasonal variability of particle emissions, as the observations were made over cropland during harvest season. The researchers also found that the particle concentration generally decreased as altitude increased, but had especially high variability in the range of temperatures in which the particles could influence cloud formation.