Simulating the impacts of pollen on cloud formation by heterogeneous ice nucleation

Primary biological aerosol particles (PBAPs) are emitted from Earth’s biosphere, including pollen, fungal spores, virus, bacteria, and plant debris. PBAPs are linked to adverse health effects and have the potential to influence ice nucleation at warmer temperatures. Anemophilous (or wind-driven) pollen is one type of PBAP, and the emitted pollen grains can rupture under high humidity to form smaller sub-pollen particles (SPP). Both pollen and SPP can be lifted to the upper troposphere under convective conditions, readily take up water and serve as cloud condensation nuclei (CCN) and ice nucleating particles (INPs), and therefore impact cloud formation and reflectivity. Although these biological aerosol have proven to be effective INPs in previous studies, they are typically not included in emission inventories. Therefore, it is difficult to quantify their effects on cloud formation and local climate.

Here, we include the emission and rupture of pollen in WRF-Chem simulations and investigate the impacts of pollen and SPP on both warm and ice clouds in the United States South Great Plains (SGP) from April 11-20, 2013, a period with high pollen emission and convective events. We update the Morrison microphysics scheme inside WRF-Chem using aerosol-aware INP parameterizations, considering different freezing mechanisms including heterogeneous freezing (immersion, contact, and deposition freezing) and homogeneous freezing. We further incorporate heterogeneous ice nucleation from pollen and SPP in the model to evaluate pollen effects on ice cloud formation. The corresponding pollen and SPP INP parameterizations are obtained by laboratory experiments that indicate pollen grains are more efficient INPs than SPP and could contribute to ice cloud formation. The model simulation results are evaluated using observational data from Atmospheric Radiation Measurement (ARM) SGP sites.  We conducted a suite of sensitivity tests to examine the impacts of pollen and SPP on one convective event (April 17-18, 2013), and compare the newly developed pollen and SPP INP parameterizations with those developed in previous literature. Our results highlight that the addition of PBAPs such as pollen could shift the convective event onset timing and vertical structure.