Little Fluffy Clouds: Aggregate Aerosols in the virga Cloud Code, Comparison to Microphysical Models, and Application to sub-Neptune Atmospheres

We introduce a new module with the open-source virga cloud modeling code to treat fractal aggregate aerosol particles. Previously, virga assumed spherical aerosol particles, while substantial evidence from the Solar System suggests that fractal particles are common. Following recent advances made to extend microphysical models to non-spherical particles, we implement a simple parametrization for dynamical and optical effects of fractal particles within the existing virga framework. We then perform a case study of the well-characterized mini-Neptunes GJ 1214 b, K2-18 b, and GJ 436 b to demonstrate how our new fractal treatment affects theoretical spectra of these worlds and how these compare to existing observations, as well as how our method compares to previous fractal aggregate particle treatments as applied to these worlds. We also provide testable predictions for future JWST observations to better characterize and understand potentially hazy or cloudy planetary atmospheres.