Submicron-size aerosol scavenging by electro-collection and ice nucleation

In this study, we investigated how submicron aerosol particles (APs) are scavenged by cloud droplets and raindrops, including the effects of ice nucleation on these particles. We used an original two-moment aerosol scheme, which contains the prognostic equations for the number and mass of APs in the air and in all types of hydrometeors. This scheme was integrated into a three-moment microphysics scheme within a three-dimensional, non-hydrostatic Advanced Regional Prediction System (ARPS) model (Vučković et al. 2022, 2023).

We analyzed the scavenging of APs through electroscavenging processes, which involve Coulomb interactions with raindrops and both Coulomb and image charge interactions with cloud droplets. We assumed a Boltzmann charge distribution of aerosols, while cloud droplets and raindrops can possess either positive or negative charges in an amount that depends on their surface area. The kernels calculated for discrete bins of APs and droplets/drops are incorporated into the bulk microphysical scheme.

Ice nucleation on the APs is also considered a scavenging mechanism. This approach allows us to analyse how these processes impact the number and mass of APs in the atmosphere, hydrometeors, and those washed out by precipitation. Our results indicate that electroscavenging is the dominant process for medium to large submicron APs, whereas Brownian diffusion primarily affects smaller particles. Nucleation scavenging caused by depositional nucleation on atmospheric particles (APs) is identified as the primary mechanism for nucleation scavenging involving APs. This process is especially significant for reducing mass, while it plays a lesser role in decreasing the number of APs in the atmosphere.

Electrostatic collection by cloud droplets and raindrops enhance the scavenging of APs regardless of their charge sign, and the presence of image charges on cloud droplets further increases this collection. Increasing the charge on hydrometeors correlates with a greater number and mass of aerosol particles removed. The sign of the charge is less significant: our findings show that even when droplets are uncharged, the collection efficiency is still high. Depositional nucleation scavenging is identified as the most important mechanism for reducing the mass of APs (Vučković et al. 2024).

This approach provides valuable insights into the redistribution of APs between the atmosphere, hydrometeors and precipitation. The results are applicable to issues related to air pollution, cloud modification, and climate modelling.

 

Acknowledgement: This research was supported by the Science Fund of the Republic of Serbia, No. 7389, Project Extreme weather events in Serbia - analysis, modelling and impacts” – EXTREMES

 

References

 

Vučković, V., D. Vujović, and A. Jovanović, 2022: Aerosol parameterisation in a three-moment microphysical scheme: Numerical simulation of submicron-sized aerosol scavenging. Atmos Res, 273, 106148, https://doi.org/10.1016/j.atmosres.2022.106148.

Vučković, V., D. Vujović, and D. Savić, 2023: Influence of electrostatic collection on scavenging of submicron-sized aerosols by cloud droplets and raindrops. Aerosol Science and Technology, 57, https://doi.org/10.1080/02786826.2023.2251551.

Vučković, V., D. Vujović, D. Savić, and L. Filipović, 2024: Impact of electro-collection and ice nucleation on aerosol scavenging. Aerosol Science and Technology, https://doi.org/10.1080/02786826.2024.2441289.