Scavenging of Sub-Micron to Micron-Sized Microbial Aerosols during Simulated Rainfall

Rachel A Moore,Brent C Christner,David G Schmale,Craig Powers,Regina Hanlon

doi:10.3390/atmos11010080

Abstract

The processes removing aerosols from the atmosphere during rainfall are generically referred to as scavenging. Scavenging influences aerosol distributions in the atmosphere, with consequent effects on cloud properties, radiative forcing, and human health. In this study, we investigated the below-cloud scavenging process, specifically focusing on the scavenging of 0.2 to 2 µm-sized microbial aerosols by populations of water drops with average diameters of 3.0 and 3.6 mm. Rainfall was simulated in convective boundary layer air masses by dispensing the water drops from a 55 m bridge and collecting them at ground level. Particles and microbial cells scavenged by the water drops were visualized, enumerated, and sized using scanning electron and epifluorescence microscopy. Aerosolized particles and DNA-containing microbial cells of 2 µm diameter were scavenged at efficiencies similar to those reported previously in empirical studies; however, the efficiencies derived for smaller aerosols were significantly higher (one to three orders of magnitude) than those predicted by microphysical modeling. Application of the derived scavenging efficiencies to cell data from rainfall implies that, on average, approximately 50 to 70% of the 1 µm microbial cells in the precipitation originated from within the cloud. Further study of submicron to micron-sized aerosol scavenging over a broader raindrop size distribution would improve fundamental understanding of the scavenging process and the capacity to estimate (bio)aerosol abundances in the source cloud through analysis of rainfall.

Highlights

Particles aerosolized from natural and anthropogenic sources are transported horizontally and vertically in the atmosphere, where they can have direct effects on the formation of clouds and precipitation by serving as cloud condensation and ice nuclei, respectively [1]
This study examined the efficiency at which two water drop sizes captured 0.2 to 2 μm aerosols while descending through the atmosphere, with the specific objective of deriving data relevant for understanding wet deposition processes of microorganisms and sized particles
We did not determine the composition of microorganisms or particles present in the air masses sampled, our results (Figure 4) indicate that microbial aerosol composition was less important than their equivalent diameter

Summary

Introduction

Particles aerosolized from natural and anthropogenic sources are transported horizontally and vertically in the atmosphere, where they can have direct effects on the formation of clouds and precipitation by serving as cloud condensation and ice nuclei, respectively [1]. In-cloud water droplets that form around aerosols (i.e., in the wet phase) are deposited in precipitation together with aerosols that are scavenged by rain drops as they descend through the atmosphere. The scavenging of aerosols by rain drops is the major process of particle deposition from the troposphere [2]. Knowledge of wet deposition processes is vital for understanding its consequences on global distributions and concentrations of aerosols in the atmosphere. Multiple mechanisms are involved in aerosol scavenging by rain drops, including inertial impaction, Brownian diffusion, and interception [1,3].

Methods

Discussion

Conclusion