Cloud-resolving simulations of mercury scavenging and deposition in thunderstorms

U. S. Nair,G. Kallos,Y. Wu,J. T. Walters,C. D. Holmes,A. Ter Schure

doi:10.5194/acp-13-10143-2013

Abstract

Abstract. This study examines dynamical and microphysical features of convective clouds that affect mercury (Hg) wet scavenging and concentrations in rainfall. Using idealized numerical model simulations in the Regional Atmospheric Modeling System (RAMS), we diagnose vertical transport and scavenging of soluble Hg species – gaseous oxidized mercury (GOM) and particle-bound mercury (HgP), collectively Hg(II) – in thunderstorms under typical environmental conditions found in the Northeast and Southeast United States (US). Mercury scavenging efficiencies from various initial altitudes are diagnosed for a case study of a typical strong convective storm in the Southeast US. Assuming that soluble mercury concentrations are initially vertically uniform, the model results suggest that 60% of mercury deposited to the surface in rainwater originates from above the boundary layer (> 2 km). The free troposphere could supply a larger fraction of mercury wet deposition if GOM and HgP concentrations increase with altitude. We use radiosonde observations in the Northeast and Southeast to characterize three important environmental characteristics that influence thunderstorm morphology: convective available potential energy (CAPE), vertical shear (0–6 km) of horizontal wind (SHEAR) and precipitable water (PW). The Southeast US generally has lower SHEAR and higher CAPE and PW. We then use RAMS to test how PW and SHEAR impact mercury scavenging and deposition, while keeping the initial Hg(II) concentrations fixed in all experiments. We found that the mercury concentration in rainfall is sensitive to SHEAR with the nature of sensitivity differing depending upon the PW. Since CAPE and PW cannot be perturbed independently, we test their combined influence using an ensemble of thunderstorm simulations initialized with environmental conditions for the Northeast and Southeast US. These simulations, which begin with identical Hg(II) concentrations, predict higher mercury concentrations in rainfall from thunderstorms forming in the environmental conditions over the Southeast US compared to the Northeast US. A final simulation of a stratiform rain event produces lower mercury concentrations than in thunderstorms forming in environments typical of the Southeast US. The stratiform cloud scavenges mercury from the lowest ~ 4 km of the atmosphere, while thunderstorms scavenge up to ~ 10 km.

Highlights

Lakes, rivers and coastal waters throughout the United States contain mercury at levels that harm wildlife and people who consume fish from these waters (Liu et al, 2008; KarounaRenier et al, 2008; EPA, 2011)
Through analysis of radiosonde data, we identify atmospheric conditions – convective available potential energy, shear and precipitable water – that differ between the Northeast and Southeast United States
In the context of the physical process settings discussed above, the experimental design utilized in this study focuses on thunderstorm morphology and evolution based on a parameter space defined by three variables, namely convective available potential energy, vertical shear of horizontal wind and precipitable water

Summary

Introduction

Rivers and coastal waters throughout the United States contain mercury at levels that harm wildlife and people who consume fish from these waters (Liu et al, 2008; KarounaRenier et al, 2008; EPA, 2011). In the Eastern United States, wet deposition is largest over the Gulf Coast region (Fig. 1), during the summer months, coinciding with the Published by Copernicus Publications on behalf of the European Geosciences Union. Nair et al.: Cloud-resolving simulations of mercury scavenging and deposition in thunderstorms peak of convective storm activity. Rainwater samples from thunderstorms contain higher mercury concentrations than rain from non-convective or weakly convective storms (Holmes et al, 2010b; Holmes, 2013). Unlike other watersoluble anthropogenic pollutants, including sulfate and nitrate, the region of high mercury wet deposition does not overlap the peak emission region, which is much farther north (EPA, 2013)

Methods

Results

Discussion

Conclusion