Cloud-resolving simulations of mercury scavenging and deposition in thunderstorms

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.

Active and widespread halogen chemistry in the tropical and subtropical free troposphere

“…These reactions modify the Hg(II) source particularly in the lower FT (a factor of 20; SI Appendix and Fig. S10), where Hg(II) is more susceptible to dry (51) and wet deposition (52). Our study indicates that atmospheric mercury is a rather chemically dynamic component of air that warrants future study.…”

Deciphering potential chemical compounds of gaseous oxidized mercury in Florida, USA

“…For example, fluxes at Naval Air Station Pensacola Outlying Landing Field (OLF) in Florida were 17.1 µg m −2 in 2012 and 21.0 µg m −2 in 2014 (MDN, 2014). A contributing factor to wet deposition in the Gulf Coast area may be related to high atmospheric convection during thunderstorms and scavenging of gaseous oxidized Hg (GOM) from the free troposphere (Nair et al, 2013), as well as down-mixing of air with high GOM from the free troposphere .…”

Mercury oxidation from bromine chemistry in the free troposphere over the southeastern US

“…The high deposition levels experienced in the SE US cannot be explained by regional anthropogenic sources of mercury alone, which are mainly located within the Ohio River valley, where the prevailing winds carry emissions northeast. Additionally, several modeling studies Nair et al, 2013) have shown that meteorological patterns above the SE US greatly influence the wet deposition of mercury and that these processes are linked with deep convective activity. This indicates that a regional emission-deposition pattern is most likely not the major source-receptor relationship for mercury entering the environment over Florida, in the SE US, meaning that other possibilities, such as enhanced atmospheric oxidation followed by deposition, need to be explored.…”

“…The rapid oxidation of mercury in the lower FT is of potential relevance in the SE US, where there have been several studies linking deep convective activity to the elevated levels of mercury found in rainwater (Guentzel et al, 2001;Landing et al, 2010;Nair et al, 2013). The bioaccumulation of methyl mercury in fish tissues is particularly relevant in this region, where it has been deemed unsafe to eat fish harvested from many lakes in the region (Engle et al, 2008;Liu et al, 2008).…”