Atmospheric wet deposition of mercury and other trace elements in Pensacola, Florida

Abstract

Abstract.In an effort to understand and quantify the impact of local, regional, and far-distant atmospheric mercury sources to rainfall mercury deposition in the Pensacola, Florida watershed, a program of event-based rainfall sampling was started in late 2004. Modified Aerochem-Metrics wet/dry rainfall samplers were deployed at three sites in the region around the Crist coal-fired power plant and eventbased samples were collected continuously for three years. Samples were analyzed for total Hg and a suite of trace elements including Al, As, Ba, Bi, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, La, Li, Mg, Mn, Na, Nb, Ni, P, Pb, Sb, Se, Si, Sn, Sr, Th, U, V, and Zn. Nutrients (ammonia and nitrate) and major anions (chloride and sulfate) were also measured on each sample. Multivariate statistical methods were used to sort these tracers into factors that represent potential source categories contributing to the rainfall chemistry. As, Hg, Sb, Se, Sn, and non sea-salt sulfate were all significantly correlated (R>0.6) with one factor which we interpret as an anthropogenic source term reflecting input from coal combustion throughout the southeastern US. Using ratios of total Hg to volatile elements, we estimate that 22-33% of the rainfall Hg results from coal combustion in the southeastern US with the majority coming from the global background.

Cloud-resolving simulations of mercury scavenging and deposition in thunderstorms

“…Of these, cloud top entrainment is more effective and is driven by fluid shear instabilities that engulf environmental air along the cloud edge caused by horizontal variations in updraft strength. Subsequent evaporation of cloud droplets within engulfed air leads to downdrafts that penetrate and mix environmental air over depths of 1-2 km (Knupp and Cotton, 1985). Larger-scale systematic lateral entrainment, under conditions without environmental shear, occurs due to increasing vertical velocity with height and associated lateral flow driven by mass continuity requirements.…”

“…Small-scale turbulent and larger-scale cloud entrainment (Fig. 2) also incorporates free tropospheric air into thunderstorms (Knupp and Cotton, 1985). Further, there are two forms of small-scale turbulent entrainment: lateral and cloud top entrainment.…”

“…2. Schematic of entrainment processes in convective clouds (Adapted from Knupp and Cotton, 1985). Turbulent eddies along the lateral and top boundaries of the cloud entrain environmental air into the cloud.…”

“…The enhancement might be due to the large volumes of boundary layer air that are sucked into the convective updraft, where scavenging can occur (Dvonch et al, 1998(Dvonch et al, , 2005White et al, 2009). Alternatively, deep convective thunderstorms may scavenge from a high-altitude reservoir of soluble mercury that is inaccessible to weak or non-convective storms (Guentzel et al, 2001;Selin and Jacob, 2008;Landing et al, 2010). Soluble mercury species (Hg(II)) consist of gaseous oxidized mercury (GOM) and particle-bound mercury (HgP), both of which can be scavenged by cloud water and precipitation.…”

Atmospheric deposition of mercury and major ions to the Pensacola (Florida) watershed: spatial, seasonal, and inter-annual variability

“…However, in order to improve water quality at the watershed level, the importance of local sources in addition to regional sources needs to be determined. Urban areas frequently have higher mercury deposition than rural areas (Mason et al, 1997;Keeler and Dvonch, 2005;Keeler et al, 2006;Van Arsdale et al, 2007), particularly areas downwind of coal-fired utilities (White et al, 2009). In addition, sampling individual rain events provides a more detailed assessment of rain composition that can be used for source tracking.…”