Grantee Research Project Results
Final Report: Dynamic Exchange of Volatile Species and Semivolatile Organic Contaminants Across the Air-Water Interface of the Chesapeake Bay
EPA Grant Number: R825245Title: Dynamic Exchange of Volatile Species and Semivolatile Organic Contaminants Across the Air-Water Interface of the Chesapeake Bay
Investigators:
Institution: University of Maryland Center for Environmental Science , Chesapeake Biological Laboratory
EPA Project Officer: Chung, Serena
Project Period: October 1, 1996 through September 30, 1999
Project Amount: $449,989
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
Atmospheric deposition is an important source of organic contaminants and mercury to surface waters, including the Great Lakes and the Chesapeake Bay. The 1990 Clean Air Act Amendments require the U.S. Environmental Protection Agency (EPA) to assess the relative importance of the atmosphere as a source of contaminants to these "Great Waters." Gas exchange of volatile pollutants across the air-water interface is an important but relatively poorly understood component of atmospheric deposition, especially near coastal urban areas. For predictive models of the fate and transport of organic contaminants and mercury in the Chesapeake Bay to be accurate, the inputs of these pollutants need to be accurately known. However, the flux is not unidirectional as both semivolatile organics and Hg, as Hg? , can be lost from the water to the atmosphere via gas exchange, or vice versa. Thus, quantifying only the depositional flux is insufficient.This study examined the short-term and long-term variability of gas exchange of these contaminants as our preliminary results suggested that both the magnitude and the direction of this flux can vary dramatically, both spatially and temporally. Therefore, to quantify the net input of contaminants from the atmosphere to the Chesapeake Bay, the magnitudes of the gas exchange processes need to be accurately assessed. Additionally, Hg may exist in the atmosphere as gaseous Hg(II) species and these highly soluble compounds would partition from the atmosphere into surface waters. However, little is known about the concentration of these species in the atmosphere and in this study we addressed this aspect of the air-water exchange of Hg as well. Furthermore, we studied the gas exchange dynamics of semivolatile organics and Hg in Baltimore as the urban environment is a significant source of these contaminants to the Bay, and gas exchange fluxes are likely an important transfer mechanism. The primary objective of this study was to determine the factors controlling the deposition to and emission of mercury and semivolatile organic contaminants from the waters of the Chesapeake Bay to the atmosphere. Specifically, the study involved: (1) quantifying the flux of these contaminants between the water of the Chesapeake Bay and the atmosphere due the exchange of gaseous species; (2) determining the importance of this gas emission and/or deposition flux in relation to wet and particulate deposition of mercury and semivolatile organics to the Chesapeake Bay; (3) assessing the magnitude of short-term and seasonal variability in gas exchange; (4) determining the fractionation and distribution of mercury between large and submicron particulate fractions and the gas phase, including an investigation of the presence of gaseous ionic mercury species; and (5) assessing the importance of the Baltimore urban area in contributing mercury and semivolatile organic contaminants to the Chesapeake Bay.
Summary/Accomplishments (Outputs/Outcomes):
The findings of the project include:1. Temperature-Dependence of the Henry's Law Constants of 13 Polycyclic Aromatic Hydrocarbons (PAHs) Between 4? and 31?C. An understanding of the temperature dependence of the Henry's Law constant for organic contaminants is critical when modeling the transport and fate of these contaminants in the environment. The Henry's law constants for 13 PAHs were experimentally determined between 4? Cand 31?Cusing a gas-stripping apparatus. The Henry's law constants ranged between 0.02 Pa m3/mol ? 0.01 Pa m3/mol for chrysene at 4?C and 73.3 Pa m3/mol ? 20 Pa m3/mol for 2-methylnaphthalene at 31?C. The temperature dependence of each PAH was modeled using the van't Hoff equation to calculate the enthalpy and entropy of phase change. For nine of the PAHs, the present study reports the first experimental measured temperature dependence of their Henry's law constant. The enthalpies of phase change ranged between 35.4 kJ/mol ? 1.9 kJ/mol for 1-methylphenanthrene and 100 kJ/mol ? 8 kJ/mol for chrysene. These data can be used to extrapolate Henry's law values within the experimental temperature ranges. For all PAHs except benzo[a]fluorene, the temperature dependence can be predicted within a relative standard error < 10 percent.
2. Air-Water Exchange of PAHs Across the Air-Water Interface of the Northern Chesapeake Bay. Air-water exchange fluxes of 13 PAHs were determined along a transect in the Patapsco River from the Inner Harbor of Baltimore, MD, to the mainstem of the northern Chesapeake Bay. Sampling took place at six sites during three sampling intensives (June 1996, February 1997, and July 1997) and at one site every ninth day between March 1997 to March 1998 to measure spatial, daily and annual variability in the fluxes. The direction and magnitude of the daily fluxes of individual PAHs were strongly influenced by the wind speed and direction, by the air temperature and by the highly variable PAH concentrations in the gas and dissolved phases. Individual fluxes ranged from 14,200 ng/m3-day net volatilization of fluorene during high winds to 11,400 ng/m3-day net absorption of phenanthrene when prevailing winds blowing from the northwest across the city of Baltimore elevated gaseous PAH concentrations over the water. The largest PAH volatilization fluxes occurred adjacent to the storm water discharges, driven by elevated dissolved PAH concentrations in surface waters. Estimated annual volatilization fluxes ranged from 1.1 g/m2-yr for chrysene to 800 g/m2-yr for fluorene.
3. Examination of Methods for the Measurement of Reactive Gaseous Mercury in the Atmosphere. Reactive gaseous Hg (RGHg), usually assumed to be HgCl2, may dominate the total Hg depositional flux due to its higher surface reactivity and water solubility. Three methods are currently used for RGHg sampling: multi-stage filter packs, refluxing mist chambers, and KCl-coated denuders, but none of these methods are considered standard. Field comparisons were performed at Chesapeake Biological Laboratory (CBL) to test if these methods could give comparable results. Mist chambers and denuders were operated continuously for 24 h in some cases to observe the diurnal variation. All methods demonstrated the dynamic fluctuation of atmospheric RGHg, ranging from a few picograms per cubic meter to more than 500 pg/m3. These methods also reported similar temporal RGHg trends. At low RGHg levels, the denuder tended to report higher values of RGHg relative to the filter pack, while mist chamber values were generally in agreement with the filter pack. Discrepancy among methods was more significant under higher RGHg levels. Considering the uncertainties associated with these methods, our data suggest that these methods did produce comparable results. The 24 hour continuous measurements showed that RGHg was usually undetectable at night. However, our data also suggest factors in addition to photochemistry, such as movement and mixing of air masses, are influencing the distribution of RGHg at CBL.
4. Exchange of Gaseous Mercury Across the Air-Water Interface of the Northern Chesapeake Bay. Samples collected from both urban and rural sites around the Chesapeake Bay demonstrate the importance of urban areas as a source of mercury (Hg) to nearby waters via both wet and dry deposition. Concentrations of Hg in wet deposition, atmospheric particulate Hg and reactive gaseous Hg are about twice those of rural sites in the region. Fluxes to the water surface are estimated based on measured concentrations and estimated deposition velocities for dry deposition, and from actual rainfall amounts for wet deposition. In addition to the impact of urban sources, dry deposition of reactive gaseous Hg, given our measurements and assumptions concerning mercury speciation in the atmosphere, is an important flux to the Chesapeake Bay, and likely to other coastal waters impacted by man's activities. The net flux to Baltimore Harbor is two to three times that at the mouth of the Patuxent River. Gas exchange of elemental mercury is an important loss for coastal waters, and, in the Chesapeake Bay, is about 15-30 percent of the net atmospheric flux. Gas exchange is relatively less important in impacted environments such as Baltimore Harbor. Overall, there is net deposition to the Chesapeake Bay from the atmosphere. Both intensive studies and long-term measurements show that the concentration of dissolved gaseous Hg (DGHg) is relatively low (typically <0.2 pM) and DGHg is a small fraction of the total Hg. Baltimore's urban air is an important source of mercury (Hg) to the northern Chesapeake Bay. Elevated atmospheric Hg concentrations were detected at a city sampling site at the Science Center (4.4?2.7 ng/m3), as compared to a rural site (Stillpond; 1.7?0.5 ng/m3). The annual dry depositional fluxes of reactive gaseous Hg (RGHg) and particulate-bound Hg (Hg-P) at sites around the northern Chesapeake Bay have been determined, with the fluxes of RGHg ranged from 7 to 121 mg/m2 and the fluxes of Hg-P ranged from 1 to 34 mg/m2. These numbers were the same magnitude as the wet depositional fluxes of Hg measured at the same sites. Local wind direction influenced the concentration of atmospheric Hg detected at the city sampling site. When air came from the SE, S and SW directions, the city sampling site tended to be impacted by the local emission sources, with higher total gaseous Hg (TGHg) and Hg-P concentrations detected.
Journal Articles on this Report : 10 Displayed | Download in RIS Format
Other project views: | All 27 publications | 11 publications in selected types | All 10 journal articles |
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Bamford HA, Offenberg JH, Larsen RK, Ko F-C, Baker JE. Diffusive exchange of polycyclic aromatic hydrocarbons across the air-water interface of the Patapsco River, an urbanized subestuary of the Chesapeake Bay. Environmental Science & Technology 1999;33(13):2138-2144. |
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Bamford HA, Poster DL, Baker JE. Method for measuring the temperature dependence of the Henry's Law Constant of selected polycyclic aromatic hydrocarbons. Polycyclic Aromatic Compounds 1999;14:11-22. |
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Bamford HA, Poster DL, Baker JE. Temperature dependence of Henry's law constants of thirteen polycyclic aromatic hydrocarbons between 4°C and 31°C. Environmental Toxicology and Chemistry 1999;18(9):1905-1912. |
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Bamford HA, Poster DL, Baker JE. Henry's law constants of polychlorinated biphenyl congeners and their variation with temperature. Journal of Chemical and Engineering Data 2000;45(6):1069-1074. |
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Harman-Fetcho JA, McConnell LL, Rice CP, Baker JE. Wet deposition and air-water gas exchange of currently used pesticides to a subestuary of the Chesapeake Bay. Environmental Science & Technology 2000;34(8):1462-1468. |
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Mason RP, Lawson NM, Lawrence AL, Leaner JJ, Lee JG, Sheu G-R. Mercury in the Chesapeake Bay. Marine Chemistry 1999;65(1-2):77-96. |
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Mason RP, Lawson NM, Sheu GR. Annual and seasonal trends in mercury deposition in Maryland. Atmospheric Environment 2000;34(11):1691-1701. |
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Nelson ED, McConnell LL, Baker JE. Diffusive exchange of gaseous polycyclic aromatic hydrocarbons and polychlorinated biphenyls across the air-water interface of the Chesapeake Bay. Environmental Science and Technology 1998;32(7):912-919. |
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Offenberg JH, Baker JE. Influence of Baltimore's urban atmosphere on organic contaminants over the northern Chesapeake Bay. Journal of the & Air Waste Management Association 1999;49(8):959-965. |
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Sheu GR, Mason RP. An examination of methods for the measurements of reactive gaseous mercury in the atmosphere. Environmental Science and Technology 2001;35(6):1209-1216. |
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Supplemental Keywords:
Henry's law constants, estuary, PAHs, PCBs, volatilization, mass balance, environmental chemistry, Midatlantic, Chesapeake Bay., RFA, Scientific Discipline, Air, Water, Geographic Area, Water & Watershed, Hydrology, air toxics, Environmental Chemistry, State, Chemistry, Air Deposition, Watersheds, Mercury, atmospheric processes, aquatic, fate and transport, exposure and effects, semivolatile organic contaminants, gas exchange, emissions, surface water, predictive model, Maryland (MD), aquatic ecosystems, air-water interface, mercury content, organic contaminants, atmospheric deposition, coastal, heavy metals, mercury vaporProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.