Abstract
Abstract. Air-water exchange fluxes of persistent, bioaccumulative and toxic (PBT) substances are frequently estimated using the Whitman two-film (W2F) method, but micrometeorological flux measurements of these compounds over water are rarely attempted. We measured air-water exchange fluxes of hexachlorobenzene (HCB) and polychlorinated biphenyls (PCBs) on 14 July 2006 in Lake Superior using the modified Bowen ratio (MBR) method. Measured fluxes were compared to estimates using the W2F method, and to estimates from an Internal Boundary Layer Transport and Exchange (IBLTE) model that implements the NOAA COARE bulk flux algorithm and gas transfer model. We reveal an inaccuracy in the estimate of water vapor transfer velocity that is commonly used with the W2F method for PBT flux estimation, and demonstrate the effect of use of an improved estimation method. Flux measurements were conducted at three stations with increasing fetch in offshore flow (15, 30, and 60 km) in southeastern Lake Superior. This sampling strategy enabled comparison of measured and predicted flux, as well as modification in near-surface atmospheric concentration with fetch, using the IBLTE model. Fluxes estimated using the W2F model were compared to fluxes measured by MBR. In five of seven cases in which the MBR flux was significantly greater than zero, concentration increased with fetch at 1-m height, which is qualitatively consistent with the measured volatilization flux. As far as we are aware, these are the first reported ship-based micrometeorological air-water exchange flux measurements of PCBs.
Highlights
Contamination of ecosystems by atmospheric deposition of persisent, bioaccumulative and toxic (PBT) substances is a continuing concern for human and ecosystem health
Gaseous concentration of PBTs was measured using lowflow (13 L min−1) denuder samplers consisting of 289 sections of capillary gas chromatograph (GC) columns packed into a deactivated stainless steel tube
A transect measurement approach that incorporates modified Bowen ratio flux measurements collected at stations with increasing fetch in offshore flow was combined with an internal boundary layer transport exchange model to provide an opportunity for critical evaluation of measured and estimated fluxes by quantitative comparison of measured and modeled www.atmos-chem-phys.net/12/4607/2012/
Summary
Contamination of ecosystems by atmospheric deposition of persisent, bioaccumulative and toxic (PBT) substances is a continuing concern for human and ecosystem health. A. Perlinger: Semivolatile organic PBT compound air-water gas exchange measurements, temperature-dependent Henry’s law constant, and mass transfer velocity parameterizations. Uncertainties in MBR flux measurements are dominated by the chemical concentration measurement precision, and depend on atmospheric variables (Rowe et al, 2011b), but knowledge of Henry’s law constant and surface mass transfer parameterizations is not required. The constant flux layer may occur in the atmospheric surface layer below the height of measurement, in particular under stable atmospheric conditions, which frequently occur in the Laurentian Great Lakes when warm air is advected over cold water (e.g., Rowe et al, 2011a). The COARE gas transfer model is applied within the IBLTE model to estimate the modification of PBT gaseous concentration, surface fluxes, and near-surface vertical gradients with fetch
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