Correspondence Comment on “Measurements of Atmospheric Mercury Species at a Coastal Site in the Antarctic and over the South Atlantic Ocean during Polar Summer” In a paper by Temme et al. (1), the authors make several statements that we feel are misleading and present a conclusion that is not substantiated by the data and analysis presented. We offer the following comments to clarify our concerns. (i) The authors cite the standard operating procedure (SOP) of Landis et al. (2) for the collection and analysis of inorganic reactive gaseous mercury (RGM) and gaseous elemental mercury (GEM). However, the authors operated their automated Tekran system at an overall flow rate of 7.5 L min-1 rather than the 10 L min-1 indicated in the SOP. As a result, the mass median aerodynamic diameter (MMAD) cut point of the impactor inlet was 2.9 µm, not 2.5 µm as indicated. Aerosols larger than 2.5 µm may not be quantitatively transported through the annular denuder and may lead to particulate-phase mercury (Hgp) being quantified as RGM. The lower flow rate also increased their method detection limit (MDL) for a 60-min sample from 6 (2) to 8 pg m-3. The authors did not present the MDL for RGM in the paper. However, they report median RGM concentrations aboard the R/V Polarstern as 5.5 pg m-3, which is below their MDL. When sampling at background locations, we recommend a 2-h sampling period at 10 L min-1 to achieve an MDL of 3 pg m-3. (ii) We feel that the authors’ reference to the total Hgp (TPM) methodology of Lu et al. (3) is a misnomer. This methodology describes collection of “TPM” through a 4 mm i.d. quartz tube onto a quartz fiber filter at a relatively low flow rate (3.5-5.5 L min-1). Lu et al. (3) did not present the aerosol collection characteristics of the apparatus. The authors used the same apparatus at flow rates ranging from 1.4 to 2.1 L min-1. We calculated the MMAD cut point for the apparatus using the aerosol transmission model of Hangal and Willeke (4). At a flow rate of 1.8 L min-1 and wind speed of 8 km h-1 (∼average wind speed in the continental United States), the cut point was 7.5 µm. At a wind speed of 24 km h-1 (∼average wind speed at the northern coast of Antarctica) the cut point is 2.5 µm, and at 72 km h-1 the cut point drops to 0.5 µm. Mercury on larger particles may be significant in the Antarctic since aerosols >2.5 µm have recently been found to be a significant portion of PM10 in the marine boundary layer (5) and recent modeling studies have indicated the importance of gas-phase mercury adsorption onto sea salt aerosols (6). In addition, this method will also present problems with RGM artifact formation on the quartz filter (2). The RGM artifact is not quantitative and therefore cannot be subtracted or corrected. (iii) On page 24, second paragraph of the Quality Control section, the authors state “Comparable results were obtained for different methods such as multistage filter packs, refluxing mist chambers, and KCl-coated denuders. These major findings ... have been recently reconfirmed by a detailed laboratory study published by Landis et al.”. We first wish to clarify that there are two distinctly different KCl-coated denuder methodologies being used for ambient RGM determinations. The tubular denuder method (7) is restricted to a relatively long sampling period at low flow rate (∼ 1 L 10.1021/es0303844 Not subject to U.S. Copyright. Publ. 2003 Am. Chem. Soc. Published on Web 06/12/2003
min-1) because of the geometry of the device. Tubular denuders are extracted with a dilute HCl and analyzed. The annular denuder method (2) combines higher collection surface area and flow rate (10 L min-1) yielding greater mass of RGM collected, thereby allowing better time resolution. Following collection, thermal desorption of the denuder decomposes the RGM to Hg0, allowing for quantification without chemical extraction or sample preparation. Thermal desorption provides for rapid analysis in the field, minimizes the possibility of contamination, and allows the denuder to be immediately reused without further preparation. The connate desorption and regeneration of the denuder was the key element necessary for successful automation of selfconsistent speciation of atmospheric mercury. Second, there were no laboratory comparisons of ambient RGM methodologies published in Landis et al. (2). A field comparison was reported in Landis et al. (2) between refluxing mist chambers and annular denuders (n ) 22). However, the comparison found that the mist chamber provided significantly higher RGM concentrations than the annular denuder (p < 0.0001). The mist chamber RGM results were, on average, a factor of 6.5 times higher. We believe the mist chamber method to be susceptible to positive artifact formation from particulate-phase mercury entrainment and the aqueous oxidation of Hg0 to RGM in the HCl and NaCl scrubbing solution (8). Other papers cited by the authors to support their contention that the refluxing mist chamber, ion exchange filter, and KCl-coated denuder methodologies are “comparable”, in fact, report differences between the methodologies. (a) Ebinghaus et al. (9) present a comparison between ion-exchange filters and tubular denuder methodologies. On average, the tubular denuder RGM results were almost a factor of 5 higher than the ion-exchange filter results. A linear regression analysis on this comparison reveals that there was not a significant relationship between the two methods. (b) The one comparison presented by Munthe et al. (10) that consisted of at least four paired observations was between tubular denuders and refluxing mist chambers. Although the mist chamber (19 ( 12 pg m-3; mean ( SD) and the tubular denuder (19 ( 6 pg m-3) were of the same order, a linear regression analysis on this comparison revealed that there was not a significant relationship between the two methods. (c) The comparisons presented in Sheu and Mason (11) were unique in that that long-duration ion-exchange filter pack samples (6-24 h) were collected and compared to the mean of a series (3-12) of short-duration (2 h) mist chamber and annular denuder samples. This sampling design helped to improve the signal-to-noise ratio for the ion-exchange filter packs (since their blank values are relatively high, >200 pg) and helped moderate the variability of the mist chamber and annular denuder samples by comparing the mean of a larger sample population. Nonetheless, the ion-exchange filter RGM results were, on average, 34% higher than those reported for the refluxing mist chambers; annular denuder RGM results were, on average, 51% higher than those reported for the ion-exchange filters with a significant intercept of 16 pg m-3. (iv) On page 24, second paragraph of the Definition of TGM section, the authors state that “... at least under Antarctic conditions with low air humidity, high levels of RGM can pass through a heated Teflon sampling line and ... subsequently detected as Hg0 in a Tekran 2537A.”. To support their statement, the authors provide results from two VOL. 37, NO. 14, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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statistical tests that indicate the “mean TGM concentration during this summer period differs significantly from the mean GEM concentration” and “TGM concentrations and the calculated sum of GEM + RGM concentrations reveal no significant difference between these two aggregate concentrations.”. Although not explicitly indicated, the authors’ reference to the Table 1 (which summarizes sampling populations of differing size) suggests that they ran hypothesis tests of independent samples rather than more appropriate paired comparisons. In addition, it is not clear whether observations with RGM concentrations below the MDL were removed prior to the hypothesis test. Our laboratory experiments indicate that RGM is not quantitatively transported through a heated Teflon sampling line. In fact, after running elevated HgCl2 concentrations through a Tekran 2537A at low humidity, we extracted the Teflon tubing from both the sampling line and the internal instrument components and found significant quantities of mercury. In addition, after exposure to elevated HgCl2 concentrations, the instrument (i) had elevated zero air concentrations; (ii) gave false positive responses to mercuryfree zero air injected with O3, elevated humidity, or elevated temperature; and (iii) had significantly elevated baseline standard deviations. The adsorption/desorption behavior of HgCl2 in the inlet line and internal analyzer components varied depending on the exact variables that scientists are using to elucidate atmospheric mercury chemistry (e.g., oxidation potential, meteorological conditions). We strongly recommend that researchers avoid allowing RGM species into the Tekran 2537A instrument by incorporating a Tekran model 1130 gas-phase speciation unit and/or a soda and lime trap into the inlet system (2) (Note: RGM absorption problems will affect the inlet line and internal components of any mercury analyzer, not just the 2537A.) (v) The authors attempt to explain the temporal variability of the mercury depletion events (MDE) observed at the Neumayer Station in Antarctica using back trajectory analysis in conjunction with sea-ice analysis charts. On page 26, second paragraph, they state that “This good agreement between the TGM measurements during MDEs, and the results from trajectory calculations in combination with seaice charts support the theory that reactive bromine, which destroys ozone and can oxidize elemental mercury in a subsequent reaction, is released from sea salt, which is associated either with sea-ice surfaces or sea salt aerosols.”. While the authors’ conclusion that MDEs at Neumayer Station
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are driven by reactive bromine radicals from sea ice is plausible, the evidence presented is insufficient to support this conclusion. The authors do not present any analysis demonstrating a statistical association between MDEs and meteorological transport, halogen radicals, or sea salt aerosols.
Acknowledgments This work has been funded by the U.S. Environmental Protection Agency Office of Research and Development. It has been subjected to Agency Review and approved for publication.
Literature Cited (1) Temme, C.; Einax, J.; Ebinghaus, R.; Schroeder, W. Environ. Sci. Technol. 2003, 37, 22-31. (2) Landis, M. S.; Stevens, R. K.; Schaedlich, F.; Prestbo, E. M. Environ. Sci. Technol. 2002, 36, 3000-3009. (3) Lu, J. Y.; Schroeder, W. H.; Berg, T.; Munthe, J.; Schneeberger, D.; Schaedlich, F. Anal. Chem. 1998, 70 (11), 2403-2408. (4) Hangal, S.; Willeke, K. Atmos. Environ. 1990, 24A, 2379-2386. (5) Malcolm, E.; Keeler, G. J.; Landis, M. S. J. Geophys. Res. (in press). (6) Hedgecock; Pirrone Atmos. Environ. 2001, 35, 3055-3062. (7) Xiao, Z.; Sommar, J.; Wei, S.; Lindqvist, O. Fresenius J. Anal. Chem. 1997, 358, 386-391. (8) Stratton, W. J.; Lindberg, S. E. Environ. Sci. Technol. 2001, 35, 170-177. (9) Ebinghaus, R.; Kock, H. H.; Temme, C.; Einax, J. W.; Lowe, A. G.; Richter, A.; Burrows, J. P.; Schroeder, W. H. Environ. Sci. Technol. 2002, 36, 1238-1244. (10) Munthe, J.; Wangberg, I.; Pirrone, N.; Iverfeldt, A.; Ferrara, R.; Costa, P.; Ebinghaus, R.; Feng, X.; Gardfelt, K.; Keeler, G. J.; Lanzillotta, E.; Lindberg, S. E.; Lu, J.; Mamane, Y.; Nucaro, E.; Prestbo, E. M.; Schmolke, S. R.; Schroeder, W. H.; Sommar, J.; Sprovieri, F.; Stevens, R. K.; Stratton, W.; Tuncel, G.; Urba, A. Atmos. Environ. 2001, 35 (17), 3007-3017. (11) Sheu G. R.; Mason, R. P. Environ. Sci. Technol. 2001, 35, 12091216.
Matthew S. Landis* and Robert K. Stevens† U.S. EPA Office of Research and Development Research Triangle Park, North Carolina 27711 ES0303844 †
With Florida DEP, assigned to U.S. EPA.
