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Project Title: Mercury Dry Deposition Measurement Intercomparison and Workshop
Chemicals Studied: Mercury
Geographic Areas: N/A
Project Start Date: May 2008
Project Status: Completed
Synopsis: As part of this project, the Mercury Dry Deposition Measurement Intercomparison Workshop was held at the University of Michigan Matthei Botanical Gardens in Ann Arbor, MI, on August 4-13, 2008 and the Mercury Dry Deposition Measurement Workshop was held in the Department of Atmospheric, Oceanic, and Space Sciences on the University of Michigan's North Campus on August 6-7, 2008. The overall goals of these GLAD funded events were:
(a) evaluation of currently utilized sampling approaches used in the quantification of mercury dry deposition;
(b) quantification of the variability between mercury dry deposition estimates obtained from the approaches used during the intercomparison;
(c) presentation of previous mercury dry deposition measurement results and interpretations by workshop participants using their respective approaches;
(d) identification of mercury dry deposition measurement research needs and opportunities for collaboration between participants, and
(e) an opportunity for interaction between scientists, federal/state/local policymakers, and private industry representatives.
The following observations can be made from the results of this project:
1. Under low ambient mercury concentrations, the mercury dry deposition rates quantified with different measurement approaches were generally in good agreement, with most estimates ranging from 0.0 to 0.5 ng/m2/hr. The upper limit of this range would equate to an annual deposition of approximately 4 ug/m2. These results would suggest that peer-reviewed published estimates of mercury dry deposition derived from these measurement approaches can be viewed as comparable, without significant methodological biases. The approaches for which consistent biases were observed have been highlighted.
2. The variability in the mercury dry deposition estimates obtained were attributed to a number of factors including: (i) differences in the sampler designs pertaining to the forms of mercury believed to be sampled by a given approach, (ii) the efficiency with which a given mercury species is "collected" by a given approach, (iii) differences in the ability for different sample media to "retain" collected mercury (e.g., potential for loss of collected mercury by volatilization under sunny conditions), and (iv) differences in the analytical methods applied to samples (e.g., direct thermal desorption into Tekran, Incorporated Model 2537A, laboratory-based cold vapor atomic fluorescence spectroscopy).
3. In studies where varying meteorological conditions are expected the UMAQL Adjusted-TSS and UNR CEM samplers maybe the most appropriate approaches for unattended sampling over extended periods of time, as these two approaches appear not to be impacted by adverse meteorological conditions (strong winds or rain). When compared directly these two approaches were in good agreement and the differences in the mercury dry deposition rates from the University of Michigan Air Quality Laboratory (UMAQL) Adjusted-TSS and University of Nevada - Reno (UNR) CEM samplers were not found to be statistically significant.
4. Given that the UMAQL TSS and UNR CEM measurement approaches were developed for use during extended and unattended sampling situations, the other sampling approaches compared in this study are more appropriate for use in short-term, process-based and/or source attribution study applications. As an example, a given measurement site can be impacted by different source regions (and thus by different natural and anthropogenic sources) on a day-to-day basis. As a result, if a particular study seeks to obtain a better understanding of the relative impact of different sources or source-types on the mercury deposition to an given site or area, short-term measurements (four to 24- hours) would be required to insure that the deposition at a give site over the sampling period was not impacted by different source regions (such as would be the case if a two-day sampling period experienced flow from the north for one day and from the south for the second day).
Preliminary project results have been presented during the 2010-11 GLAD webinar series.
Dr. Frank J. Marsik
University of Michigan Air Quality Lab
Dept. of Atmospheric, Oceanic and Space Sciences
2455 Hayward Avenue, Ann Arbor, Michigan 48109
Phone: (734) 763-5369