This collaborative award will support an inter-comparison of different measurement systems for atmospheric mercury (Hg). Hg is an important neurotoxin which bioaccumulates in the food chain. It is an important global pollutant and the atmosphere is a key site for processing and global transport. However, there are large uncertainties in our understanding of the atmospheric cycling of Hg, in part due to significant uncertainties in its measurement.
The experimental campaign will occur during the summer of 2011 at a site in Reno Nevada that has previously been used for Hg research, and will involve four new measurement systems along with the traditional method system that is currently widely in use for Hg. The new methods will be compared and challenged using a carefully designed manifold for spiking and interference tests. The specific goals for this project are: (1) to compare ambient measurements of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate mercury (PHg) by multiple groups for four weeks; (2) to examine the response of all systems to spikes of elemental Hg and mercury halides; (3) to examine the response to Hg of all systems in the presence of known or potential interfering compounds, including ozone, water vapor and other compounds; (4) to analyze the data to quantify the level of agreement and the results of interference and calibration tests for each measurement system; and (5) to publish the results in the peer-reviewed literature.
This project is aimed at improving the measurement capabilities for a very important pollutant. The development and testing of better methods to measure Hg follows on the recommendations of several national assessments and an NSF supported workshop. This project will provide training opportunities for a number of undergraduates, graduate students and post-doctoral fellows.
Introduction Mercury (Hg) is a potent neurotoxin. Despite decades of scientific study, the complex environmental cycling of Hg remains poorly understood. Industrial processes, such as coal burning and cement production release a variety of forms of Hg to the atmosphere, but there are also natural sources such as volcanoes. This Hg is then dispersed globally and introduced to ecosystems. Accurate measurements of atmospheric of Hg measurements are therefore critical for understanding how Hg cycles in the environment. However, few methods have been developed which are capable of measuring the extremely low levels of Hg present in the atmosphere (e.g., in most regions worldwide, Hg concentrations are less than one-trillionth of a gram per cubic meter of air). Most mercury scientists use a standard commercial instrument to measure atmospheric Hg, but a number of problems have been identified with this method. Several university groups, including the University of Washington team, have developed new methods to measure atmospheric Hg. In order to better understand the global Hg cycle, it is essential that we carefully evaluate the performances of both the standard and new methods. Intellectual Merit: The Reno Atmospheric Mercury Intercomparison Experiment (RAMIX) was planned to rigorously evaluate the performance of the standard and state-of-the-art instrumental methods for measuring atmospheric mercury (Hg). Three different measurement methods were tested during RAMIX, including the standard and widely used commercial instrument and two new research-grade instruments. The study was motivated by the need to better define the capabilities and limitations of each method, both individually and with respect to one another, in a real-world, field setting. This type of study (i.e., an "intercomparison") is an essential component of the development of any new measurement method, and is critical for ensuring that an adequate level of confidence exists in the corresponding measurements. In order to successfully carry out this experiment, a variety of new tools had to be developed. This included constructed a manifold so that all groups were sampling the identical airstream and systems to generate known amounts of different chemical forms of Hg that would be sampled by each instrument. Extensive testing of the new manifold system was completed at the University of Washington prior to the Reno field experiment. Study Description The RAMIX study was carried out at a suburban location in Reno, Nevada during August and September of 2011. The study location and time were chosen based on previous measurements made at the same location. Scientists from the University of Washington (UW), UNR, University of Miami (UM), University of Houston, Desert Research Institute, and State University of New York participated in the RAMIX study. Participants sampled from the common manifold during the experiment. Three different measurement systems were successfully compared during RAMIX: the standard commercial instrument, a new research-grade instrument, developed at UW for atmospheric elemental and reactive Hg measurements from aircraft and flown in several prior aircraft missions; and an additional research-grade instrument, developed at UM. Measurement accuracy was evaluated by periodically adding known quantities of elemental Hg and HgBr2 compound to the sampled airstream. The robustness of the measurement methods was further evaluated through a series of "interference" tests. For the latter tests, known quantities of two non-Hg gases, water vapor and ozone (O3), were added to the sample stream, both in the presence and absence of added Hg. The selected non-Hg gases were chosen because they are abundant in the atmosphere and have the potential to cause interference in Hg measurements in some environments. The data from all groups was collected into a single data archive and evaluated using rigorous statistical methods. Key Advances A number of significant discoveries were made as a result of RAMIX. 1) We developed a system to introduce known quantities of Hg0 HgBr2 into an airstream for calibration purposes; 2) All systems appeared to give similar results for gaseous elemental Hg, however we can now say conclusively that the commercial instrument is unreliable for measurements of oxidized Hg compounds. 3) A research-grade instrument developed at the University of Washington, Bothell performed far better, lending high confidence to the measurements from that system. Three scientific publications are being published to describe the RAMIX results. Broader Outcomes: The RAMIX study was the first Hg intercomparison to employ a common air sampling system, which greatly facilitated comparability of results obtained with each method tested. Additionally, RAMIX was the first study to employ a system to introduce standards for reactive/oxidized Hg in a real world measurement setting. The RAMIX results have important implications for our past and future measurements of Hg. This information is essential as we move towards regulating and controlling this important neurotoxin.
