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.
A detailed understanding of the biogeochemical cycling of mercury and the routes to the production of organ mercury compounds in ecosystems is a critical issue from a human health perspective. Direct exposure to mercury is primarily through the ingestion of dimethylmercury from fish consumption, however an understanding of the overall budget and mechanism of chemical transformation of mercury in both its elemental and combined forms is critically important. Wet or dry deposition of oxidized mercury is an important step in a complex process that involves both chemistry and microbiology and eventually produces alkyl mercuric compounds. The typical background concentrations of Hg(0) in unpolluted environments range from 1.5–2 ng m-3 where 1 ng m-3 is ~ 3x106 atoms cm-3 or ~ 120 ppq (parts per quadrillion) and RGM is typically less than 10% of this value, hence atmospheric measurements represent a significant challenge in ultratrace analytical chemistry. Current approaches to the measurement of both elemental and oxidized mercury rely on instruments that use preconcentration on gold followed by analysis using CVAFS (cold vapor atomic fluorescence spectroscopy). We have developed a laser-based sensor for the detection of gas phase elemental mercury, Hg(0), and reactive gaseous mercury (RGM) using sequential two-photon laser induced fluorescence (2PLIF). The instrument is capable of fast, in-situ, measurement of Hg(0) at ambient levels. By incorporating pyrolysis to convert RGM to Hg(0) it is possible to measure total gas phase mercury (TGM) and hence to measure RGM by difference. The Reno Atmospheric Mercury Inter-comparison Experiment (RAMIX) was an attempt to compare new Hg measurement systems with the Tekran systems that are currently widely in use for Hg. The RAMIX intercomparison allowed us to compare the 2PLIF instrument with several CVAFS instruments. During the RAMIX campaign the 2PLIF instrument performed well. We sampled on 18 days, typically sampling for between 4 and 6 hours. The longest period of continuous sampling lasted for 26 hours and occurred on September 1st and 2nd. Over this18 day period we sampled from the RAMIX manifold and, in addition, at the end of the campaign we sampled ambient air independently and also attempted to measure RGM by pyrolyzing the sample air and measuring the difference between Hg(0) and TGM. We typically saw good agreement for measurements of gas phase elemental mercury but not for oxidized mercury, typically referred to as RGM (reactive gaseous mercury).
