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.
Our team for the Reno Atmospheric Mercury Inter-comparison (RAMIX) was composed of Dr. Talbot’s group at the University of Houston (UH) and Dr. Mao’s group at SUNY-College of Environmental Science and Forestry (SUNY-ESF).  A major participant was Dr. Talbot’s Ph.D. student Ms. Dara Feddersen.  We had just built a new mercury instrument (UH-MERC) with NOAA funding during the previous year.  The instrument is a major improvement over the commercially available Tekran instrument.  The main objective for the development of UH-Merc was to decrease sample times for total gaseous mercury (TGM) and reactive gaseous mercury (RGM) allowing for high resolution measurements.  Current automated systems are designed for ground-based measurements and have time resolution for gaseous elemental mercury (GEM) of 2.5-5 minutes, and RGM and particulate bound mercury (PBM) of 2-3 hours.  UH-Merc measures TGM, GEM and RGM at 1 minute time resolution.  This produces more data to observe small fluctuations in mercury mixing ratios especially useful in aircraft measurements.  The basic operational principle of UH-Merc is similar to the commercial units available from Tekran (Tekran model 2537) which uses cold vapor atomic fluorescence spectrometry (CVAFS) at 253.7 nm.  UH-Merc is extremely portable featuring both the TGM and Hg0 measurements in one unit with the same dimensions as the Tekran unit.  UH-Merc operates using a combination of custom-fabricated circuitry, a highly sensitive Hamamatsu photo multiplier tube, higher heating temperatures, and increased flow rates allowing for lower detection limits (5 ppqv or  <0.05 ng/m3), lower time resolution, and increased accuracy and precision ( <5% each) with reduced channel bias, reduced noise and increased stability.  Since the measurements of mercury species by UH-MERC are based on difference methods, great emphasis was placed on obtaining high precision.  We achieved a difference between two channels of  <0.05 ng m-3, far better than the 0.2-0.3 ng m-3 offered by the Tekran instruments.  During the campaign, several instruments, including the standard Tekran automated system, sampled off a teflon manifold for delivery of Hgo and HgBr2 to the participants.  Initial results, Figure 1, show one week of data collected using UH-Merc.  The results in ambient air (average Hg0 2.1 ng/m3) match Hg0 mixing ratios detected by the other instruments on the manifold (2.2 ng/m3 Gustin et al., 2013) and previous results at the location (2.1 ng/m3 Lyman and Gustin, 2009).  There is a distinct diurnal pattern evident in the data with daytime averages of 1.8 ng/m3 and night-time averages of 2.4 ng/m3.  In addition, there were three Hg0 spikes administered into the manifold for 1 hour each in which UH-Merc was able to most accurately measure with a recovery of 91 plus/minus 10% (Figure 2).  The results show good repeatability and accuracy; a result of the instruments modifications to improve sensitivity.  It was reported in Gustin et al. (2013) that roughly 5 minutes were  necessary before the Hg0 spikes were stable in the manifold; this is evident in Figure 2 where the instrument was able to track the stabilizing of the signal in the manifold.  The instrument also shows the spike slowly exiting the manifold; it took 1 minute or less to remove all the Hg0 in the manifold. Finally, the Tekran uses a dual channel system to allow for continuous sampling using adsorption and desorption onto gold traps.  UH-Merc has two sets of dual channels for measurement of TGM and Hg0.  The gold trapping cartridges used to collect Hg0 in the Tekran are quartz tubes with ultra-pure gold absorbent beads.  The cartridges adsorb mercury at room temperature then are heated to desorb the mercury.  The UH-Merc model uses tightly rolled gold screen (39 mesh) which improves accuracy and precision by providing more surface area and allows for higher flow rates and greatly reduced pressure drops.  Overall, we believe that the measurements offered by our new instrument are at the top of the atmospheric field at the present time.  The field campaign was very successful and demonstrated that UH-MERC can measure Hg(o) with high precision and accuracy.  Publications Resulting from this work:  Gustin, M. S., Huang, J., Miller, M. B., Peterson, C., Jaffe, D. A., Ambrose, J., Finley, B. D., Lyman, S. N., Call, K., Talbot, R., Feddersen, D., Mao, H., Lindberg, S. E. (2013), Do we understand what the mercury speciation instruments are actually measuring? Results of RAMIX, Environ. Sci. Tech., doi:10.1021/es3039104.  Feddersen, D., H. Mao, P. Laine, and R. Talbot (2013), Development of a high time-resolution instrument for measurments of elemental and oxidized mercury in the atmosphere, Atmos. Meas. Tech., submitted.
