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.
The goal of this study was to conduct an inter-comparison of novel instrumentation to measure atmospheric mercury (Hg) mass concentrations with existing measurement techniques. Common technology for measuring atmospheric Hg levels is limited to low temporal resolution (several minutes). As part of the Collaborative Research Project "Reno Atmospheric Mercury Inter-comparison Experiment (RAMIX)", our group performed fast-response (25 Hz sampling rate) measurements of gaseous elemental Hg (GEM) concentrations in ambient air using a novel laser-based technique called Cavity Ring-Down Spectroscopy (CRDS). Contributions of this study included three major areas: (i) preparation of a prototype CRDS sensor for the first real-time, continuous measurements of GEM concentrations in ambient air; (ii) comparison of CRDS measurements to other sensors during a large RAMIX field campaign with other research groups; and (iii) detailed inter-comparison studies of CRDS GEM concentration measurements with a commercially-available sensor that operates at a 2.5 min. time resolution. This study contributed significant technical improvements of a current prototype CRDS sensor; including automated wavelength control and stabilization of the laser system; implementation of a differential measurement technique that allows for continuous correction of system baseline drifts; development of automated signal acquisition and analysis systems for high-speed (25 Hz sampling rate) GEM measurements; and transfer of the system from the laboratory into a mobile, trailer platform for field deployment. Unfortunately the second component of the study, the deployment of the sensor during the RAMIX field deployment with other research groups, was less successful because unexpected technical problems with the complex laser system prevented us from conducting reasonable measurements at that time. The third component was much more successful and involved the detailed comparison of our CRDS sensor with a commercially available sensor with a time resolution of 2.5 min at our institute. This comparison showed excellent agreement between the two systems and high linearity of the CRDS system during GEM spike additions to ambient air over a mass concentration range from 0 ng m-3 to 70 ng m-3 (equivalent to about 50-times ambient-air GEM levels). Calculated sensitivity of the system indicates a sensitivity of the system when operating in continuous mode in ambient air of about 1.0 ng m-3 at a 2.5 min time resolution. While this sensitivity is lower than current commercially-available instruments, the system can provide unprecedented temporal resolution that is simply not achievable with existing Hg sensors and particularly useful for the study of polluted environments and pollution sources. To our knowledge, this study provided the first, real-time and continuous measurements of GEM in ambient air using the CRDS technique. Educational activities of this research project included technical and scientific training of a post-doctoral fellow, three graduate students, and several undergraduate and high-school students. Outreach activities included collaboration with a local high-school chemistry teacher, including laboratory and facility tours at our institute and summer internships provided to his chemistry class to learn about atmospheric instrumentation, data processing and analysis, and atmospheric pollution issues.
