Comparison of KCl denuders with the pyrolysis method and calibration using HgBr2 at an in-service AMNET site

During the summer of 2013, we initiated a project to examine the performance of Tekran measurements of Gaseous Oxidized Mercury (GOM) with a pyrolysis method at the North Birmingham SEARCH site. Measurements started in June 2013 and will run until September 2013. This project responds to recent studies that indicate problems with the KCl denuder method for collection of GOM (e.g. Lyman et al., 2010; Gustin et al., 2013; Ambrose et al., 2013). For this project, we compared two GOM measurement systems, one using the KCl denuder method and a second method using high temperature pyrolysis of Hg compounds and detection of the resulting Hg⁰ vapors. Both instruments were also calibrated using an HgBr₂ source to understand the recovery of one possible atmospheric GOM constituent. Both instruments sampled from a common, heated manifold. Past work has shown that in order to fully transmit HgBr₂ sample lines must be made from PFA lines and heated to 100 °C. The transmission rate of HgBr₂ during this project is approximately 90% over 25 feet of sample tubing at this temperature. Very preliminary results from this study have found that the transmitted HgBr₂ is captured with 95% efficiency in carbon-scrubbed ambient air for both the KCl denuder and the pyrolysis method. However, the denuder method appears to be significantly less efficient in the capture of GOM when sampling unaltered ambient air versus the pyrolysis validation of total Hg⁰. Therefore, calibration of GOM measurements is essential in order to accurately correct for fluctuations in the GOM capture efficiency. We have also found that calibrations for GOM can be done routinely in the field and that these are essential to fully understand the GOM measurements. At present our calibration system is performed manually, but in principle this method could be readily automated.