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Atmospheric Hg^IIconcentrations at a given site are linearly related to the amount of Hg^IIlost from the atmosphere during precipitation. The slope from DCS measurement data (a) could be used to correct the older data which were biased low (b). 

Accurate measurement of atmospheric reactive mercury (RM) presents analytical challenges due to its reactivity and ultra-trace concentrations. In the last decade, use of the University of Nevada, Reno – Reactive Mercury Active System (RMAS) for RM measurements has increased since it has been shown to be more accurate than the industry standard, the Tekran 2537/1130/1135 system. However, RMAS measurements also have limitations, including long time resolution and sampling biases associated with membranes used for RM sampling. We therefore investigated the use of higher sampling flow rates to reduce sampling time and tested alternative membrane materials using both ambient air sampling and controlled laboratory experiments with a gaseous oxidized mercury (GOM) calibrator. Results indicated that increasing the RMAS sampling flow had a negative impact on determined RM concentrations. RM concentrations at 2 L min−1 were 10% and 30–50% lower than at 1 L min−1 in spring/summer and winter, respectively. However, the chemical composition of RM captured on membranes was not impacted by the increased flow rate. Membranes currently used in the RMAS performed better than numerous alternatives with similar composition, retaining Hg more efficiently. Both ambient air sampling and laboratory experiments revealed that membranes designed to retain only particulate-bound mercury (PBM) also retained significant amounts of GOM. PBM membranes based on borosilicate glass designs retained more than 70% of GOM. 

There is much uncertainty regarding the sources of reactive mercury (RM) compounds and atmospheric chemistry driving their formation. This work focused on assessing the chemistry and potential sources of reactive mercury measured in Reno, Nevada, United States, using 1 year of data collected using Reactive Mercury Active System. In addition, ancillary meteorology and criteria air pollutant data, Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) analyses, and a generalized linear model were applied to better understand reactive mercury observations. During the year of sampling, a fire event impacted the sampling site, and gaseous elemental Hg and particulate-bound mercury concentrations increased, as did Hg^II-S compounds. Data collected on a peak above Reno showed that reactive mercury concentrations were higher at higher elevation, and compounds found in Reno were the same as those measured on the peak. HYSPLIT results demonstrated RM compounds were generated inside and outside of the basin housing Reno. Compounds were sourced from San Francisco, Sacramento, and Reno in the fall and winter, and from long-range transport and the marine boundary layer during the spring and summer. The generalized linear model produced correlations that could be explained; however, when applying the model to similar data collected at two other locations, the Reno model did not predict the observations, suggesting that sampling location chemistry and concentration cannot be generalized. 

Mercury (Hg) researchers have made progress in under- standing atmospheric Hg, especially with respect to oxidized Hg (HgII) that can represent 2 to 20% of Hg in the atmosphere. Knowledge developed over the past ∼10 years has pointed to existing challenges with current methods for measuring atmospheric Hg concentrations and the chemical composition of HgII compounds. Because of these challenges, atmospheric Hg experts met to discuss limitations of current methods and paths to overcome them considering ongoing research. Major conclusions included that current methods to measure gaseous oxidized and particulate-bound Hg have limitations, and new methods need to be developed to make these measurements more accurate. Developing analytical methods for measure- ment of HgII chemistry is challenging. While the ultimate goal is the development of ultrasensitive methods for online detection of HgII directly from ambient air, in the meantime, new surfaces are needed on which HgII can be quantitatively collected and from which it can be reversibly desorbed to determine HgII chemistry. Discussion and identification of current limitations, described here, provide a basis for paths forward. Since the atmosphere is the means by which Hg is globally distributed, accurately calibrated measurements are critical to understanding the Hg biogeochemical cycle.