Published findings from the NOMADSS/SAS field campaign
The NOMADSS/SAS research on mercury has resulted in 7 papers published in peer-reviewed journals. These papers analyzed mercury observations collected as part of the NOMADSS/SAS campaign in the southeastern US in 2013. NOMADSS (Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks) is a National Science Foundation (NSF)-sponsored research project that was incorporated into the broader Southeast Atmosphere Study (SAS). The SAS field campaign involved a wide range of ground-based and airborne measurements directed toward assessing air quality and climate in the Southeastern US during the summer of 2013. The airborne field measurement portion of NOMADSS took place from June 1 to July 15, 2013.
Jesse Ambrose and his coauthors (2015) analyzed the mercury (Hg) emission ratios from 6 coal-fired power plants (CFPPs) in their paper Mercury emission ratios from coal-fired power plants in the Southeastern United States during NOMADSS. A key finding was that for some CFPPs, including some of the largest Hg emitters in the country, the observations suggested substantially higher Hg emissions compared to 2 national emission inventories, the National Emissions Inventory and the Toxics Release Inventory. Click here to read the full paper.
Lynne Gratz and her coauthors (2016) also evaluated Hg emission inventories in Airborne observations of mercury emissions from the Chicago/Gary urban/industrial area during the 2013 NOMADSS campaign. They used the NOMADSS observations of the Chicago-Gary plume to calculate observed Hg enhancement ratios (ERs) and compare these to emission ratios (EmRs) computed using a combination of the FLEXPART trajectory model and the National Emission Inventory. These observations also suggest that the emission inventory is biased low, in part due to the underestimate in CFPP emissions found by Ambrose et al. (2015), but also due to many small emission sources not included in the inventory and the re-emission of legacy Hg. They propose that total Hg emissions may be 65% higher than reported in the National Emission Inventory. Click here to read the full paper.
Because of the importance of gaseous elemental Hg (Hg0) oxidation in the global Hg cycle, a key NOMADSS goal was to understand the distribution and chemistry of gaseous oxidized mercury (GOM) in the atmosphere. For this reason, NOMADSS flights sampled air in the mid-upper troposphere over the southeastern US. The role of bromine in mercury oxidation was explored by Lynne Gratz and her coauthors (2015) in Oxidation of mercury by bromine in the subtropical Pacific free troposphere. Their results support the role of bromine as the dominant oxidant of Hg in the upper troposphere. They also found evidence that subtropical high-pressure systems are key locations in the global Hg cycle because of the presence of elevated bromine radicals over extended periods and sustained Hg accumulation. On one research flight over Texas, they recorded an isolated dry free tropospheric air mass in which the oxidized Hg and bromine monoxide (BrO) were above their respective detection limits. (See flight map at left.) Click here to read the full paper.
In Origin of oxidized mercury in the summertime free troposphere over the southeastern US, Viral Shah and his coauthors (2016) used a GEOS-Chem chemical transport model to interpret Hg observations. They examined the origin of oxidized Hg in the free troposphere and tested the kinetics of the bromine-initiated oxidation mechanism. They provided further support for the bromine mechanism and demonstrated that BrO mixing ratios in the model are likely too low. They also showed that the highest GOM concentrations occur in subsiding air in subtropical anticyclones, due to fast production and slow removal, making these anticyclones significant sources of global oxidized Hg. Click here to read the full paper.
Shaojie Song et al. (2016) also used the NOMADSS observations with the GEOS-Chem model to improve our understanding of the sources and sinks of Hg and to constrain the surface exchange of Hg0 over the eastern US. In Constraints from observations and modeling on atmosphere–surface exchange of mercury in eastern North America, they suggested that the northwestern Atlantic is a net source of Hg0, with high evasion fluxes in summer, likely due to high wet deposition of Hg in this region. In contrast, they found that terrestrial ecosystems in the eastern US during summer are likely a net sink of Hg0. Click here to read the full paper.
These results are consistent with the constraints on surface-atmosphere cycling of mercury found using a global suite of observations and an inverse modeling approach in Song et al., 2015—Top-down constraints on atmospheric mercury emissions and implications for global biogeochemical cycling. They reevaluated the long-term global biogeochemical cycle of mercury and showed that legacy mercury accounts for less present-day atmospheric deposition and primary anthropogenic emissions account for more (up to 23%) than previously estimated. Click here to read the full paper.
Ground-based observations of Hg during SAS were used to provide new insights into our ability to measure Hg0 and GOM using traditional, commercial instrumentation. In Evaluation of the KCl denuder method for gaseous oxidized mercury using HgBr2 at an in-service AMNet site, Crystal McClure et al. (2014) installed a novel system to measure total atmospheric mercury and a calibration system for GOM using a permeation source of HgBr2. Simultaneously, a standard suite of instruments measured Hg0, GOM and particulate Hg. Both types of instruments were calibrated using the HgBr2 permeation source. McClure et al. found that the commercial system could measure GOM accurately in zero air but suffered from significant interference from ozone and humidity in ambient air. This work strongly indicates the need for improved GOM calibration methods and further development of atmospheric Hg instrumentation. Click here to read the full paper.