Sources and Temporal Variability in Summertime Ambient Mercury in Colorado Springs, CO, USA

Mercury (Hg) is a hazardous air pollutant and potent neurotoxin emitted from both natural and anthropogenic sources. It exists in the atmosphere primarily in the elemental vapor phase (gaseous elemental Hg; GEM), which is not highly water soluble and can remain the atmosphere for weeks to months. Mercury is also present in much lower concentrations in an oxidized form, either in the gas phase (gaseous oxidized Hg; GOM) or bound to particles (particle-bound Hg; PBM), which is readily removed by wet and dry deposition. The spatial impacts of Hg emissions thus range from local to global in scale. Atmospheric deposition is the primary way that Hg is introduced to terrestrial and aquatic ecosystems, where it can undergo methylation and bioaccumulate within food chains. Therefore, characterizing the atmospheric emissions from various sources is paramount to effectively mitigating ecosystem Hg contamination. Presently in the U.S., coal combustion for electricity generation is the largest source of Hg to the atmosphere, but other combustion processes such as cement manufacturing, steel production, gold mining, and biomass burning can also emit Hg. In 2011, the U.S. EPA began regulating Hg emissions from coal-fired power plants (CFPPs) by way of the Mercury and Air Toxics Standards (MATS). Although the MATS required control technologies to be in place by 2016, in June 2015 the U.S. Supreme Court effectively put these standards on hold, and it is unclear what the future holds for national regulation of power plant Hg emissions. Colorado Springs is the second largest population center in Colorado, covering an area of nearly 200 mi². While there are relatively few Hg point sources within the city compared to some other U.S. urban areas, it is home to the Martin Drake coal-fired power plant (CFPP). This facility, located in the heart of downtown, provides a significant fraction of the electricity for the local community but has been the center of much scrutiny due to increasing concerns over the age of the plant, its impact on local air quality, and emissions of greenhouse gases that contribute to global warming. While monitoring and modeling efforts have considered the impact of Martin Drake emissions on criteria air pollutants such as sulfur dioxide (SO₂), there had until recently been no active monitoring of Hg in ambient air near the plant. According to the U.S. EPA National Emissions Inventory (NEI), Martin Drake is the third largest Hg point source in El Paso and Teller counties but emits substantially less Hg on an annual basis than the Ray Nixon CFPP (~20 miles south) and the Cripple Creek gold mine (~30 miles west). Additionally, Colorado Springs is located approximately 70 miles south of Denver where coal combustion, chemical manufacturing, oil refining, and other industries emit pollutants such as Hg to the atmosphere. Furthermore, summertime in the U.S. west is frequently impacted by wildfires. Thus ambient Hg in Colorado Springs may be influenced by various sources, ranging from natural to anthropogenic and from local to regional in origin. The objective of this study is to quantify Hg concentrations in Colorado Springs and assess the impact that Martin Drake and other sources have on the local environment. During summer 2016 we continuously measured total gaseous Hg (TGM = GEM + GOM), carbon dioxide (CO₂), SO₂, carbon monoxide (CO), and meteorological parameters at a site approximately one mile north-northwest of the Martin Drake plant. The mean (± 1σ) TGM concentration was 1.7 ± 0.3 ng m^-3, with a range of 1.0 to 3.2 ng m^-3. These values are above the estimated global background concentration of GEM, but lower than levels measured in more industrialized U.S. urban areas. We observe diurnal variability in TGM throughout much of the study, with the highest concentrations occurring overnight, suggesting the role of the nocturnal boundary layer in containing emissions near the surface. We also find that elevated TGM was associated with low wind speeds, but was not always concurrent with elevated SO₂; thus ambient TGM appears impacted by a combination of sources and meteorological conditions. Using a range of techniques, such as CO₂:CO enhancement ratios to distinguish different combustion sources as well as cluster analysis of HYbrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model back-trajectories to identify the transport pathways associated with Hg enhancements, we explore the major Hg source regions and determine whether the Martin Drake CFPP emissions significantly elevate ambient Hg concentrations in Colorado Springs.