Atmospheric transport and in situ oxidation are important factors influencing mercury concentrations at the surface and wet and dry deposition rates. Contributions of both natural and anthropogenic processes can significantly impact burdens of mercury on local, regional and global scales. To address these key issues in atmospheric mercury research, airborne measurements of mercury speciation and ancillary parameters were conducted over a region near Tullahoma, Tennessee, USA, from August 2012 to June 2013. Here, for the first time, we present vertical profiles of Hg speciation from aircraft for an annual cycle over the same location. These airborne measurements included gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), as well as ozone (O₃), sulfur dioxide (SO₂), condensation nuclei (CN) and meteorological parameters. The flights, each lasting ~3 h, were conducted typically one week out of each month to characterize seasonality in mercury concentrations. Data obtained from 0 to 6 km altitudes show that GEM exhibited a relatively constant vertical profile for all seasons with an average concentration of 1.38 ± 0.17 ng∙m⁻³. A pronounced seasonality of GOM was observed, with the highest GOM concentrations up to 120 pg∙m⁻³ in the summer flights and lowest (0–20 pg∙m⁻³) in the winter flights. Vertical profiles of GOM show the maximum levels at altitudes between 2 and 4 km. Limited PBM measurements exhibit similar levels to GOM at all altitudes. HYSPLIT back trajectories showed that the trajectories for elevated GOM (>70 pg∙m⁻³) or PBM concentrations (>30 pg∙m⁻³) were largely associated with air masses coming from west/northwest, while events with low GOM (<20 pg∙m⁻³) or PBM concentrations (<5 pg∙m⁻³) were generally associated with winds from a wider range of wind directions. This is the first set of speciated mercury vertical profiles collected in a single location over the course of a year. Even though there are current concerns that the KCl denuders used in this study may under-collect GOM, especially in the presence of elevated ozone, the collected data in this region shows the strong seasonality of oxidized mercury concentrations throughout the low to middle free troposphere.

During two intensive studies in summer 2010 and spring 2011, measurements of mercury species including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM), trace chemical species including O₃, SO₂, CO, NO, NO_Y, and black carbon, and meteorological parameters were made at an Atmospheric Mercury Network (AMNet) site at the Grand Bay National Estuarine Research Reserve (NERR) in Moss Point, Mississippi. Surface measurements indicate that the mean mercury concentrations were 1.42 ± 0.12 ng∙m⁻³ for GEM, 5.4 ± 10.2 pg∙m⁻³ for GOM, and 3.1 ± 1.9 pg∙m⁻³ for PBM during the summer 2010 intensive and 1.53 ± 0.11 ng∙m⁻³ for GEM, 5.3 ± 10.2 pg∙m⁻³ for GOM, and 5.7 ± 6.2 pg∙m⁻³ for PBM during the spring 2011 intensive. Elevated daytime GOM levels (>20 pg∙m⁻³) were observed on a few days in each study and were usually associated with either elevated O₃ (>50 ppbv), BrO, and solar radiation or elevated SO₂ (>a few ppbv) but lower O₃ (~20–40 ppbv). This behavior suggests two potential sources of GOM: photochemical oxidation of GEM and direct emissions of GOM from nearby local sources. Lack of correlation between GOM and Beryllium-7 (⁷Be) suggests little influence on surface GOM from downward mixing of GOM from the upper troposphere. These data were analyzed using the HYSPLIT back trajectory model and principal component analysis in order to develop source-receptor relationships for mercury species in this coastal environment. Trajectory frequency analysis shows that high GOM events were generally associated with high frequencies of the trajectories passing through the areas with high mercury emissions, while low GOM levels were largely associated the trajectories passing through relatively clean areas. Principal component analysis also reveals two main factors: direct emission and photochemical processes that were clustered with high GOM and PBM. This study indicates that the receptor site, which is located in a coastal environment of the Gulf of Mexico, experienced impacts from mercury sources that are both local and regional in nature.

Atmospheric mercury species (gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM)), trace pollutants (O₃, SO₂, CO, NO, NO_Y, and black carbon), and meteorological parameters have been continuously measured since 2007 at an Atmospheric Mercury Network (AMNet) site that is located on the northern coast of the Gulf of Mexico in Moss Point, Mississippi. For the data that were collected between 2007 and 2018, the average concentrations and standard deviations are 1.39 ± 0.22 ng m⁻³ for GEM, 5.1 ± 10.2 pg m⁻³ for GOM, 5.9 ± 13.0 pg m⁻³ for PBM, and 309 ± 407 ng m⁻² wk⁻¹ for mercury wet deposition, with interannual trends of −0.009 ng m⁻³ yr⁻¹ for GEM, −0.36 pg m⁻³ yr⁻¹ for GOM, 0.18 pg m⁻³ yr⁻¹ for PBM, and 2.8 ng m⁻² wk⁻¹ yr⁻¹ for mercury wet deposition. The diurnal variation of GEM shows lower concentrations in the early morning due to GEM depletion, likely due to plant uptake in high humidity events and slight elevation during the day, likely due to downward mixing to the surface of higher concentrations of GEM in the air aloft. The seasonal variation of GEM shows higher levels in winter and spring and lower levels in summer and fall. Diurnal variations of both GOM and PBM show broad peaks in the afternoon likely due to the photochemical oxidation of GEM. Seasonally, PBM measurements exhibit higher levels in winter and early spring and lower levels in summer with rising levels in fall, while GOM measurements show high levels in late spring/early summer and late fall and low levels in winter. The seasonal variation of mercury wet deposition shows higher values in summer and lower values in winter, due to larger rainfall amounts in summer than in winter. As expected, anticorrelation between mercury wet deposition and the sum of GOM and PBM, but positive correlation between mercury wet deposition and rainfall were observed. Correlation among GOM, ozone, and SO₂ suggests possible different GOM sources: direct emissions and photochemical oxidation of GEM, with the possible influence of boundary layer dynamics and seasonal variability. This study indicates that the monitoring site experiences are impacted from local and regional mercury sources as well as large scale mercury cycling phenomena.

Mercury is a toxic element that is dispersed globally through the atmosphere. Accurately measuring airborne mercury concentrations aids understanding of the pollutant’s sources, distribution, cycling, and trends. We deployed MerPAS^® passive air samplers (PAS) for ~4 weeks during each season, from spring 2019 to winter 2020, to determine gaseous elemental mercury (GEM) levels at six locations along the northern Gulf of Mexico, where the pollutant is of particular concern due to high mercury wet deposition rates and high concentrations in local seafood. The objective was to (1) evaluate spatial and seasonal trends along the Mississippi and Alabama coast, and (2) compare active and passive sampling methods for GEM at Grand Bay National Estuarine Research Reserve, an Atmospheric Mercury Network site. We observed higher GEM levels (p < 0.05) in the winter (1.53 ± 0.03 ng m⁻³) compared to other seasons at all sites; with the general pattern being: winter > spring > summer ≈ fall. Average GEM levels (all deployment combined) were highest at Bay St. Louis (1.36 ± 0.05 ng m⁻³), the western-most site nearest the New Orleans metropolitan area, and lowest at Cedar Point (1.07 ± 0.09 ng m⁻³), a coastal marsh with extensive vegetation that can uptake GEM. The MerPAS units compared reasonably well with the established active monitoring system, but gave slightly lower concentrations, except in the winter when the two methods were statistically similar. Both the passive and active sampling methods showed the same seasonal trends and the difference between them for each season was <15%, acceptable for evaluating larger spatial and temporal trends. Overall, this work demonstrates that PASs can provide insight into GEM levels and the factors affecting them along coastal regions.

Our investigation seeks to uncover the intricate nature of mercury dynamics in the free troposphere through analysis of the isotopic composition of total gaseous elemental mercury (TGM) at the high altitude Mauna Loa Observatory (MLO, 3397 m) in Hawaii, USA. By focusing on this unique site, we aim to provide essential insights into the behavior and cycling of mercury, contributing valuable data to a deeper understanding of its global distribution and environmental impacts. Forty-eight hours of TGM sampling from January to September 2022 revealed significant variations in δ²⁰²Hg (−1.86 % to −0.32 %; mean = −1.17 ± 0.65 %, 2 SD, n = 34) and small variations in Δ¹⁹⁹Hg (−0.27 % to 0.04 %; mean = −0.13 ± 0.14 %, 2 SD, n = 34) and Δ²⁰⁰Hg (−0.20 % to 0.06 %; mean = −0.05 ± 0.13 %, 2 SD, n = 34). During the sampling period, GEM was negatively correlated with gaseous oxidized mercury (GOM). However, the GOM/GEM ratio was not −1, suggesting that GEM oxidation and subsequent scavenging occurred previously. The δ²⁰²Hg isotopic compositions of TGM at MLO were different from those of reported values of high-altitude mountains; the δ²⁰²Hg of TGM at MLO was lower than the isotopic ratios that were obtained from other mountain regions. The unique atmospheric conditions at Mauna Loa, with (upslope winds during the day and downslope winds at night, likely result in the) possibly mixing of GEMs from terrestrial (and possibly oceanic GEM emission) sources with and tropospheric sources, influencing and affect the isotopic composition. During the late summer to early fall (September 14–28), negative correlations were found between relative humidity and GOM and between particle number concentrations and Δ¹⁹⁹Hg, indicating the gas-to-particle partitioning of the atmospheric mercury during this period. This study will improve our understanding on mercury dynamics of marine origin and high altitudes and shed light on its complex interactions with environmental factors.