Atmospheric mercury species (gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM)), trace pollutants (O3, SO2, CO, NO, NOY, 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 (Hg) is a potent neurotoxin that is particularly damaging to the development of fetuses, infants, and young children [
It is important to assess the long-term trends of atmospheric mercury in order to understand the relative contribution of Hg to ecosystems from various geographic regions and source types because atmospheric mercury deposition is the major pathway for the input of Hg into ecosystems. Such understanding is essential for developing and assessing regulations and control policies. Several distinct chemical and physical forms of Hg exist in the air and there are three operationally-defined Hg species: gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM). While GEM is ubiquitously observed in the troposphere, the distributions of GOM and PBM are not well documented. Mercury in aquatic ecosystems mainly results from dry and wet deposition of GOM and PBM from the atmosphere [
The northern coast of the Gulf of Mexico experiences high atmospheric wet deposition of Hg due to unique atmospheric conditions in this region, including high rainfall amounts and relatively high concentrations of Hg in rain [
In this work, we present measurements of atmospheric speciated mercury collected at a coastal site in the northern Gulf of Mexico and examine their long-term trends. Correlationships between mercury, trace gases, and meteorological parameters were also examined. The overall goal of this study is to develop source-receptor information for atmospheric mercury deposition to this sensitive coastal environment and inform policies to reduce mercury loadings.
National Oceanic and Atmospheric Administration’s (NOAA’s) Air Resources Laboratory (ARL) started to establish the mercury monitoring site at Grand Bay in 2006. The monitoring site has been described elsewhere [
Atmospheric speciated mercury, including GEM, GOM, and PBM, has been monitored at the Grand Bay site since 2007 while using two Tekran speciation systems (Tekran Instrument Corporation, Scarborough, ON, Canada,
Recent studies [
Other measurements of atmospheric constituents were made at the same site, including ozone (O3), sulfur dioxide (SO2), carbon monoxide (CO), nitric oxide (NO), total reactive nitrogen (NOY), and black carbon. Meteorological parameters (temperature, pressure, relative humidity, precipitation, wind speed and direction, and solar radiation) were also measured. Since 2010, weekly-integrated precipitation samples have been collected at the site for subsequent chemical analysis of total mercury, major ions, and trace metals, according to the NADP’s Mercury Deposition Network (MDN) and National Trend Network (NTN) protocols. Weekly mercury wet deposition was quantified by multiplying weekly precipitation totals by the total mercury concentrations that were measured in the weekly samples.
Besides the long-term monitoring, an intensive study in two phases was also conducted with the first phase in summer 2010 and the second phase in spring 2011 [
The frequency distributions of GEM, GOM, and PBM measured at the Grand Bay site from 2007 to 2018 show that GEM exhibits a nearly normal distribution (i.e., annual median and mean concentrations are about the same for each year (
There are considerable variations for GOM and PBM and mercury wet deposition, even for the quarterly average values (
The concentrations and trends of GEM, GOM, and PBM that were observed at the Grand Bay site are influenced by the spatio-temporal patterns of upwind emissions—including the poorly characterized partitioning of emissions among the different mercury forms—as well as complex atmospheric transport, dispersion, chemical transformation, and deposition phenomena that mercury is subjected to after emission to the air. While the spatio-temporal trend of GOM emissions are not well known, the overall interannual trend of GOM follows well with the decreasing trend of total mercury release into air from direct, inventoried point sources in the region that surround the Grand Bay site (
The seasonal variation of GEM shows that its concentration was the lowest in fall at the Grand Bay site; the lowest monthly GEM concentration of 1.28 ng m−3 was observed in September, while the GEM concentrations were generally constant in winter and spring and slightly lower in summer (
The seasonal variation of mercury wet deposition exhibits a broad peak from late spring to early fall and the lowest values in late fall through winter, being roughly proportional to rainfall with the highest amount in summer and the lowest amount in fall and winter (
The average diurnal variation of GEM at the Grand Bay site exhibits a few prominent features: decreasing levels after 21:00 Central Standard Time (CST)) with a minimum near daybreak (06:00 CST), increasing levels after 07:00 (CST) and reaching the maximum at around 10:00 (CST), and relatively constant levels during the day between 10:00 and 21:00 (CST) (
GOM and PBM both show significant diurnal variations with broad peaks in the afternoon, with the amplitude of PBM peak being somewhat smaller than the GOM peak. These midday peaks are likely due to the production of reactive mercury from the photochemical oxidation of GEM and/or downward mixing of air aloft to the surface in the convective boundary layer. The maximum mean hourly [GOM] of 12.2 pg m−3 and the maximum mean hourly [PBM] of 8.8 pg m−3 appear at 15:00 and 13:00 (CST), respectively, thus making the peak of total reactive mercury (TRM = GOM + PBM) appear at 14:00 (CST,
The interpretation of coastal measurements with this lens is complicated, though, as adjacent land and water surfaces often create conditions that dictate significantly different boundary layer heights and the transition zone between marine and terrestrial boundary layer regimes is relatively complex and uncertain. The sea-breeze phenomenon, which promotes flow from the ocean to the land during the day and from the land to the ocean at night, especially during the summer, is an additional meteorological factor that influences diurnal variations at the site. The higher GOM and PBM levels that were observed during the day could be, at least partially, due to the oxidation of GEM in the marine boundary layer brought onshore due to daytime sea-breeze flow. Finally, emissions from tall stacks, e.g., from coal-fired power plants in vicinity of the site, may sometimes be injected above the shallow night-time boundary layer and so be meteorologically disconnected from impacting surface measurement sites under those conditions.
Diurnal profiles in different seasons also show some interesting features (
The correlation between Hg species, ozone, and SO2 can be used to differentiate the mercury sources between direct emissions (typically with narrow plumes of SO2 and Hg fumigating the site, leading to short-term spikes) and photochemical production of GOM and PBM (typically with longer duration increases during the day) because of some common emission sources for mercury and SO2, and ozone being a tracer for photochemical oxidation [
One example is the observations on 18–21 March 2011 when elevated GOM and O3 levels together with SO2 spikes were observed (
Scatter plots of ozone versus GEM, GOM, and PBM that are colored by SO2 concentration also suggest these two sources of GOM. In general, high ozone and high GOM concentrations during the day (
The diurnal variation of a pollutant, such as carbon monoxide (CO), which is relatively inactive chemically, and so can be considered a quasi-conservative tracer, can be used to help understand the impact of the boundary layer dynamics at the site. At the Grand Bay site from 2007 to 2018, the mean CO concentration was 148 ppbv during the day (defined by with solar radiation values greater than 10 W m−2) and 183 ppbv at night (solar radiation equal to zero). While using the mean CO concentration of 134 ppbv when the winds were from south-southeasterly (SSE) to south-southwesterly (SSW) as a regional background, the enhancement of CO is approximately 49 ppbv at night and 14 ppbv during the day above the background. All things being equal, this suggests that boundary layer dynamics—at least for low-elevation emission—may contribute to a dilution of approximately, a factor of 3.5 during the day. However, diurnal variations of local/regional CO emissions and the variation in atmospheric transport time from sources to our site complicate the interpretation. The mean GOM concentration that was measured at the Grand Bay site was 7.83 pg m−3 during the day, greater than the mean GOM concentration at night (2.52 pg m−3) by a factor of 3.1. Boundary layer dynamics no doubt play a role in the diurnal variations that were observed at this site, but other processes, such as direct emissions, photochemical oxidation, as well as dry and wet deposition also likely affect the GOM concentrations.
There is no strong correlation between GOM and SO2 (
Wind direction and wind speed can play an important role in enhanced GEM, GOM, and PBM concentrations. The highest concentrations of GOM and PBM were mainly observed in air masses from two wind sectors, as shown in
On the other hand, GEM show little dependence on wind direction for its concentrations less than 10 ng m−3, while higher GEM concentrations (>10 ng m−3) were mainly observed in air masses coming from north (
Anticorrelation between GOM and RH and between PBM and RH was observed, as we expect, because of the high deposition removal of GOM and PBM under high humidity conditions. Anticorrelation between Hg wet deposition and the sum of GOM and PBM due to depletion by precipitation scavenging but positive correlation between Hg wet deposition and rainfall were observed, as we would expect (
GEM depletion events were occasionally observed in the early morning hours under high humidity (with relative humidity (RH) largely > 80%) and relatively calm wind (with a mean wind speed of less than or equal to 1.5 m s−1) conditions (
In polar regions, GEM can be quickly oxidized by reactive bromine species and its concentration reduction are well correlated with enhanced GOM concentrations [
Standard additions of GEM were conducted at the Beltsville site in Maryland during GEM depletion events by introducing a known amount of GEM to both the sampling inlet and upstream of the denuder of the sampling unit on the tower. The Beltsville site was equipped with a similar mercury instrumentation as that at the Grand Bay site [
Continuous observations of atmospheric speciated mercury and other chemical and meteorological parameters were made at the Grand Bay site in Mississippi for 12 years from 2007 to 2018. The results show that this rural site was affected by local and regional emission sources of mercury and other primary trace species with occasional transport-related episodes of higher concentrations, as well as photochemical processes and planetary boundary layer dynamics. During this period, the mean concentrations and standard deviations were 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. The GEM and GOM concentrations at this site have been slowly decreasing during this 12-year period, with a decrease rate of −0.6 ± 0.2% yr−1 for GEM and −7.1 ± 1.4% yr−1 for GOM, both being statistically significant. A slight increasing trend in PBM (3.5 ± 4.3% yr−1) was observed at this site, even though the trend is not statistically significant, given the variability of the data. Coincident with increases in the atmospheric PBM concentrations, Hg wet deposition has been slightly increasing at a rate of 2.8 ± 10.4 ng m−2 wk−1 yr−1 (0.9 ± 3.4% yr−1), although this trend is not statistically significant.
Lower GEM concentrations were observed in the early morning, especially during fog events with high humidity and weak winds in fall. The GEM depletion events were mainly observed in air masses coming from north with continental origins and likely due to the uptake by surrounding plants, which is perhaps enhanced by oxidation within surface moisture (e.g., dew) within a shallow night-time boundary layer. The slight elevation of GEM during the day is likely due to downward mixing of air masses with higher concentrations of GEM aloft. Diurnal variations of GOM and PBM both show peaks in the afternoon, likely due to the production of reactive mercury from the photochemical oxidation of GEM, but possibly also because of development of the midday convective boundary layer entraining air aloft. The seasonal variation of GEM shows higher levels in winter and spring and lower levels in summer and fall. GOM measurements show high levels in spring and consistently low levels in the other three seasons, while the PBM measurements exhibit higher levels from late fall to early spring and lower levels from late spring to fall. As expected, mercury wet deposition was the highest in the summer due to higher precipitation amounts.
Relationships between elevated GEM/GOM/PBM and wind direction indicate that mercury measurements at this site may be influenced by nearby mercury sources. Relationships among GOM, O3, and SO2 suggest different sources of GOM: direct emissions from mercury sources in situ photochemical production, and the transport of air aloft to the surface. This study indicates that the receptor site experienced impacts from mercury sources that are both local and regional in nature. The long-term monitoring data at this site will be further analyzed with the HYSPLIT trajectory and dispersion model to elucidate mercury source-receptor relationships in this coastal environment (Cohen et al., manuscript in preparation, 2020).
The following are available online at
Conceptualization, W.T.L., M.D.C., X.R.; Methodology, W.T.L., P.K., M.D.C., X.R.; Software, X.R., M.D.C., P.K.; Validation, W.T.L., P.K., X.R.; Formal Analysis, X.R., M.D.C.; Investigation, X.R., W.T.L., P.K., M.D.C., M.L.O., J.W., R.C., M.A.; Resources, X.R.; Data Curation, X.R., W.T.L., P.K., M.D.C.; Writing—Original Draft Preparation, X.R.; Writing—Review & Editing, X.R., W.T.L., P.K., M.D.C., M.L.O., J.W., R.C., M.A., A.A.S.; Visualization, X.R., P.K., M.D.C.; Supervision, W.T.L., R.A., A.A.S.; Project Administration, W.T.L., R.A., A.A.S.; Funding Acquisition, R.A., A.A.S. All authors have read and agreed to the published version of the manuscript.
This study was supported by National Oceanic and Atmospheric Administration through the Cooperative Institute for Satellites Earth System Studies (Grant NA14NES4320003) at the University of Maryland.
The authors thank the NOAA Grand Bay NERR for cooperation in facilitating the field observations. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the authors and do not necessarily reflect the views of NOAA or the Department of Commerce.
The authors declare no conflict of interest.
Mercury air emissions point sources in the Grand Bay region based on the Toxic Release Inventory [
Top panels: Frequency distributions of hourly measurements of GEM (
Time series of GEM (
Time series of point-source anthropogenic mercury air emissions in different distance ranges away from the Grand Bay site [
Boxplots showing composite seasonal variations of GEM, GOM, PBM, and Hg wet deposition measured at the Grand Bay site from 2007 to 2018. On each box, the central mark is the median, the edges of the box are the 25th and 75th percentiles, and the whiskers extend to the 5th and 95th percentiles. The linked green squares are composite monthly means. The linked black circles in the bottom panel are the composite monthly precipitation amounts (units: mm × 5).
Left: Boxplots showing composite diurnal variations of GEM (
Time series of GEM, GOM, PBM, ozone, and SO2 concentrations on three days in March 2011.
Scatter plot of ozone versus GEM (
Relationships between wind direction and GEM (
Relationships between wind direction and GEM (
GOM (
An example of GEM depletion events observed on 1–2 November 2009 (
Time series of ambient [GEM] and GEM recovery with standard additions to both the inlet and upstream of the denuder of the sampling unit on the tower at the Beltsville site on August 11 and 12, 2018. The GEM recovery is expressed as fractions and is calculated by the equation of ([GEM]sa − [GEM]amb)* Sample Volume/(GEM amount injected), where [GEM]sa is the total [GEM] concentration (ambient [GEM] + added [GEM]) during standard additions, [GEM]amb is the ambient GEM concentration, which is calculated as the mean of the GEM concentrations before and after the standard addition. The injected GEM amount is calculated from the the mercury source permeation rate times the permeation source injection time.
Statistics of measured hourly concentrations of GEM, GOM, and PBM at the Grand Bay site in Mississippi. Minimum concentrations for GOM and PBM were zero (not shown). Overall statistics are shown in bold.
Year | [GEM] (ng m−3) | [GOM] (pg m−3) | [PBM] (pg m−3) | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Min | Max | Median | Mean ± σ | Max | Median | Mean ± σ | Max | Median | Mean ± σ | |
2007 | 0.85 | 8.0 | 1.40 | 1.41 ± 0.29 | 140 | 1.2 | 4.0 ± 8.7 | 112 | 1.7 | 2.8 ± 4.3 |
2008 | 0.79 | 6.1 | 1.40 | 1.40 ± 0.16 | 256 | 2.1 | 7.5 ± 15.2 | 52 | 2.6 | 3.8 ± 4.1 |
2009 | 0.71 | 8.1 | 1.37 | 1.37 ± 0.17 | 133 | 1.7 | 5.2 ± 9.4 | 46 | 2.1 | 3.6 ± 4.4 |
2010 | 0.92 | 2.8 | 1.40 | 1.39 ± 0.12 | 130 | 1.7 | 6.1 ± 12.1 | 364 | 4.6 | 9.4 ± 17.5 |
2011 | 0.79 | 16.0 | 1.45 | 1.44 ± 0.23 | 127 | 1.7 | 6.4 ± 11.7 | 470 | 2.9 | 4.9 ± 8.7 |
2012 | 0.83 | 13.8 | 1.37 | 1.38 ± 0.25 | 98 | 1.5 | 4.8 ± 8.9 | 464 | 2.3 | 4.4 ± 10.4 |
2013 | 0.86 | 7.8 | 1.41 | 1.43 ± 0.32 | 104 | 2.2 | 5.5 ± 9.7 | 100 | 2.4 | 4.4 ± 7.5 |
2014 | 0.70 | 9.7 | 1.39 | 1.41 ± 0.34 | 94 | 1.8 | 4.5 ± 7.6 | 970 | 5.3 | 12.3 ± 28 |
2015 | 0.72 | 9.3 | 1.32 | 1.34 ± 0.23 | 71 | 1.9 | 3.5 ± 4.8 | 192 | 7.1 | 14.1 ± 19.4 |
2016 | 0.82 | 3.7 | 1.33 | 1.33 ± 0.14 | 42 | 1.4 | 2.9 ± 4.5 | 61 | 2.3 | 3.9 ± 5.2 |
2017 | 0.83 | 2.24 | 1.31 | 1.31 ± 0.14 | 46 | 0.89 | 2.7 ± 4.8 | 34 | 0.6 | 1.2 ± 2.2 |
2018 | 0.74 | 2.20 | 1.31 | 1.31 ± 0.13 | 27 | 0 | 1.5 ± 3.0 | 174 | 2.3 | 5.0 ± 12.2 |
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