U.S. Department of the Interior Fact Sheet 2007-3071

U.S. Geological Survey September 2007

Investigating Atmospheric Mercury with the 

U.S. Geological Survey Mobile Mercury Laboratory 

Sources and Forms of Mercury in 

the Atmosphere

Mercury in the atmosphere is 

derived from both natural sources, 

such as emissions from volcanoes, 

and anthropogenic sources, such as 

combustion of mercury-bearing fossil 

fuels (fig. 1). Mercury exists in several 

different forms, which remain in the 

atmosphere for different lengths of 

time before being deposited on the land 

surface, or in rivers, lakes, or the ocean. 

Most of the mercury in the atmosphere 

occurs in the gaseous state. Three 

forms of mercury are recognized: 

•	 Elemental mercury (Hgo

) makes up 

most of the mercury in the atmosphere. 

Hgo can exist in the atmosphere for up 

to a year. During this time, Hgo can 

travel around the globe, far from its 

source, before being deposited.

•	 Particulate mercury (Hgp) is mercury 

that is attached to atmospheric 

aerosols (solid or liquid fine particles 

suspended in the atmosphere).

•	 Reactive gaseous mercury (RGM) 

is charged (ionic) gaseous mercury 

that is bound to other elements and is 

chemically reactive. The properties of 

RGM cause it to be deposited rapidly 

from the atmosphere. Compared to 

Hgo

, RGM has a much shorter lifetime 

in the atmosphere (days or weeks). 

As a result, RGM is much more 

likely than Hgo to be deposited near 

the source. 

Relation Between Mercury in the 

Atmosphere and Human Exposure

At the miniscule concentrations at 

which it is present in the atmosphere, 

mercury poses no direct health hazard. 

However, deposition of mercury from 

the atmosphere into water bodies, 

together with the input of mercury from 

the surrounding land, can result in its 

accumulation in aquatic sediments, where 

it may be converted by bacteria to an 

organic form, such as methylmercury 

(MeHg) (U.S. Geological Survey, 2000). 

Bacteria containing methylmercury are 

consumed by organisms higher in the 

food web, or adsorbed onto plankton, 

which are also consumed. Because 

methylmercury tends to be retained in 

the bodies of organisms, concentrations 

increase at each higher level of the 

food web. As a result of this process, 

known as biomagnification, high-end 

consumers such as fish, fish-eating 

wildlife, and people potentially can be 

exposed to harmful concentrations of 

methylmercury even at locations that 

are far from mercury sources and in 

areas with very low rates of atmospheric 

mercury deposition. Most humans are 

exposed to mercury primarily through 

the consumption of fish, especially those 

species that are high in the food web.

Studies conducted in the Experimental Lakes Area of northwestern 

Ontario, Canada, by scientists from the 

Emission from natural 

and anthropogenic sources

RGM and Hg-P

Oxidation to RGM and

Hg-P through reaction with

oxidants (such as O3, OH.

)

RGM and Hg-P Deposition

Deposition of RGM and HgR

P

e-emission and deposition

Evasion

Inorganic Hg

Net mercury methylation at the watersediment interface by microbial

and chemical mechanisms

MeHg

bioaccumulation

and 

biomagnification MeHg

Elemental Hg Deposition

Elemental Hg

Re-emission Surface-water/

ground-water exchange

Figure 1. Diagram of mercury cycling in the atmosphere showing major sources and pathways. 

(Hg, mercury; RGM, reactive gaseous mercury; Hg-P, particulate mercury; MeHg, methylmercury)

Atmospheric mercury is thought to be an important source of mercury present in fish, resulting in numerous local, statewide, 

tribal, and province-wide fish consumption advisories in the United States and Canada (U.S. Environmental Protection Agency, 

2007a). To understand how mercury occurs in the atmosphere and its potential to be transferred from the atmosphere to the 

biosphere, the U.S. Geological Survey (USGS) has been investigating sources and forms of atmospheric mercury, especially in 

locations where the amount of mercury deposited from precipitation is above average.
USGS and various other organizations are 

investigating the connection between the 

deposition of mercury and its uptake in 

ecosystems by tracing known amounts of 

mercury added to a watershed ecosystem. 

Results of the experiment, known 

as METAALICUS (U.S. Geological 

Survey, 2007), show that recently added 

mercury is more likely to be converted to 

methylmercury and taken up in organisms 

than mercury that is already present in the 

ecosystem. This finding indicates that a 

Sites not pictured:

AK03

VI01

1 kg/ha

10 kg/ha

EXPLANATION

Less than or equal to 3

3 to 6

6 to 9

9 to 12

12 to 15

15 to 18

18 to 21

21 to 24

24 to 27

Greater than 27

National Atmospheric Deposition

 Program sampling site

Sampling site where the actual

 deposition for the station (noted

 by point fill color) differed from

 the contoured estimate

Annual sulfate deposition, in 

 kilograms per hectare (kg/ha)

Figure 2a. Map showing location of Mercury Deposition Network sampling sites and contours of 

2005 total annual mercury wet deposition. Map shows that wet deposition of mercury is greatest 

along the Gulf Coast and in Florida. (From National Atmospheric Deposition Program, 2007)

Figure 2b. Map showing 2005 contours of sulfate-ion (SO4

2-) deposition, which reflect the 

distribution of sulfur-emitting sources such as coal-fired utility power stations. This distribution is 

distinctly different from that for total wet mercury deposition (shown in figure 2a). (From National 

Atmospheric Deposition Program, 2007)

reduction in the input of anthropogenic 

mercury to the atmosphere could decrease 

the occurrence of fish consumption 

advisories in some environments. In an 

effort to reduce atmospheric emissions 

of mercury in the United States, the 

U.S. Environmental Protection Agency 

(USEPA) has adopted a plan to limit 

mercury emissions from coal-fired utility 

power stations, the largest unregulated 

anthropogenic mercury source in the 

United States. The plan, consisting 

of the USEPA Clean Air Interstate 

Rule (U.S. Environmental Protection 

Agency, 2007b) and the USEPA Clean 

Air Mercury Rule (U.S. Environmental 

Protection Agency, 2007c), would 

ultimately result in a 70-percent reduction 

in mercury emissions from this source 

by 2018. 

Measuring Deposition of Mercury 

from the Atmosphere

In the mid-1990s, the Mercury 

Deposition Network (MDN) of the 

National Atmospheric Deposition 

Program was begun to provide weekly 

estimates of the amount of mercury 

deposited in the United States from 

precipitation and the concentration of 

mercury in that precipitation (National 

Atmospheric Deposition Program, 

2007). The MDN, a cooperative effort 

of many agencies and participators, 

consists of nearly 100 sampling stations 

concentrated primarily in the eastern 

United States, where rainfall amounts, 

and the proportion of mercury-emitting 

point sources, are high. Recent MDN 

results (fig. 2a) show that wet mercury 

deposition is greatest in the southeastern 

United States and the Gulf Coast states; 

however, the density of coal-fired utility 

power stations and the proportion of 

atmospheric contaminants derived from 

coal combustion, such as sulfur dioxide, 

are greatest in the Ohio Valley, MidAtlantic, and northeastern states (fig. 2b). 

Possible explanations for this discrepancy 

include (1) the MDN does not measure 

mercury that is deposited when there is 

no rainfall (dry deposition); (2) reactions 

may be occurring in coal-fired power 

plant plumes that convert most of the 

mercury to the elemental form, which is 

less easily deposited; and (or) (3) there 

may be unique conditions or sources of 

mercury in the southeastern states, or 

near coastal settings, that contribute to 

high rates of mercury deposition there. 

Deployment of the USGS Mobile 

Mercury Laboratory to Measure 

Atmospheric Mercury

In order to improve understanding 

of the pattern of mercury wet deposition, 

especially in the southeastern United 

States, the USGS mobile mercury lab was 

deployed to sites in Alabama and South 

Carolina in 2005 and 2006, respectively 

(fig. 3). These sites were chosen to study 

the interaction of atmospheric mercury 

from sources on land with air masses 

Site not pictured:

Hidalgo, Mexico 01 

Less than or equal to 4

4 to 6

6 to 8

8 to 10

10 to 12

12 to 14

14 to 16

16 to 18

Greater than 18

Mercury Deposition Network

 sampling site

Sampling site where the actual

 deposition (noted by point

 fill color) differed from the

 contoured estimate

Annual total mercury

 deposition, in micrograms

 per square meter (μg/m2) 

EXPLANATION

12.3 μg/m2

#
coming off the Gulf of Mexico or the 

Atlantic Ocean. In addition, wet deposition of mercury is high or moderately 

high at these sites, and coastal ecosystems are known to be highly sensitive to 

the input of atmospheric mercury.

The mobile mercury lab is equipped 

with instruments not available at 

most MDN sites. These instruments 

Figure 3. USGS Wisconsin Water Science Center mobile mercury laboratory deployed at Weeks 

Bay National Estuarine Reserve, Alabama. Instruments on mast at left rear of the trailer include 

the front end of the mercury speciation unit (large box) and meteorological sensors. Instrument on 

rooftop tripod collects fine particulate matter (<2.5 micrometers in diameter), whose mass per unit 

volume of air is determined by an instrument inside the trailer.

Figure 4a. Plot showing continuous daily variation in the concentration 

of reactive gaseous mercury at the Weeks Bay, Alabama, site during an 

8-day period in April 2005.

Figure 4b. Plot showing concentrations of particulate mercury 

(particles < 2.5 micrometers in diameter) measured by the mobile 

mercury lab and total particulate mercury measured in samplers 

with no size limitation. The difference between the two values equals 

the concentration of particulate mercury missed by the mobile lab 

analyzer. The plot shows that a large proportion of particulate mercury 

is bound to particles larger than 2.5 micrometers in diameter. 

continually measure the amounts 

and proportions of Hgo

, Hgp, and

RGM in the atmosphere. The mobile 

mercury lab also measures important 

atmospheric components such as sulfur 

dioxide, ozone, nitrogen oxides, and the 

mass of fine particulate matter, while 

making continuous meteorological 

measurements. In addition to the 

mobile mercury lab, separate particulate 

samplers were set up in each study area 

to determine the proportion of mercury 

and trace metals bound to particles larger 

than 2.5 μm (micrometers) in diameter—

particles missed by the mercury sampling 

unit in the mobile lab because of its 

upper particle-size limit of 2.5 μm.

Model for Enhanced Mercury 

Deposition at Coastal Sites

Daily results for the Weeks Bay 

sampling site in southern Alabama are 

shown in figure 4. Figure 4a shows 

the results of continuous sampling for 

RGM, with daily elevation of RGM 

concentrations during the afternoon 

hours, when sunlight and ozone levels 

are greatest. These conditions favor 

formation of RGM by sunlight-induced 

oxidation of Hgo

 derived from natural and 

anthropogenic sources on land. A similar 

pattern of afternoon maximum RGM 

concentration, but with less prominent 

highs, was found in Cape Romain, South 

Carolina, in 2006. 

Particulate mercury (Hgp

) 

concentrations measured in Weeks Bay 

atmospheric aerosol particles were 

significantly lower for particles equal to 

or less than 2.5 μm in diameter than for 

aerosol particles of all sizes (fig. 4b), 

indicating that a large fraction of Hgp

is bound to coarse particles. During 

sampling for Hgp

 at Weeks Bay, some 

air masses came directly off the land; 

others came onshore from over the 

ocean; and still others came off the land, 

spent time over the ocean and returned 

DATE

April 15 April 17 April 19 April 21 April 23 

0

20

40

60

80

100

CONCENTRATION, IN PICOGRAMS PER

CUBIC METER

DATE

June 7 June 9 June 11 June 13 June 15 June 17 

0

5

10

15

20

25

Uncertainty error bar

CONCENTRATION, IN PICOGRAMS

PER CUBIC METER

Particulate mercury <2.5 micrometers

Total particulate mercury
onshore as ocean breezes. On days when 

air masses coming off the land return 

onshore in the form of ocean breezes 

or come directly off the ocean into the 

RGM-rich nearshore environment, some 

of the RGM and (or) Hgo appears to be 

scavenged and incorporated onto large 

aerosol particles, which contain sea salt 

from the ocean air. Combining mercury 

and air contaminant sources on land with 

onshore ocean breezes that are warm and 

humid and contain sea salt provides an 

environment that may allow for enhanced 

deposition of mercury along the Gulf 

Coast compared to inland areas or sites 

over the open ocean (fig. 5). 

Other Research Sites

Additional studies with the 

mobile mercury lab are planned to 

test the effect of ocean temperature 

and sampling-site location on the 

model developed for the Gulf Coast 

by deploying the lab at more northerly 

coastal sites in the Mid-Atlantic region 

and possibly New England. Our overall 

hypothesis is that the marine boundary 

layer (the intrinsic characteristics of 

the atmosphere overlying the ocean) 

interacts with atmospheric mercury in 

near-coastal settings to enhance mercury 

deposition, and the strength of this 

interaction depends on factors such as 

ocean temperatures and wind patterns. 

Thus, the marine boundary layer is 

hypothesized to have a smaller effect on 

mercury deposition in cooler oceanic 

settings such as the Mid-Atlantic or New 

England states than at the warmer Gulf 

Coast and southeast Atlantic sites. 

At existing air-quality-monitoring 

sites, the capabilities of the mobile 

mercury lab can be nearly duplicated 

by adding an analyzer that measures 

forms of atmospheric mercury to the 

instruments that are already in place. 

This approach was taken in 2006 at 

Shenandoah National Park-Big Meadows 

in Virginia and in 2007 in western 

Pennsylvania and is planned for other 

study areas. Studies at these inland 

sites are intended to illustrate what is 

happening in the atmosphere nearer to 

mercury sources such as coal-fired utility 

power stations, in order to understand 

the factors that control the reactivity of 

atmospheric mercury and the mechanisms 

by which mercury begins to separate 

from other atmospheric constituents 

derived from coal combustion.

Summary 

Mercury deposited from the 

atmosphere is added to aquatic 

ecosystems, where it can be converted 

to toxic organic forms such as 

methylmercury and taken up in the food 

web, ultimately resulting in unsafe levels 

for human consumption of fish. Our work 

in coastal Alabama shows that, in some 

cases, air masses containing mercury 

derived from the land mix with marine air 

masses, generating high concentrations 

of reactive gaseous mercury that can be 

deposited directly or possibly adhere 

to sea-salt particles that are deposited 

or dissolved in precipitation. This 

reactive gaseous mercury and particulate 

mercury is available to be deposited in 

mercury-sensitive coastal ecosystems, 

which may explain in part why mercury 

deposition is greater along the Gulf 

Coast than in the Ohio Valley, even 

though coal use for power generation 

is much more prevalent in the Ohio 

Valley than along the Gulf Coast. 

References Cited

National Atmospheric Deposition Program, 

2007, Mercury deposition network: A NADP 

network: NADP Program Office, Illinois 

State Water Survey, accessed July 30, 2007, 

at http://nadp.sws.uiuc.edu/mdn/.

U.S. Environmental Protection Agency, 2007a, 

Fish advisories: U.S. Environmental Protection Agency, accessed July 30, 2007, at http://

www.epa.gov/waterscience/fish/advisories/.

U.S. Environmental Protection Agency, 2007b, 

Clean Air Interstate Rule: U.S. Environmental Protection Agency, accessed July 30, 

2007, at http://www.epa.gov/cair/.

U.S. Environmental Protection Agency, 2007c, 

Clean Air Mercury Rule: U.S. Environmental 

Protection Agency, accessed July 30, 2007, at 

http://www.epa.gov/air/mercuryrule/.

U.S. Geological Survey, 2007, Mercury cycling 

in aquatic ecosystems: U.S. Geological Survey, accessed July 30, 2007, at http://infotrek.

er.usgs.gov/mercury/toxics.html#metaalicus

U.S. Geological Survey, 2000, Mercury in the 

environment: U.S. Geological Survey Fact 

Sheet 146-00, accessed July 30, 2007, at 

http://www.usgs.gov/themes/factsheet/146-00

Hgo + oxidants + light

Elevated RGM

Emission of seasalt aerosols and

release of halogens

Hg sorption by 

sea-salt aerosols

Hgo oxidation

Ocean breeze returns

Enhanced Hg deposition

Estuarine

ecosystem

Hg bound to 

sea-salt aerosols

Continental Hg

+ oxidants

Continental

winds

Gulf of

Mexico

Emission from natural and

anthropogenic sources

Figure 5. Conceptual model for enhanced mercury (Hg) deposition along the Alabama Gulf 

Coast, based on results from atmospheric sampling with the USGS mobile mercury lab and 

separate aerosol samplers. Model shows formation of sea-salt aerosols, primarily by action 

of wind on the ocean, and release to the atmosphere of halogen elements such as chlorine, a 

major component of sea salt. Wet deposition of mercury in coastal areas may be enhanced by 

formation of reactive gaseous mercury (RGM) from elemental mercury (Hgo) and oxidants derived 

from sources on land, and return of mercury onshore as RGM and mercury adsorbed onto seasalt aerosols that are easily dissolved. 

For additional information, please contact:

Allan Kolker and Mark A. Engle, 

USGS, Eastern Energy Resources Team, 

MS 956, Reston, VA 20192

David P. Krabbenhoft and Mark L. Olson, 

USGS, Wisconsin Water Science Center, 

Middleton, WI 53562