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Particulate Mercury in the Atmosphere: Its Significance, Transport,
Transformation and Sources
G. KEELER, G. GLINSORN and N. PIRRONE
The University of Michigan 4Jr Quality Laboratory, Ann Arbor, MI 48109-2029
Abstract.
The importance of particulate mercury (Hg(p)) in the transport, chemistry and deposition of this toxic
metal has long been underestimated and largely ignored. While it was once believed to constitute/t small percentage
of total atmospheric mercury, Hg(p) may contribute a significant portion of the deposition of this metal to adjacent
natural waters. Recent measurements of Hg(p) in several urban/industrial areas have documented that Hg can be
associated with large particles (>2.5 pan) and in concentrations similar to those of the vapor phase Hg (ng/m3). As
part of ongoing effort to diagnose the sources, transport and deposition of Hg to the Great Lakes and other Great
Waters, the University of Michigan Air Quality Laboratory (UMAQL) has investigated the physical and chemical
properties of particulate-phase Hg in both urban and rural locations. It appears that particulate Hg may be the one of
the most difficult of the Hg measurements to perform, and perhaps the one of the most important for deposition and
source apportionment studies. Particulate Hg concentrations measured in rural areas of the Great Lakes Region and
Vermont ranged from 1 to 86 pg/m 3 whereas Hg(p) levels in urban/industrialized areas were in the range 15 pg/m 3
to 1.2 ng/m 3.
Keywords:
Mercury, dry deposition, particle phase mercury, size distribution, Great Lakes, Lake Champlain
I. Introduction
In recent years, the behavior of hazardous air pollutants (HAPs) has been receiving a
great deal of attention from the scientific community. This is largely the result of recent
changes in regulations including the Clean Air Act Amendments of 1990 and due to the
scientific interest associated with compounds with inherent chemical and physical
complexities. Mercury (Hg) continues to be of special concern because of its multitude of
controllable sources, its volatility, mobility and strong tendency to bioaccumulate. In the
Great Lakes Region, research on the sources, transport and deposition of atmospheric
mercury has gained increasing attention as it is now believed by many to be the most
important pathway for inputs to the natural waters.
The processes and deposition rates by which mercury enters the water column are still
not adequately understood. In particular, the role of the various physical/chemical forms
of mercury deposited from the atmosphere has yet to be determined. While vapor phase
mercury is thought to constitute the vast majority of the atmospheric mercury burden,
particle-phase mercury may actually play a disproportionately large role in the amount of
Hg in the various environmental compartments. While the relative importance of wet
deposition versus dry deposition in delivering Hg to the earth's surface is largely
unknown, and location specific, most researchers agree that the particulate form of Hg is
critical in understanding the cycling of this metal in the environment.
Water, Air, and Soil Pollution
80: 159-168, 1995.
9 1995
Kluwer Academic Publishers. Printed in the Netherlands.
160 G. KEELER, G. GLINSORN AND N. PIRRONO
An important aspect of this research was the ability to accurately collect and
effectively analyze Hg(p) without artifacts. Measurements of Hg(p) has been limited over
the past two decades but with recent advances in instrumental sensitivity and the
application of clean techniques our knowledge of Hg(p) concentrations and behavior has
improved. The precise determination of ultra-trace environmental concentrations
(pg/m ~) of Hg(p) are now feasible. Until now, the number of studies with high quality
Hg(p) data has been limited to a few intensive efforts.
The Air Quality Laboratory at the University of Michigan (UMAQL) has
developed (Keeler, 1994; Lamborg
et al.,
1994) and continues to improve the methods to
reliably collect and analyze size fractionated Hg(p). These techniques are presently
being utilized to gain a wider understanding of the atmospheric mercury cycle. Various
analytical techniques have been utilized including dual-amalgamation preconcentration
and cold vapor atomic fluorescence spectrometric (CVAFS) detection performed on
Hg(p) extracted from glass fiber and other types of filters, and instrumental neutron
activation analysis (1NAA) performed on Teflon filters. The different techniques have
distinct advantages and have been used to quantify Hg(p) in recent studies (Keeler
et al.,
1994; Lamborg
et al.,
1994; Ohnez
et al.,
1994).
Atmospheric Hg measurements reported here were performed as part of several
UMAQL studies including: 1) ongoing urban atmospheric chemistry and deposition
studies in Detroit, MI (Keeler
et al.,
1994); 2) a two-year multi-site atmospheric Hg
transport and deposition study in the State of Michigan (Hoyer
et al.,
this volume); 3) a
long-term Hg and pollutant cycling study in the Lake Champlain Basin of Vermont
(Burke
et al.,
this volume); and 4) a study of the atmospheric Hg levels in Broward
County, Florida (Dvonch
et al.,
this volume). These investigations have provided an in-
depth look at the relationship between particulate mercury and other aerosol constituents.
In addition, mercury bound to particulate matter in precipitation is currently under
investigation in event precipitation samples collected at the rural locations in Michigan
and in the Lake Champlain Basin. This paper aims to evaluate the Hg(p) data collected
by the UMAQL to date and to provide evidence of it's importance in the cycling of this
critical pollutant.
2. Sampling and Analysis
Ultra-clean sampling and analysis techniques were required to oOtain reliable Hg(p)
data. Sampling equipment including filter packs, forceps, vials, petri dishes as well as
other field sampling equipment were rigorously acid-cleaned in a 5-step, 11-day process.
All sampling equipment, including filter packs and cyclones, were constructed of Teflon
or were Teflon coated. Glass-fiber filters were pre-fired at 500 ~ for > I-hour prior to
use in sampling. During sample collection, particle-free gloves were worn when field
equipment was handled. Since outdoor concentrations of Hg in all forms are typically
lower than indoor concentrations, most of the handling of filters and filter packs was
done outdoors.
Total particulate mercury (Hg(p)) was collected using an open-faced Teflon filter pack
onto 47 mm glass fiber filters (Gelman Type A/E) for 24 hours at a nominal flow rate of
30 L min "1. Mercury in the fine particle size range(<2.5 ktm) was collected onto 47 mm
diameter glass-fiber filters using Teflon coated aluminum cyclones (URG, Carboro NC)
PARTICULATE MERCURY IN THE ATMOSPHERE 161
to remove larger particles upstream of the filter. Filters were placed into acid-cleaned
petri dishes immediately after sampling, Teflon-taped and then stored at -40~ until
analysis.
In addition, a microorifice cascade impactor (MOI) was used to collect size-
fractionated aerosols (Marple and Rubow, 11984). This impactor was chosen because of
its moderately high flow rate, 30 L min , and relatively low pressure drop. With an
ambient pressure of 0.973 atm the measured pressures at the nozzle exit of the five stages
are 0.973, 0.971, 0.942, 0.929, and 0.893 atm, respectively. Regulation of the pressure
drop is important as vaporization of particle associated water inside the impactor can
result in a distortion of the size distribution (Biswas
et.,al.,
1987). This water loss could
cause the particles to become smaller, resulting in an underestimate of the particles
aerodynamic size. Experiments carried out with sulfuric acid droplets showed maximum
changes in particle size of 3%. Since it is unclear whether the particulate Hg is
associated with the sulfur containing particles in the atmosphere the actual size distortion
of the Hg laden particles may be different but should not exceed that of highly
hygroscopic sulfate. Teflon membrane filters (2 ~tm pore size) and glass-fiber filters
were both utilized as impaction surfaces as they have low blanks for Hg. The particle
cut-off diameters for the first 5 impactor stages are 5.0, 2.5, 1.0, 0.6, and 0.18 ~tm,
respectively. The last stage collects all particles below 0.18 p-m is aerodynamic size.
The filter extraction and analysis was performed in a Class 100 cleanroom using
reagents that required further purification to maintain the consistently low blank values
and detection limits. The present UMAQL protocol, utilized in the analysis of the MOI
and Detroit filters, involved the extraction of each glass fiber filter in 30 mL of 10%
HNO 3 followed by a digestion of the filter for 20 minutes at 160~ using a CEM MDS-
2000 computer controlled microwave unit. The samples were then allowed to react for
12-hours at room temperature. After digestion, 10 mL of extract were removed with a
pipet and placed into 30 mL acid cleaned polyethylene bottle for trace metals analysis
using a Perkin Elmer ELAN 5000 ICP-MS. The remaining extract was utilized for Hg
analysis by prior addition of 0.25 mL of BrCI to oxidize all the Hg to Hg 2§ The glass
fiber filters used in the MOI were extracted with only 10 mL of 10% HNO 3 and were
then treated as described above.
The UMAQL standard particulate protocol, applied to all filters collected in the
Michigan Network, Vermont Studies, and Florida Study utilized acid digestion/CVAFS
analysis of the samples extracted in a 10% solution of a 70% nitric acid/30% sulfuric
acid mixture (approximately 2N) in Teflon vials. Extraction was performed by placing
the vials in a sonic bath for 30 minutes. After extraction, the solution was oxidized with
BrCI for one hour, converting all forms of Hg present into the inorganic, +2 oxidation
state. The sample was reduced with NH2OH and SnC12 was added to convert the Hg 2§ to
Hg ~ which is volatile and liberated from solution by bubbling with Hg-free N2. The Hg
released in this way was collected on Au-coated sand traps. The Hg was subsequently
analyzed using the dual-amalgamation CVAFS method described by Fitzgerald
et al.
(1979). A calibration curve was generated by spiking vials containing blank filters with
varying amounts of a 2 ng/mL standard (in 1% BrCI).
Flow rates through the sampling systems were measured using both calibrated
rotameters with filter packs used only for flow-tests to prevent contamination, and
162 G. KEELER, G. GLINSORN AND N. PIRRONO
frequently calibrated dry test meters. Sampling pumps with mass flow-controllers were
typically used to pull ambient air through the sampling equipment. All flow checking
devices are calibrated before and after all intensive field projects with primary flow
calibration equipment (e.g. spirometer).
2.1 QUALITY CONTROL AND QUALITY ASSURANCE
The UMAQL utilizes ultra-clean technique in all facets of the collection and analysis
of our environmental samples (Keeler
et al.,
1994). All equipment and supplies used in
sampling are rigorously acid-cleaned in a 5-step, 11-day procedure (Rossman and Barres,
1991). Sample bottles, Au-sand traps and glass-fiber filter containers are Teflon-taped
and triple-bagged before and after each use in the field. Particle-free gloves are always
worn when handing the samples in the field as well as in the Class 100 clean laboratory
at the University of Michigan.
Field and storage blanks were collected regularly with the particulate Hg samples.
The field blanks were collected by loading the acid-cleaned filter packs and assemblies,
connecting the sampling equipment, and then placing the filter pack assemblies or
impactors in the sampling box for two minutes without drawing air through the system.
Field and storage blanks for particulate Hg averaged < 7 pg Hg per filter (equivalent of
<0.17 pg/m 3 for a 24-hour sample). Storage blanks for particulate mercury were
obtained by placing an unused pre-fired glass fiber filter in a petri dish and shipping it to
UMAQL for analysis.
A reagent blank was analyzed on each day of particulate Hg analysis. The appropriate
amounts of reagents were analyzed to determine the contribution of the reagents to the
concentration of rig obtained for the sample. All samples were blank corrected using the
corresponding reagent blank analyzed that day. The detection limit calculated as three
times the standard deviation of reagent blanks, was less than 1 pg/m 3 for total particulate
Hg. Initially, all particulate Hg samples were routinely analyzed in duplicate, and more
recently, 50% of all samples were analyzed in duplicate. The analytical precision
calculated from these replicate analyses was better than 10% for the routine analysis of
Hg(p) in all of the studies. An initial analytical comparison was performed to compare
the UMAQL extraction techniques to INAA performed on a "whole" undigested sample.
Standard Reference Material No. 1648 from the National Institute of Standards and
Technology (NIST) was obtained for this purpose. Urban Paniculate Material (UPM)
was extracted using the routine protocol as well as by INAA at the MIT Nuclear Reactor
Laboratory. The two techniques gave equivalent results (1.02+.05 vs. 1.07+.1) for the
UPM (Olmez, personal communication). However, this does not guarantee that
atmospheric aerosol samples would behave identically, therefore, additional experiments
with collocated ambient filter samples are being completed to investigate this question.
3. Results and Discussion
3.1 MERCURY SIZE DISTRIBUTION IN URBAN DETROIT
The levels of vapor and particulate Hg have been previously measured in the City of
Detroit during a short duration study in 1992 (Keeler
et al.,
1994). Levels of particulate
PARTICULATE MERCURY IN THE ATMOSPHERE 163
Hg varied greatly from one site to the other with maximum concentrations at both sites of
greater than 1 ng/m 3. In the present study the atmospheric Hg levels were measured at
only one site during the spring of 1994. The fine and total Hg(p) concentrations
measured in Detroit for 18 consecutive days during March of 1994 are displayed in
Figure 1.
e~
E
O'J
250
200
150
1 O0
50
~-- r-- r-- r--
Figure 1. Fine and coarse particulate Hg concentrations measured in Detroit in March, 1994.
Ambient samples were collected every day during the study period for both fine
particles (< 2.5 pro) and for total suspended particulates (TSP). The average total
particulate Hg concentration during the study period was 94 pg/m-with a range of 22 to
225 pg/m 3. The percent of particulate Hg found in the fine size range (<2.5 p.m) varied
from about 60% to 100% during the 18 days of sampling. The mean %fine for the
period was 88% as compared to Cd, another anthropogenically derived element which
was measured concurrently, had an average of only 72% of its mass in the fine fraction:
The complete size distribution of the particulate Hg was measured with the MOI for
each of the 18 days of sampling in downtown Detroit. The results indicate that
particulate Hg was mostly collected on the fourth and fifth impactor stages. The mass
median diameters (MMD) for these stages are 0.60 and 0.18 Itm, respectively. Particles
less than 0.18 pan were collected on the back-up filter. The results from the
measurements in Detroit were compared to those compiled by Milford and Davidson
(1985) from three references to particulate Hg measurements made in the late 1970s and
early '80s. A total of five measurements in the three studies were utilized to calculate a
MMD of 0.61 Ima. The range in the five concentrations measured was 0.08-81 ng/m 3
with a geometric mean concentration of 1.9 ng/m 3. The MMD calculated from the 18
days of measurements in Detroit was 0.80 prn. The larger MMD observed in Detroit was
associated with a much lower mean concentration than the older studies reported in the
review paper by Milford and Davidson (1985).
The distribution of particulate Hg in Detroit was bimodal with an obvious fine and
coarse mode. The average particle size for Hg(p) in the fine and coarse fractions was
164 G. KEELER, G. GLINSORN AND N. PIRRONO
determined using stages one and two of the MOI to calculate the average particle size of
coarse fraction (> 2.5 ~tm), and stages three through six to calculate the particle size of
fine fraction (< 2.5 lun). The average particle size of the Hg(p) in each mode was 0.68
Inn and 3.78 pan for fine and coarse particles, respectively. The observation of the coarse
particle mode was somewhat unexpected as previous studies have suggested that Hg (p),
being primarily a combustion aerosol, should be submicron in size. In Detroit adsorption
of vapor phase Hg onto existing aerosols was apparent with a positive relationship
between Hg(p) and the total particulate mass in the atmosphere. The importance of the
coarse particle Hg can be seen in the relative contribution of these large particles to the
dry deposition flux. Modeling the dry deposition flux using the size distributions from
this study demonstrated that the flux of coarse particle Hg was 4-5 times greater than the
fine particle flux (Pirrone
et al.,
this volume).
An analysis of Hg(p) concentrations observed in Wayne County, MI at nine sites
revealed that Hg levels increased by 11% annually over the period from 1986-1992
(Pirrone
et al.,
1994). The significant increase in the annual particulate Hg levels was
directly related to a 3% annual increase in coal consumption in Michigan together with
an increase of 13% in the quantity of wastes being incinerated in the City of Detroit.
The concentrations observed in Detroit are similar to those recently reported for a
long-term study of atmospheric particles in urban areas of the United Kingdom (Lee
et
al.,
1994). Quarterly average Hg(p) concentrations were in the range of 90 to 540 pg/m 3
for the ten UK sites discussed. The highest Hg(p) concentrations were observed at a site
located near a smelter which also resulted in the highest concentrations observed for a
variety of other heavy metals. The levels of particulate Hg in the UK study as well as
those reported here are at the lower end of those reported for urban locations by
Schroeder
et al.
(1987). The elevated concentrations of particulate Hg in urban areas
suggests that more attention should be given to both nonferrous metal smelters and
incinerators as sources of Hg(p) to the atmosphere.
3.2 PARTICULATE MERCURY 1N RURAL MICHIGAN
In the previous section the levels of Hg(p) measured in the urban/industrial area of
Detroit, MI were discussed. The typical levels in the urban/industrial areas and the
variability of these levels was much greater than those typically observed at the more
rural sites in Michigan, Pellston (PEL), South Haven (SHA), and Ann Arbor (ANN).
Ambient measurements were performed every sixth-day for one-year at three rural sites
..... 3
m Michigan (Hoyer
et al.,
this volume). Parlaculate mercury levels averaged 10.5 pg/m
3 3 .
atPEL (n=47), 22.4 pg/m at SHA (n=52) and 21.9 pg/m at ANN (n=54). The range m
particulate mercury concentrations observed at the South Haven and Ann Arbor sites was
much greater than that recorded at Pellston (Figure 2).
Particulate Hg displayed a seasonal behavior at the rural Michigan sites. The
maximum particulate mercury concentrations were recorded during the winter and early
spring with a maximum 24-hour concentration of 32.2 pg/m 3 observed at Pellston (8
Apr. 94), 85.7 pg/m 3 at South Haven (20 Jan 94) and 76.9 pg/m 3 at Ann Arbor (8 Jan
94).
PARTICULATE MERCURY IN THE ATMOSPHERE 165
The range in the Hg(p) was also not as dynamic at the site in the northern-most part of
the lower peninsula of Michigan. The maximum concentrations of Hg(p) at Pellston in
northern Michigan were less than 50% of those observed at the two southern Michigan
sites. Particulate Hg concentrations exceeded 30 pg/m 3 only 2 times during the year of
measurement at Pellston. Air mass trajectories calculated for these days revealed that
elevated concentrations were associated with transport was from the urban areas to the
southwest and southeast to the site. Elevated Hg(p) measured at South Haven and Ann
Arbor were typically associated with transport from the east and the southwest.
5O
,15
A
~ 35
,- 30
o
25
9 1= 20
e,.
(p
o 15
e.
0
10
0
5
0
DATE
Figure 2. Monthly averaged particulate Hg concentrations at three Michigan sites.
3.3 PARTICULATE MERCURY IN RURAL VERMONT
Atmospheric Hg samples were collected twice per week with a total of 103 particulate
phase Hg samples collected during 1993. The annual arithmetic average Hg(p) and
vapor phase Hg concentration was 11.2 pg/m 3 and 2.0 ng/m 3, respectively. While the
vapor phase concentrations at Underhill displayed no strong seasonal behavior a seasonal
trend was observed in the Hg(p) with elevated concentrations during the winter months.
This was especially evident in February where all samples were above the annual
average. Increasing concentrations in November and December of 1993 provide further
support for a seasonal influence on particulate Hg concentrations at this site. A similar
increase in the concentration of other metals such as As and Se measured at Underhill
(NESCAUM data) during the winter months was also observed. A similar increase in
particulate Hg concentration during the winter months was observed at the rural sites in
Michigan, as discussed in the previous section.
The seasonally averaged particulate Hg concentrations further illustrate this trend
(Table I). The average for the winter months is significantly greater than the annual
166 G. KEELER, G. GLINSORN AND N. PIRRONO
mean, and the spring and autumn averages are somewhat higher than the average for
summer.
Concurrent measurements of Hg in precipitation and ambient air (daily vapor and
particulate) were obtained at the Underhill site (Burke
et al.,
this volume). Simple
correlations were calculated between ambient and precipitation concentrations for the
sampling days when precipitation occurred during ambient measurements. Ambient
particulate Hg was correlated with reactive Hg in precipitation (r=0.654, p<0.02, n=13).
TABLE
I
Seasonal averages for particulate Hg at Underhill, VT during 1993
Season N Hg(p)
(pg/m 3)
Winter 24
15.8
Spring 26 9.7
Summer 26 9.4
Autumn 25 10.0
3.4 PARTICULATE MERCURY IN BROWARD COUNTY SOUTH FLORIDA
Particulate Hg samples were taken concurrently at three sites during the period 25
August to 7 September (Dvonche
et al.,
this volume). The average concentrations at the
inland locations, sites 2 and 3, were 51 pg/m 3 and 49 pg/m 3, respectively. The average
Hg(p) measured at site 1 near the beach, located about 9 km east of sites 2 and 3, was 34
pg/m 3. Particulate phase Hg comprised less than 5% of the total atmospheric Hg (vapor
and particulate) which was consistent with values reported in northern locations (Burke
et al.,
this volume). The levels of rig(p) in_Broward County were generally higher than
those typically measured (about 10-30 pg/m ) at the rural sites m the Great Lakes Basin,
discussed earlier. The levels measured in Broward County were not as high as other
measurements made in large urban/industrial source areas such as Detroit, where short-
term average particulate Hg concentrations were found to be near 100 pg/m 3. However,
the elevated levels of particulate Hg observed in Broward County during were higher
than those measured at rural sites with concentrations never exceeding 100 pg/m 3. The
elevated levels in Broward county are suggestive of a local source influence. The
elevated levels of Hg(p) in South Florida were somewhat surprising. While the average
vapor phase Hg levels were 2-3 times higher in Broward County than those measured
elsewhere, the PM10 and TSP levels were not significantly elevated with typical
concentrations of PM10 in the range 15-20 ~tg/m 3 .
PARTICULATE MERCURY IN THE ATMOSPHERE 167
4. Conclusions
Measurements of Hg(p) documented the importance of this form of the compound in
the atmosphere. The levels and behavior of Hg(p) was investigated at several locations
in the Great Lakes, Lake Champlain basin, and in South Florida. The magnitude and
particle size of the observed Hg(p) varied dramatically from site-to-site as well as from
day-to-day. Seasonal variability was observed in the levels of Hg(p) with higher
concentrations typically found in the winter months than those measured in the summer.
The observed correlation between the operationally-defined reactive Hg species and CI"
in precipitation provides support for the speculation that this species may be HgCI 2.
Also, the correlation observed between ambient particulate Hg and reactive Hg species in
precipitation on days when ambient measurements were conducted and precipitation
occurred, implies that this species may be associated with particles.
The average particle size of the Hg(p) measured in each mode was 0.68 lam and 3.78
~tm for fine and coarse particles, respectively. The observation of the coarse particle
mode was somewhat unexpected as previous studies have suggested that Hg(p) should be
submicron in size. Near source adsorption of vapor phase Hg onto existing aerosols was
apparent in Detroit with a positive relationship between Hg(p) and total particulate mass
in the atmosphere. The importance of large particle Hg(p) should not be underestimated
in determining the dry deposition flux. Modeling the dry deposition demonstrated that
the flux of coarse particle Hg was 4-5 times greater than the fine particle flux in this
study. The relative importance of Hg(p) in the fine and coarse fractions to the total dry
deposition flux was site specific and varied with time and meteorological conditions.
Acknowledgments
This work was supported by the Michigan Great Lakes Protection Fund, the University of Michigan, Cooperative
Institute of Liminology and Ecosystems Research (CILER) under cooperative agreement from the Environmental
Research Laboratory (ERL), National Oceanographic and Atmospheric Administration (NOAA)., US Department of
Commerce under Cooperative Agreement No. NA90RAH00079, the State Of Florida Dep~ent of Environmental
Protection, and cooperative research agreements with the US EPA -Atmospheric Research and Exposure Assessment
Laboratory (AREAL). The success of these projects was the result of the hard work of our dedicated site operators:
Robert Vande Kopple (Pellston), Mary Barden (South Haven) and Katherine Beverstock (Dexter), Joanne
Cummings and Carl Waite (Underhill, VT). We would also like to acknowledge the contribution of several
researchers at the University of Michigan Air Quality Laboratory who performed the field sampling and sample
analysis including Janet Burke, Tim Dvonch, Marion Hoyer, Anne Rea, and Alan Vette.
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