min for 4 min. All of the dimethylmercury was recovered on the gold siection. No methylmercury(I1) chloride was produced. Dimethylmercury is rapidly drawn through all of the absorption tube sections thus allowing little time for reaction. A similar experiment was conducted with 1 ng of mercuric chloride and 3-ng samples of elemental mercury. No loss of mercury metal was noted indicating no reaction between it and mercuric chloride. No indication of reaction has been noted between dimethylmercury and elemental mercury or between mercuric chloride and methylmercury(I1) chloride. A limitation of the method is the necessity of considerable care in handling of the absorption tubes prior and during analyriis so as to avoid mercury contamination. The authors found it necessary to do the analytical work in an office area (100 ng of Hg/m3 in air) to avoid contamination in their usual laboratory area (300-800 ng of Hg/m3). Handling of mercury compounds and spilling the more concentrated stock solutions also should be avoided. Finally, for the most accurate work, occasional performance tests of the Chromosorb-W columns and blank determinations must be made. Applications. This method has been used to study the distribution of forms of mercury out of doors ( 5 ) and in houses and buildings. Data on the distribution of mercury in a large lecture hall and in the home of one of the authors (RSB) are given in Table IV. Elemental mercury is the major form found but methylmercury(I1)-type compounds are also found. Total mercury in buildings and homes is much higher than outside ambient mercury in air (3-6 ng/m3), also noted by Foote (10) but without speciation data. ’4lthough elemental mercury is the predominant mercury form found, significant amounts of the methylated mercury compounds are also found. With reasonable care, this method is suitable for the
analysis of ambient forms of mercury in air down to the 0.5-ng/m3 concentration. Analyses require approximately 0.5 hr per stack of absorption tubes and sampling times u p to 2 hr for the lower ambient concentrations of mercury. It is not a continuous analysis method. Nevertheless, information on the distribution of volatile forms of mercury in air can be obtained in a study area, something heretofore unavailable. Further work on the use of the technique in environmental chemistry studies is under way.
Acknowledgment The authors wish to acknowledge the assistance of the following individuals who helped in the development of the technique: A. D. Shumaker, J. L. Bricker, M. Ammons, and C. C . Foreback. Literature Cited (1) Imura, N., Pan. E . S. S., Kim, K . S . J., Ukita, T. K. T., Science, 172,1248-9 (1971). ( 2 ) Geological Survey Professional Paper 713, Mercury in the E n vironment. 1971. (3) Long, S. J.. Scott, D. R., Thompson, R. J . , Anal. C h e m , 45, 2227-33 (1953). 14) “Instrumentation for Environmental Monitoring-Air.” Environmental Instrumentation Group. Lawrence BeFkeley Laboratory, May 1, 1972. ( 5 ) Johnson, D . L., Braman, R. S., Enuiron. Sei. Technol. 8, 1003 11- 974) _ .
(6) Braman, R. S.,Dvnako. A,, Anal Chem 40,95-106 (1968) ( 7 ) Braman, R. S., i i i d , 13,1462-7 (1971). (8)Foreback, C. C., PhD Thesis, “Some Studies on the Detection and Determination of Mercury, Arsenic and Antimony in Gas Discharges.” University of South Florida, Tampa, Fla., J u n e 1973. (9) Braman, R. S., Foreback, C. C., Science, 182,1247-9 (1973). (10) Foote, R. S., ibid., 177,513-14 (1972). Receiced for recieu. January 7. 1974. Accepted June 24, 1974. Presented at 166th Annual Meeting o f the American Chemical Socie t ) . Au,gust 26-31, 197c3. This uork teas supported b\ ,Vational Science Foundation Grant GI-.34794X (RAiV.\’Proeram).
Distribution of Atmospheric Mercury Species Near Ground David L. Johnson” and Robert S. Braman
Department of Chemistry, University of South Florida, Tampa, Fla. 33620
A recently developed technique makes possible the routine analysis of atmospheric samples for “particulate” and “volatile” mercury. The “volatile” fraction can be analyzed for se7ieral chemical species. This work presents the results of some Tampa Bay area analyses and diurnal studies of atmospheric mercury speciation. The mercury in air in the area investigated was primarily “volatile” (>go%) and was composed of significant proportions of mercury(I1)-type compounds, methylmercurv(I1)-type compounds, and elemental mercury. Dimethylmercury was rarely observed. Results were quite variable suggesting a variety of sources and irregular wind transport processes. The data indicate that background mercury concentrations and the percentage distribution of mercury species in air in a local area may be established by mercury emanation:; from the ground or from adjacent bodies of water.
Mercury is a prominent environmental pollutant with a variety of natural and man-made sources (1-6). Because of the high volatilitv of elemental mercury and some of its compounds, it is widely dispersed in the earth‘s atmosphere. Nevertheless. few data are available describing its distribution between volatile and particulate fractions and no data exist relating to the chemical forms of mercury in air. The large number of volatile and stable organic and inorganic mercury compounds, as well as the apparent extensive involvement of mercury, in biological systems makes it imperative that we obtain speciation data in order to acquire an adequate understanding of the geochemical cycling of mercury, the extent to which man is altering that cycle, and the degree to which toxic mercury compounds pose an environmental threat. While the concentration of total mercury in terrestrial air free of obvious pollution is generally in the range of 1-10 ng Hg/m3 (7-9). the total range of atmospheric mercury values reported in the literature is almost lo7! \Villiston observed a moderate variation. two- to fivefold. in Volume8, Number 12, November 1974
1003
atmospheric mercury a t a single location during the course of a windy day in the San Francisco Bay area. This probably reflects wind transport from various local sources. The burning of fossil fuels, smelting operations, and general industrial activity all contribute mercury to the air, and Williston noted that elevated mercury concentrations coincided with high smog levels. A number of natural sources also release mercury. Measurements in air over the Hg-mineralized area near Ord mine in Arizona showed up to 600 ng Hg/m3 (9). Neville (IO) reported levels of over 106 ng Hg/m3 in a mercury mine. In Hawaii and Iceland, studies indicate that volcanic activity increases local mercury in air by as much as 100,000-fold over the ambient levels mentioned above. Mercury metal, mercuric oxide, and mercury halides are all potential chemical forms of mercury emanations from both natural and man-made sources. A wide range of inorganic mercury compounds can be methylated by biological systems, and methylation is known to occur in soil, sediments, and aquatic organisms ( I 1-13 and others). This may lead to the release of dimethylmercury and methylmercury(I1) compounds to the atmosphere. Evidently biodemethylation reactions are also widespread (14 and others). Further, dimethylmercury reacts with mercuric chloride (disproportionates) to produce methylmercury(I1) chloride. An extremely sensitive analytical technique for the several chemical forms of mercury a t ambient levels in the atmosphere has been developed. Details of the analytical methods are given elsewhere (15). We present here the results of some limited Tampa Bay area survey studies and diurnal studies. The occurrence and distribution of particulate mercury [Hg,, 1, defined as that portion of the total retained by a Gelman Instrument Co. Type A glass fiber filter, mercury(I1)-type compounds [Hg(II)], methylmercury(I1)-type compounds (MMC), mercurv metal (Hg”), and dimethylmercury (DMM) are reported. Experimental Samples of air were drawn through a connected series of 10-mm quartz tubes containing selective absorbers for the various chemical species of atmospheric mercury. The first component is a Gelman Instrument Co. Type A glass fiber filter to retain “particulate” mercury. The “volatile” mercury species, which pass this filter, enter a tube containing 45-60 mesh Chromosorb-W which has been siliconized (5% by weight SE-30 methyl silicone) and then treated with HCl vapors. This acid-washed column retains Hg(I1)-type compounds and passes other volatile mercury forms. Beneath this is a second Chromosorb-W column treated with sodium hydroxide to retain methylmercury(I1)-type compounds while passing Hg” and DMM. Mercury metal is collected by forming an amalgam with silver on silvered, 60-80 mesh glass beads. Dimethylmercury passes through the silver and is retained by a column of glass beads coated with gold. In the development of the series of selective absorption tubes (speciation stacks), mercuric chloride and methylmercury(I1) chloride were used as the “model” compounds for, respectivelv, mercury(I1)-type and methylmercury(I1)-type compounds. [In validating the analysis technique, mercury compounds other than the “model” types were also tested ( 1 5 ) . ] The reader is reminded that the determination of mercury speciation in this study is based on an operational definition. Consequently, any mercury from environmental samples on the Chromosorb-W (NaOH) tube is called methylmercury(I1) type. Mercury found on the Chromosorb-W (SE30, HC1) tube is called mercury(I1) type. A 2-cm goldcoated glass bead column is used when measurements of total mercury only are required since gold will amalga1004
Environmental Science & Technology
mate with all of the above-mentioned mercury compounds. The components of this stack are connected together with 1.0 cm of PTFE Teflon sections and taped to ensure a leakproof fit. For sampling, the speciation stack was fitted into a pump trap-a short column containing silvered glass beads to protect against back diffusion from the pump-and air was drawn through the stack by means of a small diaphragm pump which created a vacuum of about 25 in. of Hg. Air flow through the stack was about 1.5 l./min and was monitored by means of a rotameter inserted between the pump trap and the pump. Calibrations of flow rate through the speciation stacks were made with a Precision Scientific Co. wet test meter. Corrections for temperature and water saturation were made in accordance with ASTM test ZD 3195-73. The mercury collected by the various components of the speciation stack was analyzed by dc discharge emission spectroscopy (16, 17). Sections of the speciation column were placed in line with the helium carrier gas and heated to desorb or deamalgamate the mercury they contain. Mercury was swept into the discharge tube, excited by a helium plasma, and the Hg emission line a t 253.65 nm was observed in a conventional spectrometer, photomultiplier, amplifier, and recording system. Standardization was achieved by the use of 100-500 p1 samples of air saturated with Hg” vapor a t a known temperature. The noiselimited limit of detection of this system is approximately 5 x 1 O - l z gram of Hg. Blanks for the various sections of the speciation stack vary. The glass fiber filters for particulate collection were prefired before use and thereafter exhibited blank values of less than 5 X 10 l2 gram of Hg. The gold tubes had similar values. Blanks for the silvered glass bead columns, base-treated Chromosorb-W, and acid-washed Chromosorb-W tubes averaged 5000 vehicles/hr o n 1-75 Wind, N E 2-8 K t , 23.5”C 33°C W i n d , E 1-2 K t ; 20.5”C 34°C Wind, N E 2-5 Kt; 25°C Wind, S E 5-10 K t ; 34°C Wind, N E 5-7 K t ; 24°C Wind, S 10 K t ; 28°C Wind, E 1-10 K t ; 22°C Wind, SE 5 K t ; 29°C Wind, E 4-10 K t ; 21°C Wind, S E 10 K t ; 30°C Wind, E 4-8 K t ; 24°C Wind, SSE 5-10 K t ; 26°C Wind, E N E 1-3 K t ; 20.5”C Wind, SW 5 K t ; 30°C Wind, N N E 5-10 K t ; 25°C
pumpings; 3 November--2-hr purnpings. For precision, see text.
Volume8, Number 12, November 1974
1005
than 2.5 days during which 33 consecutive 2-hr samples were taken using the speciation stacks a t a height of 1 meter above the ground surface. Periodically air was sampled for total mercury a t the same time as the speciation stack but 1-2 meters away. A 2-cm gold tube was used. This comparison was made on 10 samples. The mean value for the speciation stacks was 6.8 ng Hg/m3 and that for the 10 gold tubes was 7.4 ng Hg/m3. These two means are not significantly different a t the 99% level; however, some of the individual comparisons showed differences in excess of a factor of two. Figure 2 shows the results of the diurnal study. In the lower portion of the figure is plotted total mercury as a bar graph indicating that each sample was integrated over a 2-hr period. The error bars indicate an uncertainty of 1 std dev. The smooth curve is a running average of three consecutive 2-hr samplings and is intended to help illustrate the general trend in total mercury with time. Values for Hg(1I) type, MMC type, and Hg” are plotted in the upper portion of Figure 2. Uncertainties in these determinations, while not indicated graphically, are &670 for mercury metal, *l8% for mercury(I1)-type, and &13% for methylmercury(I1)-type compounds a t 1 0. Dimethylmercury was observed in only nine of the 33 samples with an average value of 0.4 ng/m3 for the nine samples in which it was found. “Particulate” mercury was observed more frequently, averaging 0.4 ng/m3 (0.0-1.4), but showed no obvious trends with time of day. As in the geographic study, the data in Figure 2 show considerable variability. In view of what we believe to be reasonable estimates of the analytical uncertainty, a significant portion of this variability would appear to be real. We consider only two of the 33 samples to be questionable-the sample from 16h00-18h00 on September 25 which was 80% MMC type, and the 00h00-02h00 sample on September 26 which showed 42 ng/m3 of Hg(I1) type, and 23 ng/m3 of Hg”. Such variability of the results makes the elucidation of possible trends with time somewhat difficult. In
0 Hg”
OMMC
AHg(ll)
..
CmM
123c 24SEPT
Figure 2.
‘BOO
ow0
06W
200
18OC
“
0:an
I
0601
iiSt’T
Diurnal variation
of
,Ir 25 i i i -
1mc
laOD
WOUi
D””
total mercury and mercury species
at station 1
this study, however, total mercury was consistently higher a t night than in the daytime. This also appeared to be true for mercury metal, and mercury(I1)-type and methylmercury(I1)-type compounds. Table I1 shows the mean values for daytime and nighttime concentrations of Hg(I1) type, MMC type, and Hg”, as well as the other species measured. The differences in these mean values for all but DMM and “particulate” mercury are statistically significant a t the 95% confidence level. In contrast to the concentrations, the percentage distribution of the mercury species in the day vs. night groupings do not show significant differences. Figure 3 shows the short-term variability of total mercury which was investigated a t station 2. Two gold tubes were alternately pumped and analyzed on a 10-min time scale. The field apparatus was located about 10 meters from the entrance of the laboratory building and 20 meters from the edge of a parking lot, with the tip of the sampling tubes approximately 0.5 meter above the grass. Between llh50 and 13h00 on October 5 , a third 2-cm gold sampling tube was pumped tip-to-tip with the two which were switched every 10 min. During this period, the single tube pumped for $0 min indicated an average mercury content of 6.3 ng/m3, while the seven 10-min samples gave an average of 5.7 ng/m3 which includes the one value which reached 24.4 ng/m3. If this extreme value were excluded from the 70-min average, a concentration of 4.4 ng/m3 would be computed. Between the hours of 20h00 and 21h00 on October 8, a thunderstorm occurred. The rain appears not to have affected the total mercury content of the air. During all three segments of this study a noticeable wind was blowing except between 05h30 and 07h00 on October 9. The nine samplings in this calm period exhibited a maximum deviation of * l l % from the mean value, as compared with the &lo% uncertainty a t the 2-rr level for this study. To assess the possible effects of near-ground micrometeorological conditions on our sampling procedures, the diurnal variation of total mercury in a 10-meter vertical profile was observed a t station 2. Here sampling was conducted utilizing a 35-ft tower of 1.25-in. diam antenna masting hinged a t ground level so that it could be raised and lowered to change the gold sampling tubes. Samplers were connected to pumps on the ground by means of tubing and when the tower was in the raised position air was drawn directly into the gold tubes from heights of 0.1, 1.0, 5.0. and 10.0 meters above the ground. To obtain nearly continuous sampling, two sets of four gold tubes each were employed, and the flow rate through each tube in a given position on the tower was measured. Table I11 gives the results of 32 profiles taken over a 28-hr period. For samples 1-6, results were 10-20 times higher than total mercury over the profile for the rest of the study-perhaps representing contamination from a local source, or the ending of a high pollution inversion condition in the Tampa area. They are not considered further here. Samples 7 through 32 averaged 3.2 ng Hg/m3 as determined by integrating the area under each profile. Over half of these samples showed a pronounced gradient
Table II. Mean Values of Day and Night Mercury Species Concentration. and Percentages of Total Mercury a t Station 1 for Period September 24-26, 1973 Time
Hgrv
Hg(ll) type
0.27 (6%) 0.86 (19%) 0.63(14%) 0.17 (2%) 1.581 (19%) 1.56 (19%) ngjms. ’1 E x c l u d i n g the values for t h e s a m p l e 0000-0200 hr S e p t e m b e r 26.
Day
Night
0
MMC type
1006
Environmental Science 8. Technology
Hgo
2.67 (60%) 5. 03b (60%)
DMM
1: Hg
No. of values
0.05 (1%)
4.48 8.40
19 13
0.06(
