Environ. Sci. Technol. 2005, 39, 9253-9262
Automated Speciated Mercury Measurements in Michigan MARY M. LYNAM* AND GERALD J. KEELER University of Michigan Air Quality Laboratory, Ann Arbor, Michigan 48109
Automated speciated mercury measurements were made at a rural (Dexter, MI) and an urban (Detroit, MI) site in Michigan during selected times from 1999 to 2002 to assess the concentrations of elemental (Hg0), reactive gaseous (RGM), and particulate mercury (Hgp) in these environments. Here we present the first-ever reported values for RGM in Michigan. Median RGM concentrations were 2.21-2.93 pg m-3 at Dexter and were 3-11 times higher in Detroit at 6.41-22.0 pg m-3. Maximum RGM concentrations of 38.7 pg m-3 and 270 pg m-3 were observed in Dexter and Detroit, respectively. Measured RGM/Hg0 ratios were in the range of 0.04-11.60% indicating that at times RGM comprises greater than the currently held view of 5% of total gaseous mercury in the air. Well-pronounced diurnal patterns of RGM were observed at the rural site, whereas the urban site exhibited patterns that were influenced by nighttime emissions and regional transport. An analysis of RGM/Hgp ratios at the urban site when combined with trajectory analysis suggests that the site receives mercury inputs from both local and regional sources. Episodes of elevated ozone concentrations which were accompanied by increases in RGM concentrations were observed to occur in the late afternoon and overnight. These may be evidence of advection of ozone and RGM over long distances to the site.
Introduction In its 1998 Mercury Report to Congress, the U.S. EPA provided evidence of a causal link between anthropogenic releases of mercury, Hg, from industrial and combustion sources in the United States and the presence of methylmercury in fish tissue (1). Methylmercury, when consumed in the diet, is readily absorbed into the blood stream and is distributed to all tissues including the brain (2). Additionally, it can pass through the placenta to the fetus and to the fetal brain. Mercury is a powerful neurotoxin and the effects of exposure to methylated forms have been studied in populations whose sole source of protein is fish (3, 4). Mercury thus presents potential for great harm to public health as well as to top predator species such as wading birds (5) and Florida panthers (Felix concolor coryi) a sub-species of the puma species (Felix concolor) (6). There are three environmentally relevant species of mercury in the ambient air. Elemental mercury, Hg0, is present in ng m-3 concentrations and is believed to have a lifetime on the order of months (1). Divalent reactive gaseous mercury, RGM, is present in pg m-3 concentrations and is * Corresponding author phone (919) 541-1120; fax (919) 541-0960; e-mail
[email protected]. 10.1021/es040458r CCC: $30.25 Published on Web 11/03/2005
2005 American Chemical Society
believed to deposit on a local scale (on the order of 100 km from its source), while particulate phase mercury, Hgp, also present in pg m-3, has a lifetime which is particle-size dependent (1). The discovery of mercury-contaminated fish in pristine lakes not proximate to major sources of mercury suggested that inputs of mercury were being received by these lakes as a result of atmospheric deposition (7, 8). Atmospheric deposition of mercury has subsequently been verified to be a principal pathway for contamination of the aquatic environment in the Great Lakes region (9, 10). The Lake Michigan Urban Air Toxics Study and the Atmospheric Exchange over Lakes and Oceans study provided evidence that near-shore urban areas are contributing significantly to atmospheric deposition of pollutants to adjacent water bodies (11). Modeling studies carried out using monitoring data collected during the Lake Michigan Mass Balance Study (LMMBS), calculated that 84% of the mercury entering Lake Michigan was predominantly from the atmosphere (12). Results from hybrid receptor modeling estimated that the contributions from wet and dry deposition to Lake Michigan were almost equivalent, 614 ( 186 kg (10.6 ( 3.2 µg m-2) and 559 ( 177 kg (9.7 ( 3.1 µg m-2), respectively, on an annualized basis. A 1996 study conducted in Detroit, MI, directly measured wet and dry deposition and found approximately equal amounts of mercury (0.7-3 µg m-2 month-1) were deposited by wet and dry deposition to the surrounding area (13). During LMMBS, dry deposition due to divalent reactive gaseous mercury, RGM, was estimated to be 676 ( 156 kg (88% of the dry deposition). The overall uncertainty in this deposition estimate was unknown because at the time the study was conducted ambient RGM measurement methodologies were not routinely available. Furthermore, source apportionment analysis for the Detroit study revealed that more than half of the particulate mercury could not be attributed to sources (13). The unexplained portion was hypothesized to be related to RGM adsorption to particulate matter. Again however, the capacity to monitor this species was not available during the study. The findings from both these studies illustrated the critical need for speciated mercury measurements, in particular, RGM measurements. A major recommendation of both the LMMBS and the Detroit studies was the fundamental need to monitor RGM in urban areas adjacent to water bodies to accurately access the contribution of this species to atmospheric deposition in these areas (12). RGM is present in trace amounts (pg m-3) in the atmosphere and is believed to exist in the gas phase or associated with particles (14). It is 5 orders of magnitude more soluble in water than the predominant form gaseous elemental mercury. This characteristic solubility combined with its high surface activity engenders the likelihood that this species will undergo both wet and dry deposition (15) and may dominate the depositional flux of mercury to the environment (16). Previously an expert panel, as well as the U.S. EPA, have articulated the need to research the amount and primary form of RGM emitted from sources as well as information regarding atmospheric levels of mercury at receptor sites proximate to anthropogenic sources (1, 17). This information is required in order to understand what becomes of these primary emissions chemically and physically. Stack testing has shown that greater than 90% of the mercury emissions from municipal and medical waste incinerators was RGM (18); recent stack sampling in coalfired power plants (19, 20) has revealed that RGM constitutes from 40 to 80% of the mercury species in the flue and RGM VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Automated Speciated Mercury Measurements in North America location
monitoring site
reference
Barrow, AK Pompano Beach, FL Cheeka Peek, WA St. Francois, PQ Tuscaloosa, AL
marine marine marine rural urban
Lindberg et al., 2002 (24) Malcolm et al., 2003 (25) Weiss-Penzias et al., 2003 (26) Poissant et al., 2004 (27) Gabriel et al., 2005 (28)
concentrations as high as 485 pg m-3 were found in the cell room of a mercury cell chlor-alkali plant (21). In the late 1990s manual methods including a multi-stage filter pack method (22), a refluxing mist chamber method (16), and a KCl-coated denuder method were developed for the measurement of RGM (23). Soon after, automated highresolution (1-2 h) mercury speciation methods became possible with the introduction of the Tekran 1130/1135 Hg speciation unit (Toronto, Canada). Table 1 provides a summary of published data from automated Hg sampling campaigns carried out in North America. Notably, the majority of these published studies are from pristine areas while only a single urban study has been carried out in a temperate to subtropical southeastern U.S. airshed (28). Here, we present automated speciated mercury measurements from two sites in Michigan made in response to the call for information on mercury concentrations in the vicinity of anthropogenic sources. These sites are representative of rural and urban locations in Michigan and have differing source inputs. Michigan’s location in the temperate middle latitudes, far from any oceanic influence, gives rise to its continental climate that is reflected in a daily weather pattern greatly influenced by the passage of different air masses that flow across the midwest. These speciated measurements are the first of their kind made in Michigan using automated sampling. They provide higher resolution data for improvement and validation of models and should offer additional insight on an urban airshed in this area of the Great Lakes. They contain 20% of the world’s freshwater, have 33 million inhabitants in the surrounding watershed, and, based on modeling studies, are predicted to have among the highest rates of atmospheric mercury deposition from anthropogenic and global contributions in the United States (1).
Materials and Methods Site Characterization. Two sites were selected for automated monitoring: one located in Dexter, MI approximately 83 km west of the city of Detroit, while the other was located in the city of Detroit. The Dexter site (42° 24′ N, 83° 54′ W) is one of 70 monitoring stations across the United States that comprise the Clean Air Status and Trends Network (CASTNET). Established in 1987, the stations are predominantly in rural areas with a goal of providing information on background levels of pollutants where urban influences are minimal. Sampling at Dexter was carried out on a platform 2 m above ground. The Detroit site (42° 19′ N, 83° 05′ W) was located in a heavily industrialized portion of the city and monitoring was conducted on the roof of a mobile laboratory at an elevation of 6 m. This site was in close proximity to the Rouge Industrial complex and the Ambassador Bridge, which experiences heavy cross-border traffic between the United States and Canada. Figure 1 shows the locations of the site and the surrounding sources. Sampling and Analysis. Sampling was carried out from November 2000 to March 2001 (Winter) and May 2001 (Spring) at Dexter, MI and for two-week intensive sampling periods in July 2000, September 2000, July 2001, and July 2002 in Detroit, MI using the Tekran (Toronto, Canada) 2537A, 9254
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1130, and 1135 mercury speciation units for the determination of Hg0, RGM, and Hgp as described by Landis et al (23). The 1130 unit was configured to capture 1-h integrated RGM samples on a KCl-coated quartz annular denuder at a flow rate of 10 L min-1. Concomitant with 1-h sampling for RGM, 5-min Hg0 samples were continuously quantified by the 2537A analyzer. Upon completion of the 1-h sampling, the sampling lines were flushed with mercury-free air followed by heating of the particulate unit to 800 °C for Hgp desorption and quantification as Hg0. The RGM collected on the denuder was then thermally desorbed at 500 °C and quantified as Hg0. Freshly coated denuders were replaced on a weekly basis. Internal calibrations were performed on a daily basis using an internal permeation tube. Calibration of the permeation tube was carried out prior to each intensive campaign. The detection limit for RGM was 4.00 pg m-3 expressed as three times the standard deviation for the blank. Meteorological and chemical variables were also monitored continuously at the study sites. Ozone was monitored using a Thermo Environmental model 49 UV photometric analyzer. Data processing was carried out using SAS v. 8.1 (SAS Institute, Cary, NC). Tests for normality in the distributions were carried out using the Shapiro-Wilk test and were found to violate the assumptions of normality. Therefore, summary statistics are presented as median, variance, and range. Nonparametric tests were performed at a significance level of R ) 0.05. Trajectory analysis was carried out using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HY-SPLIT) model (29).
Results and Discussion Dexter Mercury Speciation. Table 2 summarizes the Hg data from the Dexter, MI site for both the winter and spring seasons. The median values for Hg0 during winter and spring were found to be 1.51 and 1.49 ng m-3 respectively, and are in agreement with typical values observed at rural locations (1). Median Hg0 concentrations for winter and spring at the Dexter site were found to be statistically different, p < 0.0001 (Wilcoxon test). Table 3 presents observed concentrations at various rural monitoring locations in the Great Lakes environs. It can be seen from this table that our results from automated sampling of Hg0 in Dexter compare well with previous results from manual sampling at Dexter and at other sites around the Great Lakes. Median levels for RGM in Dexter were in the range of 2.00-3.00 pg m-3 which are below the detection limit of 4.00 pg m-3, while maximum levels reached 38.7 pg m-3 during the spring at rural Dexter. There was a slight increase in the median, 2.9 pg m-3, and a 20% increase in the maximum RGM concentration (38.7 pg m-3) on going from the winter to spring months. This may be a reflection of higher levels of photochemistry and/or higher concentrations of gas species due to higher ambient temperatures. RGM concentrations (geometric mean) measured at three rural sites in New York were 2.5, 3.4, and 3.2 pg m-3 at the Potsdam, Stockton, and Sterling sites, respectively (33). These values are based on 24-hour integrated samples made using KClcoated manual denuders and are in good agreement with median concentrations observed at the rural Michigan site using the automated system which collects 1-h samples. Maximum summertime RGM concentrations at the New York sites were higher (60.0-80.0 pg m-3) than those observed during the spring in Dexter. In the bay St. Franc¸ ois wetlands, mean RGM concentrations of 3.6 pg m-3 were observed, while maximum concentrations were found to be 22 pg m-3 (27). Correlation between elemental mercury and reactive gaseous mercury at the Dexter site was not significant during the winter season (p ) 0.174) and weakly negatively correlated, r ) -0.174, (p ) 0.037), during spring.
FIGURE 1. Location of sampling sites used in this study, including major sources of mercury in the vicinity of the Detroit sampling site.
TABLE 2. Hg0 and RGM Concentrations (Corrected to Standard Temperature and Pressure) in Dexter, MI Hg0 (ng m-3)
RGM (pg m-3)
season
N
median
variance
range
median
variance
range
winter (Nov 1999 - Mar 2000) spring (May 2000)
169 175
1.51 1.49
0.099 0.020
1.09-4.39 1.20-1.94
2.21 2.92
19.0 20.0
0.19-31.0 0.30-38.7
Detroit Mercury Speciation. Table 4 summarizes the data from Detroit, MI during the four intensive sampling periods for the three species measured. Although measurements made at Dexter and Detroit were not made simultaneously it is still useful to compare them to gain some insight into the differences in Hg concentrations between a rural and an urban site in Michigan. Elemental Mercury. Median elemental mercury concentrations (1.68-3.13 ng m-3) were found to be higher by as much as a factor of 2; the maximum observed concentration of 39.53 ng m-3 was 10 times higher in Detroit in comparison to that in rural Dexter. This finding points to considerable enhancement of elemental mercury above the rural background level and indicates that the site is being impacted by
TABLE 3. Total Gaseous Mercury Concentrations at Rural Locations around the Great Lakes
location
total gaseous Hg (ng m-3)
date
ref
rural Great Lakes Dexter, MI Eagle Harbor, MI Perch River, NY Potsdam, NY Stockton, NY Sterling, NY St. Franc¸ ois, PQ
1.6-2.1 1.5 1.2 2.5 1.51 1.48 2.40 1.4
1994-1996 1997-1998 1997 1991-1992 2000-2001 2000-2001 2000-2001 2004
30 31 31 32 33 33 33 27
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TABLE 4. Hg0, RGM, and Hgp Concentrations (Corrected to Standard Temperature and Pressure) in Detroit, MI Hg0 (ng m-3)
July 2000 Sept 2000 July 2001 July 2002
RGM (pg m-3)
n
med (var)
range
med (var)
range
med (var)
range
126 99 133 63
1.82 (11.94) 1.68 (2.65) 1.99 (2.81) 3.13 (5.42)
1.17-39.53 1.09-15.74 1.39-13.97 2.35-20.76
6.41 (102) 9.71 (335) 9.14 (1011) 22.0 (112)
0.64-51.0 0.62-155 1.94-270 9.49-52.0
N/A 18.3 (135) N/A 19.8 (63.0)
N/A 5.70-60.1 N/A 8.43-46.8
TABLE 5. Meteorological Parameters and Ozone Concentrations (average ( std. dev.) during the Four Hg Sampling Campaigns in Detroit
July 2000 Sept 2000 July 2001 July 2002
temp (°C)
relative humidity (%)
wind speed (m s-1)
ozone (ppb)
21 ( 3 15 ( 5 25 ( 4 21 ( 3
74 ( 15 73 ( 14 72 ( 17 68 ( 12
0.9 ( 0.6 1.2 ( 0.6 1.1 ( 0.6 1.5 ( 0.6
28 ( 17 17 ( 14 40 ( 25 32 ( 16
local and regional sources. Gildemeister (34) observed mean elemental mercury concentrations in the range 2.3-2.8 ng m-3 at three different sites in Detroit, while mean Hg0 concentrations of 2.0 ng m-3 were observed in Chicago (12). Measurements of Hg0 in south Florida were in the range of 2.4-4.0 (35), while a mean value of 1.83 ( 0.43 ng m-3 was obtained during several sampling studies at the Chesapeake Biological Laboratory, CBL (22). A four-year study at sites in Tennessee and Indiana found mean TGM concentrations of 3.46 ( 1.7 ng m-3 (16). A recent study which used the Tekran automated sampling system over a 35-day period in Tuscaloosa, Al found an average Hg0 concentration of 4.05 ng m-3 and in the range 2.00-11.8 ng m-3 (28). Therefore, Hg0 concentrations during the four sampling periods in Detroit are comparable to measurements made at other sourceimpacted sites in the continental United States. Median Hg0 levels in Detroit were found to vary considerably during the 2000-2002 studies and are likely a reflection of differing source impacts, meteorological regimes, and oxidation levels of the atmosphere. Table 5 summarizes meteorological parameters and ozone for the four intensive periods. The median Hg0 concentration was higher during Jul 2000 (1.82 ng m-3) as compared to September 2000 (1.68 ng m-3), p < 0.026. Table 5 shows that the average temperature at the sampling site was higher, average wind speeds were lower, and average ozone concentrations were higher during the July 2000 campaign when compared to September 2000. The significantly higher summer Hg0 concentrations are likely to be due to a combination of factors including variability in source emissions (industrial, coalfired power plants, waste incineration) wind patterns, surface heating, mixing height, rates of deposition, atmospheric chemistry, and re-emissions of previously deposited mercury. Higher ambient temperatures would be expected to lead to faster rates for photochemical reactions, higher rates of volatilization and re-emissions of previously deposited mercury species from land surfaces and from water bodies (36, 37). Seasonal variation in TGM concentrations has also been observed during a four-year study conducted in Tennessee and Indiana where concentrations were approximately 10% higher in the warmer months (16). Reactive Gaseous Mercury. Median RGM concentrations at the Detroit site ranged from 6.41 to 22.0 pg m-3, i.e., a factor of 3-11 times higher than at the rural site, while the maximum RGM concentration of 270 pg m-3 observed in Detroit was a factor of 7 times higher than that at Dexter. 9256
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Measurement of RGM using automated sampling made at Tuscaloosa, Al found an average concentration of 13.6 pg m-3 and a maximum of 162 pg m-3 over a 35-day period (28). The measured RGM/Hg0 ratios show a range of 0.01-2.57% in Dexter and 0.04-11.60% in Detroit. Measurements in rural Indiana and Tennessee found a ratio of 3% (16) while those at CBL showed a range of 0.2-29.5%, and for 6 out of 9 sampling events the ratio was less than 5% (22). These results show that during certain episodes in an urban environment RGM can comprise far greater than the often cited 1-5% of total gaseous mercury in the air and therefore would lead to even greater amounts of deposition to the surrounds. The only significant correlation between Hg0 and RGM species in Detroit was found during July 2000, r ) 0.2887; p < 0.001. The fact that a correlation was observed during one season only between elemental and reactive gaseous mercury concentrations measured in Detroit suggests the presence of a multitude of sources for these species. Particulate Phase Mercury. Median values for Hgp in Detroit were 18.3 pg m-3 in September 2000 and 19.8 pg m-3 in July 2002. The ranges were 5.70-60.1 pgm-3 and 8.4346.8 pg m-3, respectively. Previous measurements of fine particulate mercury in Detroit (34) showed mean values in the range 26-33 pg m-3, while mean values during the LMMBS in Chicago were 49 pg m-3 in July 1994 and 77 pg m-3 in January 1995 (12). It is worth noting that the Hgp values measured previously in Detroit and in Chicago are significantly higher than those measured using the 1130/ 1135 Tekran system in Detroit during the 2000-2002 sampling campaigns. The most likely explanation is the fact that the previously collected filters did not have denuders located upstream of the filter which would lead to the collection of RGM as well as Hgp on the filter. This is because denuder technology was not available for use in mercury measurements at the time the study was carried out. Two recent studies (23, 38) have shown that a positive artifact in the measurement of Hgp occurs if a denuder is not utilized upstream of a quartz filter. In this speciation study Hgp measurements were made in Detroit by passing the sample stream through a KCl-coated denuder (1130) unit for RGM removal and then into the particulate sampling unit (1135 unit) where Hgp collected on the filter was not likely to have a positive artifact because the sample has been denuded of RGM. The even higher concentrations of Hgp that were observed in Chicago in January 1995 are a reflection of the temporal variation that has been observed for this species by others (39-41) where ambient concentrations were found to be higher in the winter time likely due to lower ambient temperatures which may promote stronger partitioning toward the condensed phase. Automated sampling for Hgp in Tuscaloosa, AL found average concentrations of 16.4 pg m-3 and a maximum concentration of 181 pg m-3. The average concentration at Tuscaloosa is similar to the median concentrations observed in Detroit, while the maximum concentration at Tuscaloosa is a factor of 3 times higher than that observed in Detroit during this study period. Average wind speed in Tuscaloosa was less than that in Detroit (0.47 m s-1 vs 1-1.5 m s-1) and may have given rise to less mixing in the boundary layer. In addition, average relative humidity was higher in Tuscaloosa and this may have promoted more
FIGURE 2. Observed mercury speciation in Detroit, MI July 24-29, 2002. gas to particle conversion of RGM to Hgp resulting in the higher concentrations observed. Temporal Variation in Hg Speciation. Figure 2 is a plot of elemental, RGM, and Hgp on selected days in July 2002 in Detroit. The profile for elemental mercury typifies our observations while monitoring this species from 2000 to 2002 in Detroit, namely, that of a fairly constant profile which is interrupted by episodes with large increases in the concentration due to the impact of a plume on the site. For example, on July 27th, a large increase (maximum concentration 21 ng m-3) was observed. Elemental mercury concentrations as high as 22 ng m-3 have also been observed during the LMMBS at the Illinois Institute of Technology site (12).
The profiles for RGM and Hgp are far more complex, exhibiting episodes when both species appear to be highly correlated (July 25, 26, 28, and 29) and not correlated (July 24 and 27) at other times. This suggests that the sources of these species may not always be related. The overall correlation between RGM and Hgp for the period was found to be positive, r ) 0.43, p ) 0.0004. Diurnal Patterns for RGM. On the basis of our knowledge of physical and chemical properties of RGM, as well as previous measurements, RGM has been observed to exhibit a well-defined diurnal cycle (16, 42, 25). Figure 3 shows diurnal cycles in Dexter and Detroit, MI during selected times. At Dexter during the first 4 days RGM exhibits a well-defined diurnal profile whose maxima coincide with maximum levels of solar radiation, which then decrease with decreasing radiation. Sustained high levels of RGM in the air are apparent overnight on May 6 and 7, 2000. On May 8, 9, 11, and 12, the maxima were lower, while higher daytime maxima were observed on May 13 and 14. Precipitation and dew formation (Supporting Information) were recorded at Dexter on May 7, 10, and 11 and likely scavenged RGM, resulting in the lower daily maxima which we observed at this site. This scavenging of RGM by dew and precipitation has previously been documented in Dexter, MI and the Florida Everglades (31). Precipitation was found to reduce ambient RGM concentrations by as much as 53% after the onset of rain (43). The diurnal profile for Detroit shows a less pronounced diel cycle for RGM; Hgp is also included in the plot and illustrates that there are significant amounts of both oxidized
FIGURE 3. Diurnal profiles in (a) Dexter, MI (May 1-15, 2000) and (b) Detroit, MI (July 24-29, 2002). VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Correlation between RGM/Hgp ratio and wind direction in July 2002 in Detroit. and particulate mercury present during the overnight hours. In general, the maximum concentrations of RGM were found to occur in the late afternoon to evening hours and on occasion high concentrations were also observed overnight or in the very early morning hours; e.g., on July 26, 2002, RGM levels of 50 pg m-3 were measured between 6 and 7 a.m. This suggests that nocturnal emissions and transport occurring within the boundary layer are impacting the site. Deposition velocities for RGM are known to range from 0 up to 1.6 cm s-1 and for Hgp range from 0.14 to 0.24 cm s- (31). Therefore we would expect that RGM would be deposited close to its source while Hgp can remain for longer periods in the atmosphere before undergoing deposition. In addition, RGM may also undergo gas-particle transfer thus slowing down its deposition. An increased residence time for particles may result in transport away from sources and later deposition. On the basis of this rationale it is instructive to examine the measured ratio of RGM to Hgp as it may provide clues regarding the age of mercury in the plumes that impacted the site in Detroit. The RGM/Hgp ratio was found to vary with the wind direction for both September 2000 and July 2002. In September 2000, mean RGM/Hgp ratios were 0.94 (n ) 33) for east/southeasterly flow and 0.72 (n ) 12) for west/ southwesterly flow and were not significantly different (p ) 0.1066). In July 2002 the mean ratios were 1.63 (n ) 19) and 1.06 (n ) 35), respectively, for east/southeasterly and west/ southwesterly flow and were found to be significantly different (p ) 0.0005, Wilcoxon test). Figure 4 shows a plot of RGM/ Hgp ratios and wind direction at the site in July 2002; the average relative humidity for the west/southwesterly and east/southeasterly flow is also included in the plot. Back trajectories (Figure 5) corresponding to the days with the maximum value RGM/Hgp of 3.29 on July 25, 2002 and the minimum RGM/Hgp of 0.47 on July 27, 2002 were examined to establish the source of the air flow. A 72-h back trajectory shows the path that the air took before arriving from the east to Detroit on July 25, 2002. The air which arrived in the lowest altitudes, i.e., 100, 500, and 1000 m, originated aloft at an altitude of 3-4000 m over northern Ontario and descended on its approach to Detroit. As it descended it was subject to influences from local sources to the east of the site including steel manufacturing and medical waste incinerators (Figure 1) that release mercury-containing emissions. Flow from the east was drier (average RH ) 56%) and had higher concentrations of RGM relative to Hgp. A second 72-h back trajectory showing the path traversed by air before it arrived to the site on July 27 reveals air which originated in the boundary layer over Ohio, Indiana, and Iowa and subse9258
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FIGURE 5. 72-h Back trajectories for Detroit on July 25 and 27, 2002. quently passed over industrial areas as well as the urban areas of Chicago and Milwaukee before arriving to Detroit. This air underwent regional transport through industrialized
regions and populated areas where conditions for production of secondary pollutants from primary emissions were highly favored. Flow from the south/southwesterly direction was moister (average RH ) 74%) and had higher concentrations of Hgp relative to RGM. Therefore the occurrence of a higher proportion of particulate mercury relative to reactive gaseous mercury and hence a lower RGM/Hgp ratio with regional flow is not surprising and suggests that this type of flow brings an aged plume depleted in RGM to the Detroit site. A recent study over a two-week period in Detroit (44) found that the major source contributions to PM2.5 were secondary sulfate 40%, and iron/steel and motor vehicle emissions 40%, while 16% was attributed to incineration and oil refining. This implies that the site is influenced by both local (iron/steel, motor vehicles, incineration, and oil refining) sources and regional sources (coal combustion/sulfate). Since mercury is known to be present in all of these emission types this suggests that mercury deposition in Detroit will contain a significant contribution from regional sources, e.g., transboundary pollution which emanates from within the Great Lakes watershed as well as the Ohio River Valley. RGM and Ozone in the Urban Environment. Mercury monitoring in pristine environments has revealed that Hg0 can undergo oxidation to produce RGM (24, 45-46). Specifically, studies in the Arctic have shown mercury depletion events during polar sunrise during which ambient concentrations of elemental mercury and ozone decrease while those of RGM increase dramatically (24, 45-46). The background concentrations of elemental mercury can decrease to below limits of detection during these events (46). Reactive halogen species released from the snowpack are believed to be responsible for the oxidation of elemental mercury to RGM (24). Studies during the Antarctic summer have shown production of RGM believed to be mediated by OH, HO2, O, or NO3 radicals (47). Recent modeling and monitoring studies in the Mediterranean region support the premise that RGM is produced in the marine boundary layer (48) although the exact mechanism is unknown. In contrast, studies in the marine boundary layer in Florida (25) and the Northwestern United States show no evidence for the production of RGM in the marine boundary layer (26). RGM concentrations in an urban environment are likely the result of a complex mixture of sources and may undoubtedly contain a contribution due to photochemical processes. The presence of ozone in the atmosphere is generally an indication of photochemical processing and an oxidizing atmosphere. An oxidizing atmosphere may contain oxidants which can oxidize Hg0 to RGM. Figure 6 shows a plot of RGM and ozone during monitoring campaigns in July 2000-2002. In general, we found that higher RGM and ozone concentrations occurred in the afternoon or early evening hours. Inspection of the ozone profiles reveals that, in addition to the peak corresponding to maximum solar radiation, there are also peaks which occurred later in the afternoon or overnight hours. The fact that these peaks in ozone occurred after solar noon is strong evidence that ozone may have undergone long-range transport to the site. Ozone is known to be advected over long distances reaching urban areas during late afternoon and evening hours (49). These episodes with secondary ozone peaks were also accompanied by increases in RGM concentrations (Figure 6a-e). Table 6 provides further information for each of these episodes, including time of occurrence, and shows the magnitude of the increases in RGM and ozone concentrations at the site during these episodes. The presence of higher RGM concentrations coupled with increases in ozone during these episodes suggests that RGM may have been produced in situ during transport in a strongly oxidizing air mass. Recent studies in the Pacific Northwest show removal of Hg0 from a pollution plume on a rapid time scale
during transport from the industrialized Vancouver-Seattle corridor to the receptor site at Cheeka Peak Observatory (26). The largest and most frequent losses occurred in the presence of increased ozone and when the air was more photochemically processed as judged by propane/ethane ratios. During these losses Hg0 was found to be strongly negatively correlated with ozone which the authors suggest may implicate a photochemical mechanism for Hg0 loss. Correlation between ozone and Hg0 measured in Detroit during all sampling campaigns is shown in Table 7 and reveals a weak negative relationship for these two species. Thus the observation of high levels of RGM and ozone in the afternoon and overnight hours in Detroit may be due to depletion of Hg0 from photochemically processed air masses during transport to the site. Significance of this Work. Speciated mercury measurements made during a series of intensive campaign from 2000 to 2002 in Dexter and Detroit, MI indicate that elemental, RGM, and particulate phase mercury concentrations are significant in the urban Detroit atmosphere and as expected are present at levels which are higher than those observed for a rural background site in Michigan. At the Dexter, MI site the median Hg0 concentration for both intensive periods was 1.5 ng m-3 which is close to the Northern Hemisphere background of 1.7 ng m-3 (50). At the urban Detroit site the median Hg0 concentration varied, and during one sampling period exhibited a median concentration of 3.13 ng m-3 which is approximately 46% higher than the Northern Hemisphere background and is consonant with anthropogenic sources of mercury impacting the site. Median RGM concentrations in Detroit were a factor of 2-7 times higher than those observed for rural Dexter. Maximum concentrations of RGM, 155 pg m-3 (Sept 2000) and 270 pg m-3 (July 2001), were similar to a concentration of 211 pg m-3 observed using the automated speciated system at a site approximately 4 km downwind of a municipal incinerator in Baltimore, MD (51). Using the range of median RGM concentrations observed in Detroit during the sampling campaigns and a deposition velocity on the order of 1-5 cm s-1 (16) we estimate that deposition of this species is in the range of 0.23-3.96 ng m-2 h-1 which is similar to estimates made by Lindberg (16) for deposition to the Walker Branch Watershed and by Sheu (22) at the Chesapeake Biological Laboratory. Median values for fine particulate mercury were lower than those observed by Landis et al. in Chicago (12) and Gildemeister in Detroit (34). However, they did not use denuders upstream of the filters during collection and a subsequent study in Detroit found evidence for a positive artifact on filters due to RGM when the filters were undenuded (38). The Tekran 1130/1135 automated system has a KClcoated denuder upstream of the particulate filter to prevent RGM artifacts during collection of particulate phase mercury. Diurnal patterns for RGM are less well-pronounced in urban Detroit when compared to rural Dexter, a fact which suggests that significant local nighttime emissions as well as regional transport within the boundary layer are occurring. The high-resolution data have permitted an assessment of RGM/Hg0 ratios which show values as high as 12% in Detroit. This ratio is a factor of 2 higher than the previously assumed upper limit of 5% (16); it is therefore recommended that these data be used for model performance evaluation. The dynamics for the three mercury species are very complex, suggesting that the source profiles are multi-factorial. Additionally, the concentrations of both RGM and Hgp were found to be of a similar magnitude, suggesting that both will play an important role in deposition to the surrounding environment. Furthermore, analysis of the observed RGM/ Hgp ratios at the Detroit site implies that the site is being impacted by both local and regional transport. VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. RGM and ozone profiles during selected times in Detroit, MI.
TABLE 6. Episodes date
time
episode
RGM (pg m-3)
ozone (ppb)
7/25/2000 7/17/2001 7/22/2001 7/25/2002 7/28/2002
17:00 17:00 19:00 17:00 01:00
(a) (b) (c) (d) (e)
19 f 51 68 f 208 50 f 270 37 f 41 13 f 32
56 f 63 56 f 70 30 f 70 41 f 50 13 f 37
TABLE 7. Summary of Correlation Coefficients among Hg0, RGM, and Ozone (Asterisks Denote Significant Correlation) date
Hg0 vs O3
HgII vs O3
July 2000 Sept 2000 July 2001 July 2002
-0.1042 (p ) 0.2457) -0.1304 (p ) 0.1982) -0.2669 (p ) 0.0021)* -0.3269 (p ) 0.0089)*
0.3363 (p ) 0.0001)* -0.0100 (p ) 0.9212) 0.3368 (p < 0.0001)* -0.0421 (p ) 0.7431)
The observed behavior of RGM and ozone in Detroit suggests that Hg0 may be depleted from air masses by oxidation processes during regional transport to the Detroit site. This implies that the lifetime for elemental mercury with respect to reaction with oxidants may be reduced owing 9260
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to atmospheric conditions during regional transport. WeissPenzias et al. (26) have suggested that injection of urban emissions containing oxidants into the boundary layer enhances reactivity with respect to Hg0 and may reduce its lifetime, leading to deposition not only from local emissions but also increasing deposition of global background Hg0. However, recent speciated measurements at a rural site downwind of Atlanta, GA (52) designed to elucidate Hg speciation in coal-fired power plant plumes show less RGM than expected based on coal analysis. These findings were interpreted to mean that RGM was undergoing reduction to Hg0 during transport to the receptor site. The aqueous reduction of divalent mercury by sulfite has been demonstrated (53-55), and it is generally believed that sulfite controls the reduction reactions of divalent mercury (17). Atmospheric mercury behavior and chemistry thus appears to be very complex and further understanding of the factors controlling oxidation and reduction reactions of mercury as it is dispersed from its sources is greatly needed. Continued long-term monitoring in urban Detroit is ongoing and will lead to further time-resolved information on the dynamics of all three Hg species during all seasons to glean how seasonal changes may affect the dynamics of these species. This should also provide further information regarding the depositional
burden of mercury species to the surrounding environs on a seasonal basis.
Acknowledgments This work has been funded in part by the Michigan Great Lakes Protection Fund, the U.S. Environmental Agency’s Great Lakes National Program Office and U.S. EPA Region 5. We thank Dr. Tim Dvonch, University of Michigan, for assistance with setup of the monitoring equipment and Dr. Matt Landis (U.S. EPA) for a loan of a Tekran 1135 particulate unit during the September 2000 intensive measurement campaign. The assistance of Drs. Frank Marsik, Fuyuen Yip, and Masako Morishita during the field intensive sampling is much appreciated. The assistance of Dr. Khalid Al-Wali with the generation of the trajectory plots is much appreciated. Thanks also to Dr. Amy Gildemeister for providing us with an ArcView map of Detroit.
(16)
(17)
(18)
(19) (20)
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Supporting Information Available Surface wetness and precipitation measured at Dexter, MI in May 2000. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review July 1, 2004. Revised manuscript received August 1, 2005. Accepted September 13, 2005. ES040458R
