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Temporal and spatial variability of total gaseous mercury in Canada: results from the Canadian Atmospheric Mercury Measurement Network (CAMNet)
AE International – North America Atmospheric Environment 37 (2003) 1003–1011
Temporal and spatial variability of total gaseous mercury in Canada: results from the Canadian Atmospheric Mercury Measurement Network (CAMNet) Markus Kellerhalsa,*, Stephen Beauchampb, Wayne Belzerc, Pierrette Blanchardd, Frank Froudee, Bruno Harveyf, Karen McDonalda, Martin Pilotef, Laurier Poissantd, Keith Puckettd, Bill Schroederd, Alexandra Steffend, Rob Tordonb a
Environment Canada, Prairie and Northern Region, 4999-98 Ave., Edmonton, AB, Canada T6B 2X3 b Environment Canada, Atlantic Region, 45 Alderney Dr., Dartmouth, NS, Canada B2Y 2N6 c Environment Canada, Pacific and Yukon Region, #700, 1200 W. 73rd Ave., Vancouver, BC, Canada V6P 6H9 d Environment Canada, Meterological Service of Canada, 4905 Dufferin St., Downsview, Ont., Canada M3H 5T4 e Environment Canada, Centre for Atmospheric Research Experiments, Egbert, Ont., Canada L0L 1N0 f Environment Canada, Qu!ebec Region, 105 rue McGill, 7e e!tage (Youville), Montr!eal, Qu!e., Canada H2Y 2E7 Received 2 January 2002; received in revised form 24 October 2002; accepted 24 October 2002
Abstract Continuous measurements of total gaseous mercury (TGM) concentration were taken during 1997–1999 at 10 rural sites across Canada, ranging from 431 to 821N and from 621 to 1231W. Overall median TGM concentrations ranged among the sites from 1.32 to 1.83 ng m 3. The spatially averaged median concentration among all sites was 1.6070.15 ng m 3. Maximum hourly average concentrations, on the order of 10 ng m 3, were observed at several sites located near major sources of anthropogenic emissions. Minimum hourly average concentrations were observed in springtime at the arctic site where concentrations dropped below the detection limit of the analyzer (50 pg m 3) on several occasions. Seasonal variability in TGM concentrations was observed at all sites. At most sites monthly median concentrations were highest in late winter and lowest in fall. Diurnal variations in TGM concentration were also observed at most sites. The most common pattern of diurnal variability was a diel cycle of minimum concentrations just before sunrise and maximum concentrations around solar noon. The diel cycle was seasonally modulated, reaching maximum amplitude during spring or summer at all sites. Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. Keywords: Atmospheric mercury; Total gaseous mercury; Temporal variability; Seasonal variability; Canada
1. Introduction Mercury is a highly toxic chemical, found throughout the global troposphere. Though the levels of mercury *Corresponding author. Tel.: +1-780-951-8626; fax: +1780-495-2444. E-mail address: [email protected] (M. Kellerhals).
generally found in the atmosphere are well below the threshold considered to cause direct impacts on human health, mercury is still a pollutant of concern because of its propensity to accumulate and concentrate in biota. At higher trophic levels in aquatic food webs, mercury may be found in elevated concentrations, sometimes exceeding the guidelines for human consumption (USEPA, 1997). Atmospheric input of mercury has been
1352-2310/03/$ - see front matter Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 9 1 7 - 2
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M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
recognized as the main source responsible for contamination of remote sites such as lakes in northern Canada (Lockhart et al., 1995). Mercury in the atmosphere is found in both gaseous and particulate form. Of the total gaseous mercury (TGM) component, the dominant form is gaseous elemental mercury (Hg0) (Slemr et al., 1985). Hg0 has been estimated (Slemr et al., 1985) to have a mean atmospheric residence on the order of 1 yr. This residence time is sufficient for Hg0 to become relatively well mixed throughout the Northern Hemisphere troposphere, implying that TGM concentrations should be of similar magnitude at a wide variety of background sites. This has been verified by several TGM measurement studies (e.g. Schmolke et al., 1999; Slemr et al., 1981). Despite the relative uniformity of TGM concentrations there are still important temporal and spatial variations in TGM concentration that provide insight into the processes governing atmospheric mercury concentration and deposition. The Canadian Atmospheric Mercury Measurement Network (CAMNet) was established with the goal of providing accurate, long-term measurements of TGM
concentration across Canada. These measurements are being used to investigate processes governing atmospheric concentrations of mercury, temporal and spatial variability of atmospheric mercury, and sources and sinks of atmospheric mercury. The measurements will also provide a high quality data set that may be used to verify global mercury models. This paper describes the network and summarizes results from 2 yr of measurements.
2. Experimental 2.1. Sites CAMNet was initiated in 1996 to unify TGM measurement initiatives then underway in different regions of Canada. CAMNet currently consists of 11 monitoring sites (see Fig. 1), chosen to represent major geographical and ecological regions of Canada. For the purposes of this study, each site was classified as either rural remote or rural affected. Rural-affected sites are those sites expected to be significantly impacted by
Fig. 1. Location map of CAMNet sites: (1) Alert, (2) Kejimkujik, (3) St. Andrews, (4) Mingan, (5) Kuujjuarapik, (6) St. Anicet, (7) Point Petre, (8) Egbert, (9) Burnt Island, (10) Esther, and (11) Reifel Island.
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Table 1 Location, site classification and measurement start dates for the 11 CAMNet sites. Station
Location
Site
Code
Province
Latitude
Longitude
Alert Kejimkujik St. Andrews Mingan St. Anicet Point Petre Egbert Burnt Island Esther Reifel Island Kuujjuarapik
ALT KEJ STA MIN ANI PPT EGB BNT EST RFL KUJ
NU NS NB QC QC ON ON ON AB BC QC
82.50N 44.43N 45.09N 50.27N 45.12N 43.83N 44.22N 45.80N 51.67N 49.10N 55.28N
62.30W 65.21W 67.00W 64.23W 74.28W 77.15W 79.78W 82.95W 110.20W 123.16W 77.75W
Start date
Classification
Jan 1995 Jul 1995 Nov 1995 Jan 1996 Jul 1994 Nov 1996 Sep 1996 Mar 1998 Jun 1998 Mar 1999 Sep 1999
Rural-Remote Rural-Remote Rural-Remote Rural-Remote Rural-affected Rural-affected Rural-affected Rural-Remote Rural-Remote Rural-affected Rural-affected
The site codes are the station abbreviations used in Fig. 3 and in the text.
nearby anthropogenic mercury emissions. Table 1 lists the location, classification and site code for all CAMNet sites. The ten sites used in this study are briefly described below: *
*
*
*
*
*
*
The Alert site is located near the northeast coast of Ellesmere Island, far from any sizable human settlement or industry. The surrounding area is largely devoid of vegetation. The Kejimkujik site is located 60 km inland from the Atlantic coast in a national park that is itself surrounded by sparse rural development. The site is surrounded by mixed softwood forest. The St. Andrews site is on the St. Croix River adjacent to the Bay of Fundy near the Canada/US border in an area of mixed softwood forest 1 km outside the town of St. Andrews (pop. B1500). The Mingan site is located 1 km from the north shore of the Gulf of Saint Lawrence, 50 km E of Sept-Iles (pop. B25 000) and 25 km W of Havre-St Pierre (pop. B3500). The site is surrounded by boreal black spruce forest. The St. Anicet site is located in the St. Lawrence River valley, 3 km south of St. Anicet (pop. B1000), 30 km SW of a major industrial area, and 90 km SW of the metropolitan area of Montreal (pop. B3 000 000). The site is surrounded by fields and deciduous wood lots. The Point Petre site is located on the north shore of Lake Ontario. The site is surrounded by the lake and deciduous forest. Lake Ontario is ringed by urban and industrial centres. Toronto (pop. B3 000 000) lies about 200 km WSW of the site. The Egbert site is located near Barrie, Ontario. The surrounding area is farmland and wood lots with moderately dense rural settlement. Toronto lies about 100 km south of the site.
*
*
*
The Burnt Island site is located in a sparsely populated rural area near the north shore of Lake Huron. The site is surrounded by coniferous forest. The Esther site is located in a sparsely populated, grassland and grain-farming area of the Canadian Prairies. The nearest sizable community is 40 km SSW of the site. The Reifel Island site is located at the mouth of the Fraser River, on the east shore of the Strait of Georgia. The major urban center of Greater Vancouver (pop B2 000 000) is spread out immediately N, E and SE of the site.
2.2. Network operation The standard operating procedures for the network are described in Steffen and Schroeder (1999). Each site uses a Tekran 2537A analyzer to measure ambient air concentration of total gaseous mercury. This measurement method is described in Poissant (1997). The Tekran instrument has been shown (Ebinghaus et al., 1999) to compare well with other methods of measuring TGM. TGM measurements are made over 15 min intervals, except at St. Andrews and Kejimkujik where TGM is measured over 5 min intervals, and Alert where TGM is measured over 30 min intervals for most of the year and over 5 min intervals during spring. At each site the monitor is located in a temperature-controlled building. Sample intake heights range among the sites from 3 to 6 m above ground level with Teflon inlet lines of 10–15 m in length. The lines are tested quarterly for efficiency by injection of TGM at the sample inlet. The analyzers are automatically calibrated every 23 or 25 h using internal permeation sources that emit vapor mercury at a constant rate. In addition to the automated
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
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calibrations, the permeation sources are programmed to inject a known amount of mercury into the ambient sample stream at regular intervals. These injections are used to verify sample recovery rates. The nominal emission rate of the internal permeation source at each site is verified during a quarterly manual calibration. In this procedure, a known amount of mercury vapor from a Tekran 2505 calibration unit is injected into a zero air sample stream. The instrument response to a series of manual injections can then be compared to its response to injections from the internal permeation source. The ratio of these two numbers is then used to calculate the actual emission rate of the permeation source. If the actual and nominal emission rates are different the nominal emission rate is manually updated. Once every 2 yr an audit of the network takes place. During the audit, a network calibration unit is used to calibrate the permeation source in each instrument and also to provide an intercomparison of the regional calibration units. The audit protocol is described in detail in Poissant and Casimir (1999). The precision of the measurement method at a given site has been estimated to be better than 2% (Poissant, 2000). Comparison across the network also requires an estimate of inter-site variability, which is caused by two main factors: (1) inter-site variability of the difference between nominal permeation source emission rate and actual emission rate, and (2) inter-site variability of the difference between nominal sample volume and actual sample volume. Based on the results of the 1998 and 1999 audits, the RMS deviation of emission rates across the network was 4.1%, while the RMS deviation of sample volumes was 1.6%. Combining all three sources of error yielded an estimated ‘‘network error’’ of 75%. Results from the 1999 audit showed that a significant portion of the inter-site variability was caused by differences in the calibration of the syringes used in the manual calibrations. If it is possible to largely eliminate this source of error, the network error can be reduced to o3% in the future.
2.3. Data management TGM measurements from CAMNet are quality controlled using the Research Data Management and Quality Control (RDMQt) system. RDMQ operates under the SASt statistical analysis system. It provides an automated quality control system that assigns each TGM datum a validity flag based on programmed criteria for sample volume, baseline voltage, baseline standard deviation, difference from adjacent values and absolute magnitude. Hourly average TGM concentrations were calculated for all hours with at least 25% data completeness. These hourly average TGM data are available from Environment Canada’s NATChem database. Data from 1997 and 1998 were used for this study, as those years had the maximum number of stations in operation (Table 1). For the two western Canadian stations (EST, DEL) data from 1999 were included in the analysis as there were few data from 1997 to 1998.
3. Results and discussion An overall average median concentration of 1.6070.15 ng m 3 for the ten sites was calculated by averaging together the ten site medians. Table 2 provides a statistical summary of TGM results from each site. The four rural-affected sites had significantly (po0.005) higher median concentrations (mean B1.70 ng m 3, range B1.65 to 1.83 ng m 3) than did the six ruralremote sites (meanB1.54 ng m 3, range B1.32 to 1.69 ng m 3). This difference is also significant in comparison with the estimated network error of 75%. Maximum concentrations were substantially higher at the rural-affected sites (6.15–10.19 ng m 3) versus the remote sites (2.64–4.57 ng m 3). TGM concentrations were also more variable at the rural-affected sites versus the rural-remote sites. The one exception is Alert, a remote site that also had high variability.
Table 2 Statistical summary of TGM concentrations. Station
N
Mean
Standard Deviation
Median
Minimum
Maximum
Alert Kejimkujik St. Andrews Mingan St. Anicet Point Petre Egbert Burnt Island Esther Reifel Island
15021 15730 15866 15530 16665 16390 14536 5229 10766 6971
1.55 1.33 1.43 1.62 1.72 1.90 1.65 1.58 1.69 1.69
0.39 0.25 0.20 0.22 0.40 0.43 0.31 0.23 0.19 0.30
1.59 1.32 1.42 1.67 1.67 1.83 1.65 1.56 1.69 1.67
o0.05 0.26 0.68 0.36 0.59 0.97 0.87 0.65 1.11 1.14
3.04 2.69 2.64 3.07 9.62 8.50 6.15 3.04 4.57 10.19
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
observed in Europe. Slemr and Scheel (1998) reported a median concentration of 1.93 ng m 3 at a mountaintop site in Germany in 1996. Lee et al. (1998) reported a mean concentration of 1.68 ng m 3 at a rural site in the UK in 1995–1996. Schmolke et al. (1999) reported median concentrations between 1.54 and 1.93 ng m 3 on a north–south measurement transect between Stockholm and Berlin in the summer of 1995. The north-south TGM gradient observed in Europe (Schmolke et al., 1999) was not seen in this study. The mean median concentration for the nine mid-latitude sites (1.6170.16 ng m 3) was not significantly different than the median concentration at the one arctic site (1.59 ng m 3). However, median concentrations at the Atlantic sites (KEJ, STA) averaged lower than at remote sites elsewhere in Canada (MIN, ALT, BNT, EST)—1.37 ng m 3 versus 1.63 ng m 3. This difference was statistically significant (po0.005) and also significant compared to the estimated network error. The relatively low concentrations in Atlantic Canada are surprising, considering that this region is downwind of significant mercury emissions from the northeastern US. Fig. 3 illustrates the probability distribution of TGM concentrations at the CAMNet sites. The four ruralaffected sites all had right-skewed probability distributions that approximate log normality. The log-normal distribution has been shown to be the theoretical distribution of pollutant concentrations for a passive (no atmospheric sources or sinks) pollutant that is emitted and then subject to successive random dilutions (Ott, 1995). Given the long atmospheric lifetime of elemental mercury, it might be expected to act as a passive pollutant on the time scale of transport from nearby sources to a monitoring site. Four of the ruralremote sites (EST, BNT, STA, KEJ) had roughly symmetrical, near normal probability distributions. Rather than reflecting successive random dilutions, concentrations at these remote sites seem to fluctuate
Higher variability of TGM concentrations at the rural-affected sites appeared to be caused by the alternating exposure of these sites to anthropogenic TGM emissions, depending on wind direction and atmospheric mixing. For example, Fig. 2 shows the dependence of TGM concentration on wind direction at the Reifel Island site. Winds from the north through southeast traveled over the metropolitan area of Vancouver before reaching the site, and had higher TGM concentrations on average than did winds from the south through northwest, which traveled over ocean prior to reaching the site. In contrast, high variability of TGM concentrations at Alert was primarily due to springtime episodes of mercury depletion (Schroeder et al., 1998). Median TGM concentrations at CAMNet sites were in general similar to or slightly lower than those
0 30
1.8
60
1.6 1.4
90
240
120 210
150 180
3
Percentiles 99th 95th 75th Median 25th 5th 1st
2
1
EGB
PPT
ANI
MIN
STA
KEJ
0 ALT
TGM Concentration (ng/m3)
Fig. 2. Dependence of TGM concentration on wind direction at Reifel Island. Radial axis is TGM concentration in ng m 3. Angular axis is wind direction.
RFL
270
EST
300
BNT
330
1007
Site Fig. 3. Probability distribution of TGM concentration at CAMNet sites.
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
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around a regional background concentration. The fluctuations represent the effects of multiple processes such as chemical depletion, surface deposition, volatilization from surrounding surfaces, and occasional transport of mercury rich air from source regions. Two of the rural-remote sites (ALT, MIN) had strongly left-skewed probability distributions. At Alert, this distribution arose because TGM was fairly constant through fall and winter, and was subject to large multiday depletion events in late winter and early spring. At Mingan, the left-skewed distribution arose due to relatively constant concentrations from November through April, followed by frequent strong nocturnal depletion May–October.
To investigate seasonal variability of TGM concentration, median monthly concentrations were calculated for each site for years with 10 months or more of data. These data are shown in Fig. 4. A slight seasonal trend is seen for eight of the nine mid-latitude sites over the 2-yr study period, with higher concentrations observed in winter and spring, and lower concentrations in summer and fall. The only exception was the Point Petre site where there was a very strong summer peak in 1998. Point Petre is downwind of major anthropogenic source regions and is right on the shore of Lake Ontario, which remains cold all summer long. The combination of sources and meteorology will be investigated to find an explanation for the different values observed at this site.
TGM Concentration (ng/m3)
2.5
2
1.5
1
0.5 Jan
(a)
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Alert 1997 Alert 1998 Median of Medians: Mid-Latitude Stations
3
TGM Concentration (ng/m )
2.5
2
1.5
1
0.5
(b)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Fig. 4. Monthly medians of TGM concentration at CAMNet sites. Data from the mid-latitude sites are shown in the top panel, with Point Petre 1998 as a dashed line. Data from Alert are shown in the lower panel. The line with square symbols is the median of the midlatitude monthly median data shown in the top panel.
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
Seasonal cycles in TGM observed elsewhere also show a winter maximum in concentration. For example, measurements of TGM in Scandinavia (Iverfeldt, 1991) found median concentrations in winter to be 33% higher than in summer. In contrast, the seasonal cycle at Alert, also shown in Fig. 4, is distinctly different from that observed at the mid-latitude sites. Annual hourly mean TGM concentrations for each site are shown in Fig. 5. All sites showed some evidence of a diel cycle in concentration. The mean amplitude of the diel cycle ranged among the sites from 3 to 13 percent of site mean concentration. Seven of the 10 sites experienced a cycle of maximum concentrations near solar noon (1000–1400 local standard time) and minimum concentrations in the early morning hours (0300– 0700). The diel cycle observed at seven CAMNet sites contrast with the diel cycle observed at several European sites (Lee et al., 1998; Schmolke et al., 1999). TGM concentrations at the European stations peaked at night. This nighttime peak was attributed to soil mercury emissions building up under the nocturnal inversion. In contrast, the diel cycle at the CAMNet sites must be attributed to nighttime depletion of TGM in the lowermost atmosphere. Overnight a shallow TGMdepleted layer is formed underneath the nocturnal inversion layer. Shortly after sunrise there is a rapid increase in near surface TGM concentration as the nocturnal inversion breaks down and undepleted air is mixed down to the surface. The continued increase in TGM concentration through the morning and into the early afternoon is likely caused by surface emission of TGM, which has been observed to increase with increasing solar radiation (Poissant and Casimir, 1998). The three sites where different diel cycles were observed were Alert, St. Andrews and Reifel Island.
TGM concentration at Alert varied by only 3% of mean concentration, with a slight peak near solar noon and a broad minimum between 1700 and 0100. Diurnal variability is quite limited at Alert since the site experiences months of continuous darkness, followed by months of continuous daylight. Nonetheless, during the boreal spring and summer there are significant daily variations in solar radiation as the solar zenith angle varies by approximately 151 each day. The resulting variations in insolation may be sufficient to drive a daily cycle of deposition and re-volatilization and also to affect the rate of the photochemical reactions that have been observed to deplete TGM at arctic sites (Lindberg et al., 2000). Reifel Island experienced maximum concentrations in the early morning and minimum concentrations in the evening. Reifel Island provides an example of how local wind systems may cause diurnal TGM variability. During daytime hours, the onshore sea-breeze advected low-TGM air from the Strait of Georgia past the analyzer, while at nighttime the land breeze advected TGM-rich urban air from Greater Vancouver past the site. The relationship between wind direction and TGM concentration at Reifel Island is shown in Fig. 2. Fig. 6 demonstrates the diurnal shift in wind direction during the summer months when the sea breeze is most prominent. Finally, at St. Andrews TGM concentrations peaked around 1800 and were minimum around 0600. This diel cycle is similar to the other sites, except that peak concentrations occurred significantly later at St. Andrews. This apparently delayed peak arises in part because during winter there is actually a mid-day minimum at St. Andrews, therefore on an annual average basis mid-day concentrations are depressed relative to concentrations later in the afternoon. The diel cycle was seasonally modulated at all sites. At all nine mid-latitude sites, the diel cycle was strongest
2
TGM Concentration (ng/m3)
1009
Point Petre Reifel Island St. Anicet Esther Egbert Mingan Alert Burnt Island St. Andrews Kejimkujik
1.8
1.6 1.6
1.4
1 3 5 7 9 11 13 15 17 19 21 23 Hour Fig. 5. Diel cycle of TGM concentrations at CAMNet sites based on annual average concentrations for each hour.
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
1010
TGM Concentration (ng/m3)
% variation from seasonal mean
Fig. 6. Wind rose at Reifel Island during July through September for (a) daytime hours (1300–2000) and (b) nighttime hours (0100– 0800).
6 4 2 0 -2 Winter Summer
-4
Spring Fall
-6 1
3
5
7
9
11
13 15 Hour
17
19
21
23
Fig. 7. Seasonal modulation of diel TGM cycle at Esther expressed as percent deviation from seasonal mean concentration for each hour.
June–September, while at Alert the diel cycle was strongest in April and May. Fig. 7 shows the seasonal variation of the diel cycle at Esther, a fairly typical case. At Esther, the amplitude of the diurnal cycle during summer was approximately 10% of the mean TGM concentration, while in winter the cycle was largely absent. Other sites showed an even more dramatic seasonal effect. For example, at Mingan the amplitude of the diurnal cycle changed from near zero during winter to 20% of mean concentration during summer. There are several possible reasons for a stronger diel cycle in summer including: stronger diurnal variations in insolation and temperature, stronger daily cycle of mixed layer development, greater role of uptake/emission by vegetation, increased volatilization of mercury from nearby water bodies, and chemical reactions enhanced by higher temperatures or greater solar radiation. The annual and seasonal average diel cycles tend to understate the strength of the diurnal variability by averaging together many cycles of differing phase and amplitude. For instance, the three Atlantic coastal sites (MIN, KEJ, STA) each had one or more months in
2 1.6 1.2 0.8 0.4 0 203
208
213 Julian Day
218
Fig. 8. Time series of TGM concentration at Mingan from 22 July, 1997 to 10 August 1997 (Julian day 203–222). Vertical grid lines are at 00 h.
winter when mean TGM concentrations were lowest near solar noon (perhaps due photochemical oxidation of TGM in a process similar to the arctic springtime depletions), producing a diel cycle opposite in phase to the summer cycle. Even during the summer months when the diel cycle is strongest, most sites had periods when the diel cycle was nearly absent and other periods when its amplitude was a large fraction of mean concentration. This is illustrated in Fig. 8, a time series plot of TGM concentration at Mingan during a typical 20-day period in summer 1997. On 12 of the 20 days there were strong diel cycles. These diel cycles consisted of nocturnal depletion of TGM, by an average of 0.8 ng m 3, or approximately 50% of mean TGM concentration. On several other days in the period the diel cycle was absent. The strong nocturnal depletion of TGM that occurred at Mingan and several other sites indicates a significant removal of atmospheric mercury over night. For example, on the 12 nights described above TGM concentration was reduced by an average of 0.8 ng m 3 over a 10-h period. If this depletion was confined to a 20-m layer under a strong nocturnal inversion, the inferred deposition flux is 1.6 ng m 2 h 1. A similar calculation was reported in Poissant (2000), however,
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
that study assumed a very deep nocturnal inversion layer and hence calculated a much higher deposition rate. Existing measurements of TGM fluxes over natural surfaces in Canada indicate that emissions are more prevalent than uptake (Poissant and Casimir, 1998; Beauchamp et al., 2000). Nonetheless, depositional fluxes of the same order of magnitude as this inferred flux have been observed (Beauchamp et al., 2000). Further measurements of TGM fluxes as well as vertical profiles of TGM are needed to better define the significance of this nocturnal removal of mercury.
4. Conclusions Measurements of TGM were taken at 10 rural sites across Canada. Median TGM concentrations were found to be slightly higher at four sites located in proximity to anthropogenic sources. The only geographic difference in median TGM concentrations was a tendency for lower TGM concentrations in Atlantic Canada. TGM concentrations at the mid-latitude sites exhibited a slight seasonal pattern with higher median concentrations in late winter and lower concentrations in late summer. A diel cycle in TGM concentration was observed at all sites. The most common pattern was maximum concentration near solar noon and minimum concentration immediately before sunrise. The diel cycle was seasonally modulated at all sites, reaching maximum amplitude between June and September at the mid-latitude sites. The nocturnal decreases in TGM observed at most of the sites in summer may indicate a significant removal of mercury from the atmosphere.
References Beauchamp, S., Tordon, R., Phinney, L., Pinette, A., Rencz, A., Dalziel, J., Wong, H.T.K., 2000. Air-surface exchange of mercury over natural and impacted surfaces in Atlantic Canada. Proceedings International Conference on Heavy Metals in the Environment, Ann Arbor, MI, USA, 6–10 August 2000. Ebinghaus, R., Jennings, S.G., Schroeder, W.H., Berg, T., Donagh, T., Guentzel, J., Kenny, C., Kock, H.H., Kvietkus, K., Landing, W., Muhleck, . T., Munthe, J., Prestbo, E., Schneeberger, D., Slemr, F., Sommar, J., Urba, A., Wallschl.ager, D., Xiao, Z., 1999. International field intercomparison measurements of atmospheric mercury species at Mace Head, Ireland. Atmospheric Environment 33, 3063–3073. Iverfeldt, A., 1991. Occurrence and turnover of atmospheric mercury over the Nordic countries. Water, Air, and Soil Pollution 56, 251–265.
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Lee, D.S., Dollard, G.J., Pepler, S., 1998. Gas-phase mercury in the atmosphere of the United Kingdom. Atmospheric Environment 32, 855–864. Lindberg, S.E., Brooks, S., Lin, C.-J., Meyers, T., Chambers, L., 2000. The Barrow Arctic Mercury Study (BAMS): Recent measurements of the production of reactive gaseous mercury during mercury depletion events at Point Barrow, Alaska. Proceedings International Conference on Heavy Metals in the Environment, Ann Arbor, MI, USA, 6–10 August 2000. Lockhart, W.L., Wilkinson, P., Billeck, B.N., Hunt, R.V., Wagemann, R., Bruskill, G.J., 1995. Current and historical inputs of mercury to high latitude lakes in Canada and to Hudson Bay. Water, Air, and Soil Pollution 80, 603–610. Ott, W.R., 1995. Environmental Statistics and Data Analysis. CRC Press, Boca Raton. Poissant, L., 1997. Field observations of total gaseous mercury behavior: interactions with ozone concentration and water vapour mixing ratio at a rural site. Water, Air, and Soil Pollution 97, 341–353. Poissant, L., 2000. Total gaseous mercury in Qu!ebec (Canada) in 1998. Science of the Total Environment 259, 191–201. Poissant, L., Casimir, A., 1998. Water-air and soil-air exchange rate of total gaseous mercury measured at background sites. Atmospheric Environment 32, 883–893. Poissant, L., Casimir, A., 1999. Audit Protocol for Total Gaseous Mercury Measurements, Environment Canada, Montreal. Schmolke, S.R, Schroeder, W.H., Kock, H.H., Schneeberger, D., Munthe, J., Ebinghaus, R., 1999. Simultaneous measurements of total gaseous mercury at four sites on a 800 km transect: spatial distribution and short-time variability of total gaseous mercury over central Europe. Atmospheric Environment 33, 1725–1733. Schroeder, W.H., Anlauf, K.G., Barrie, L.A., Lu, J.Y., Steffen, A., Schneeberger, D.R., Berg, T., 1998. Arctic springtime depletion of mercury. Nature 394, 331. Slemr, F., Scheel, H.E., 1998. Trends in atmospheric mercury concentrations at the summit of the Wank Mountain, southern Germany. Atmospheric Environment 32, 845–853. Slemr, F., Schuster, G., Seiler, W., 1985. Distribution, speciation, and budget of atmospheric mercury. Journal of Atmospheric Chemistry 3, 407–434. Slemr, F., Seiler, W., Schuster, G., 1981. Latitudinal distribution of mercury over the Atlantic Ocean. Journal of Geophysical Research 86, C2, 1159–1166. Steffen, A., Schroeder, W.H., 1999. Standard Operating Procedures Manual Procedure for Total Gaseous Mercury Measurements—Canadian Atmospheric Mercury Measurement Network (CAMNet), Environment Canada, Toronto. US Environmental Protection Agency(US EPA), 1997. Mercury study report to congress, Vol. IV: an assessment of exposure to mercury in the United States. EPA 452/R97-006.
Temporal and spatial variability of total gaseous mercury in Canada: results from the Canadian Atmospheric Mercury Measurement Network (CAMNet) Markus Kellerhalsa,*, Stephen Beauchampb, Wayne Belzerc, Pierrette Blanchardd, Frank Froudee, Bruno Harveyf, Karen McDonalda, Martin Pilotef, Laurier Poissantd, Keith Puckettd, Bill Schroederd, Alexandra Steffend, Rob Tordonb a
Environment Canada, Prairie and Northern Region, 4999-98 Ave., Edmonton, AB, Canada T6B 2X3 b Environment Canada, Atlantic Region, 45 Alderney Dr., Dartmouth, NS, Canada B2Y 2N6 c Environment Canada, Pacific and Yukon Region, #700, 1200 W. 73rd Ave., Vancouver, BC, Canada V6P 6H9 d Environment Canada, Meterological Service of Canada, 4905 Dufferin St., Downsview, Ont., Canada M3H 5T4 e Environment Canada, Centre for Atmospheric Research Experiments, Egbert, Ont., Canada L0L 1N0 f Environment Canada, Qu!ebec Region, 105 rue McGill, 7e e!tage (Youville), Montr!eal, Qu!e., Canada H2Y 2E7 Received 2 January 2002; received in revised form 24 October 2002; accepted 24 October 2002
Abstract Continuous measurements of total gaseous mercury (TGM) concentration were taken during 1997–1999 at 10 rural sites across Canada, ranging from 431 to 821N and from 621 to 1231W. Overall median TGM concentrations ranged among the sites from 1.32 to 1.83 ng m 3. The spatially averaged median concentration among all sites was 1.6070.15 ng m 3. Maximum hourly average concentrations, on the order of 10 ng m 3, were observed at several sites located near major sources of anthropogenic emissions. Minimum hourly average concentrations were observed in springtime at the arctic site where concentrations dropped below the detection limit of the analyzer (50 pg m 3) on several occasions. Seasonal variability in TGM concentrations was observed at all sites. At most sites monthly median concentrations were highest in late winter and lowest in fall. Diurnal variations in TGM concentration were also observed at most sites. The most common pattern of diurnal variability was a diel cycle of minimum concentrations just before sunrise and maximum concentrations around solar noon. The diel cycle was seasonally modulated, reaching maximum amplitude during spring or summer at all sites. Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. Keywords: Atmospheric mercury; Total gaseous mercury; Temporal variability; Seasonal variability; Canada
1. Introduction Mercury is a highly toxic chemical, found throughout the global troposphere. Though the levels of mercury *Corresponding author. Tel.: +1-780-951-8626; fax: +1780-495-2444. E-mail address: [email protected] (M. Kellerhals).
generally found in the atmosphere are well below the threshold considered to cause direct impacts on human health, mercury is still a pollutant of concern because of its propensity to accumulate and concentrate in biota. At higher trophic levels in aquatic food webs, mercury may be found in elevated concentrations, sometimes exceeding the guidelines for human consumption (USEPA, 1997). Atmospheric input of mercury has been
1352-2310/03/$ - see front matter Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 9 1 7 - 2
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recognized as the main source responsible for contamination of remote sites such as lakes in northern Canada (Lockhart et al., 1995). Mercury in the atmosphere is found in both gaseous and particulate form. Of the total gaseous mercury (TGM) component, the dominant form is gaseous elemental mercury (Hg0) (Slemr et al., 1985). Hg0 has been estimated (Slemr et al., 1985) to have a mean atmospheric residence on the order of 1 yr. This residence time is sufficient for Hg0 to become relatively well mixed throughout the Northern Hemisphere troposphere, implying that TGM concentrations should be of similar magnitude at a wide variety of background sites. This has been verified by several TGM measurement studies (e.g. Schmolke et al., 1999; Slemr et al., 1981). Despite the relative uniformity of TGM concentrations there are still important temporal and spatial variations in TGM concentration that provide insight into the processes governing atmospheric mercury concentration and deposition. The Canadian Atmospheric Mercury Measurement Network (CAMNet) was established with the goal of providing accurate, long-term measurements of TGM
concentration across Canada. These measurements are being used to investigate processes governing atmospheric concentrations of mercury, temporal and spatial variability of atmospheric mercury, and sources and sinks of atmospheric mercury. The measurements will also provide a high quality data set that may be used to verify global mercury models. This paper describes the network and summarizes results from 2 yr of measurements.
2. Experimental 2.1. Sites CAMNet was initiated in 1996 to unify TGM measurement initiatives then underway in different regions of Canada. CAMNet currently consists of 11 monitoring sites (see Fig. 1), chosen to represent major geographical and ecological regions of Canada. For the purposes of this study, each site was classified as either rural remote or rural affected. Rural-affected sites are those sites expected to be significantly impacted by
Fig. 1. Location map of CAMNet sites: (1) Alert, (2) Kejimkujik, (3) St. Andrews, (4) Mingan, (5) Kuujjuarapik, (6) St. Anicet, (7) Point Petre, (8) Egbert, (9) Burnt Island, (10) Esther, and (11) Reifel Island.
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Table 1 Location, site classification and measurement start dates for the 11 CAMNet sites. Station
Location
Site
Code
Province
Latitude
Longitude
Alert Kejimkujik St. Andrews Mingan St. Anicet Point Petre Egbert Burnt Island Esther Reifel Island Kuujjuarapik
ALT KEJ STA MIN ANI PPT EGB BNT EST RFL KUJ
NU NS NB QC QC ON ON ON AB BC QC
82.50N 44.43N 45.09N 50.27N 45.12N 43.83N 44.22N 45.80N 51.67N 49.10N 55.28N
62.30W 65.21W 67.00W 64.23W 74.28W 77.15W 79.78W 82.95W 110.20W 123.16W 77.75W
Start date
Classification
Jan 1995 Jul 1995 Nov 1995 Jan 1996 Jul 1994 Nov 1996 Sep 1996 Mar 1998 Jun 1998 Mar 1999 Sep 1999
Rural-Remote Rural-Remote Rural-Remote Rural-Remote Rural-affected Rural-affected Rural-affected Rural-Remote Rural-Remote Rural-affected Rural-affected
The site codes are the station abbreviations used in Fig. 3 and in the text.
nearby anthropogenic mercury emissions. Table 1 lists the location, classification and site code for all CAMNet sites. The ten sites used in this study are briefly described below: *
*
*
*
*
*
*
The Alert site is located near the northeast coast of Ellesmere Island, far from any sizable human settlement or industry. The surrounding area is largely devoid of vegetation. The Kejimkujik site is located 60 km inland from the Atlantic coast in a national park that is itself surrounded by sparse rural development. The site is surrounded by mixed softwood forest. The St. Andrews site is on the St. Croix River adjacent to the Bay of Fundy near the Canada/US border in an area of mixed softwood forest 1 km outside the town of St. Andrews (pop. B1500). The Mingan site is located 1 km from the north shore of the Gulf of Saint Lawrence, 50 km E of Sept-Iles (pop. B25 000) and 25 km W of Havre-St Pierre (pop. B3500). The site is surrounded by boreal black spruce forest. The St. Anicet site is located in the St. Lawrence River valley, 3 km south of St. Anicet (pop. B1000), 30 km SW of a major industrial area, and 90 km SW of the metropolitan area of Montreal (pop. B3 000 000). The site is surrounded by fields and deciduous wood lots. The Point Petre site is located on the north shore of Lake Ontario. The site is surrounded by the lake and deciduous forest. Lake Ontario is ringed by urban and industrial centres. Toronto (pop. B3 000 000) lies about 200 km WSW of the site. The Egbert site is located near Barrie, Ontario. The surrounding area is farmland and wood lots with moderately dense rural settlement. Toronto lies about 100 km south of the site.
*
*
*
The Burnt Island site is located in a sparsely populated rural area near the north shore of Lake Huron. The site is surrounded by coniferous forest. The Esther site is located in a sparsely populated, grassland and grain-farming area of the Canadian Prairies. The nearest sizable community is 40 km SSW of the site. The Reifel Island site is located at the mouth of the Fraser River, on the east shore of the Strait of Georgia. The major urban center of Greater Vancouver (pop B2 000 000) is spread out immediately N, E and SE of the site.
2.2. Network operation The standard operating procedures for the network are described in Steffen and Schroeder (1999). Each site uses a Tekran 2537A analyzer to measure ambient air concentration of total gaseous mercury. This measurement method is described in Poissant (1997). The Tekran instrument has been shown (Ebinghaus et al., 1999) to compare well with other methods of measuring TGM. TGM measurements are made over 15 min intervals, except at St. Andrews and Kejimkujik where TGM is measured over 5 min intervals, and Alert where TGM is measured over 30 min intervals for most of the year and over 5 min intervals during spring. At each site the monitor is located in a temperature-controlled building. Sample intake heights range among the sites from 3 to 6 m above ground level with Teflon inlet lines of 10–15 m in length. The lines are tested quarterly for efficiency by injection of TGM at the sample inlet. The analyzers are automatically calibrated every 23 or 25 h using internal permeation sources that emit vapor mercury at a constant rate. In addition to the automated
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
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calibrations, the permeation sources are programmed to inject a known amount of mercury into the ambient sample stream at regular intervals. These injections are used to verify sample recovery rates. The nominal emission rate of the internal permeation source at each site is verified during a quarterly manual calibration. In this procedure, a known amount of mercury vapor from a Tekran 2505 calibration unit is injected into a zero air sample stream. The instrument response to a series of manual injections can then be compared to its response to injections from the internal permeation source. The ratio of these two numbers is then used to calculate the actual emission rate of the permeation source. If the actual and nominal emission rates are different the nominal emission rate is manually updated. Once every 2 yr an audit of the network takes place. During the audit, a network calibration unit is used to calibrate the permeation source in each instrument and also to provide an intercomparison of the regional calibration units. The audit protocol is described in detail in Poissant and Casimir (1999). The precision of the measurement method at a given site has been estimated to be better than 2% (Poissant, 2000). Comparison across the network also requires an estimate of inter-site variability, which is caused by two main factors: (1) inter-site variability of the difference between nominal permeation source emission rate and actual emission rate, and (2) inter-site variability of the difference between nominal sample volume and actual sample volume. Based on the results of the 1998 and 1999 audits, the RMS deviation of emission rates across the network was 4.1%, while the RMS deviation of sample volumes was 1.6%. Combining all three sources of error yielded an estimated ‘‘network error’’ of 75%. Results from the 1999 audit showed that a significant portion of the inter-site variability was caused by differences in the calibration of the syringes used in the manual calibrations. If it is possible to largely eliminate this source of error, the network error can be reduced to o3% in the future.
2.3. Data management TGM measurements from CAMNet are quality controlled using the Research Data Management and Quality Control (RDMQt) system. RDMQ operates under the SASt statistical analysis system. It provides an automated quality control system that assigns each TGM datum a validity flag based on programmed criteria for sample volume, baseline voltage, baseline standard deviation, difference from adjacent values and absolute magnitude. Hourly average TGM concentrations were calculated for all hours with at least 25% data completeness. These hourly average TGM data are available from Environment Canada’s NATChem database. Data from 1997 and 1998 were used for this study, as those years had the maximum number of stations in operation (Table 1). For the two western Canadian stations (EST, DEL) data from 1999 were included in the analysis as there were few data from 1997 to 1998.
3. Results and discussion An overall average median concentration of 1.6070.15 ng m 3 for the ten sites was calculated by averaging together the ten site medians. Table 2 provides a statistical summary of TGM results from each site. The four rural-affected sites had significantly (po0.005) higher median concentrations (mean B1.70 ng m 3, range B1.65 to 1.83 ng m 3) than did the six ruralremote sites (meanB1.54 ng m 3, range B1.32 to 1.69 ng m 3). This difference is also significant in comparison with the estimated network error of 75%. Maximum concentrations were substantially higher at the rural-affected sites (6.15–10.19 ng m 3) versus the remote sites (2.64–4.57 ng m 3). TGM concentrations were also more variable at the rural-affected sites versus the rural-remote sites. The one exception is Alert, a remote site that also had high variability.
Table 2 Statistical summary of TGM concentrations. Station
N
Mean
Standard Deviation
Median
Minimum
Maximum
Alert Kejimkujik St. Andrews Mingan St. Anicet Point Petre Egbert Burnt Island Esther Reifel Island
15021 15730 15866 15530 16665 16390 14536 5229 10766 6971
1.55 1.33 1.43 1.62 1.72 1.90 1.65 1.58 1.69 1.69
0.39 0.25 0.20 0.22 0.40 0.43 0.31 0.23 0.19 0.30
1.59 1.32 1.42 1.67 1.67 1.83 1.65 1.56 1.69 1.67
o0.05 0.26 0.68 0.36 0.59 0.97 0.87 0.65 1.11 1.14
3.04 2.69 2.64 3.07 9.62 8.50 6.15 3.04 4.57 10.19
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
observed in Europe. Slemr and Scheel (1998) reported a median concentration of 1.93 ng m 3 at a mountaintop site in Germany in 1996. Lee et al. (1998) reported a mean concentration of 1.68 ng m 3 at a rural site in the UK in 1995–1996. Schmolke et al. (1999) reported median concentrations between 1.54 and 1.93 ng m 3 on a north–south measurement transect between Stockholm and Berlin in the summer of 1995. The north-south TGM gradient observed in Europe (Schmolke et al., 1999) was not seen in this study. The mean median concentration for the nine mid-latitude sites (1.6170.16 ng m 3) was not significantly different than the median concentration at the one arctic site (1.59 ng m 3). However, median concentrations at the Atlantic sites (KEJ, STA) averaged lower than at remote sites elsewhere in Canada (MIN, ALT, BNT, EST)—1.37 ng m 3 versus 1.63 ng m 3. This difference was statistically significant (po0.005) and also significant compared to the estimated network error. The relatively low concentrations in Atlantic Canada are surprising, considering that this region is downwind of significant mercury emissions from the northeastern US. Fig. 3 illustrates the probability distribution of TGM concentrations at the CAMNet sites. The four ruralaffected sites all had right-skewed probability distributions that approximate log normality. The log-normal distribution has been shown to be the theoretical distribution of pollutant concentrations for a passive (no atmospheric sources or sinks) pollutant that is emitted and then subject to successive random dilutions (Ott, 1995). Given the long atmospheric lifetime of elemental mercury, it might be expected to act as a passive pollutant on the time scale of transport from nearby sources to a monitoring site. Four of the ruralremote sites (EST, BNT, STA, KEJ) had roughly symmetrical, near normal probability distributions. Rather than reflecting successive random dilutions, concentrations at these remote sites seem to fluctuate
Higher variability of TGM concentrations at the rural-affected sites appeared to be caused by the alternating exposure of these sites to anthropogenic TGM emissions, depending on wind direction and atmospheric mixing. For example, Fig. 2 shows the dependence of TGM concentration on wind direction at the Reifel Island site. Winds from the north through southeast traveled over the metropolitan area of Vancouver before reaching the site, and had higher TGM concentrations on average than did winds from the south through northwest, which traveled over ocean prior to reaching the site. In contrast, high variability of TGM concentrations at Alert was primarily due to springtime episodes of mercury depletion (Schroeder et al., 1998). Median TGM concentrations at CAMNet sites were in general similar to or slightly lower than those
0 30
1.8
60
1.6 1.4
90
240
120 210
150 180
3
Percentiles 99th 95th 75th Median 25th 5th 1st
2
1
EGB
PPT
ANI
MIN
STA
KEJ
0 ALT
TGM Concentration (ng/m3)
Fig. 2. Dependence of TGM concentration on wind direction at Reifel Island. Radial axis is TGM concentration in ng m 3. Angular axis is wind direction.
RFL
270
EST
300
BNT
330
1007
Site Fig. 3. Probability distribution of TGM concentration at CAMNet sites.
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
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around a regional background concentration. The fluctuations represent the effects of multiple processes such as chemical depletion, surface deposition, volatilization from surrounding surfaces, and occasional transport of mercury rich air from source regions. Two of the rural-remote sites (ALT, MIN) had strongly left-skewed probability distributions. At Alert, this distribution arose because TGM was fairly constant through fall and winter, and was subject to large multiday depletion events in late winter and early spring. At Mingan, the left-skewed distribution arose due to relatively constant concentrations from November through April, followed by frequent strong nocturnal depletion May–October.
To investigate seasonal variability of TGM concentration, median monthly concentrations were calculated for each site for years with 10 months or more of data. These data are shown in Fig. 4. A slight seasonal trend is seen for eight of the nine mid-latitude sites over the 2-yr study period, with higher concentrations observed in winter and spring, and lower concentrations in summer and fall. The only exception was the Point Petre site where there was a very strong summer peak in 1998. Point Petre is downwind of major anthropogenic source regions and is right on the shore of Lake Ontario, which remains cold all summer long. The combination of sources and meteorology will be investigated to find an explanation for the different values observed at this site.
TGM Concentration (ng/m3)
2.5
2
1.5
1
0.5 Jan
(a)
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Alert 1997 Alert 1998 Median of Medians: Mid-Latitude Stations
3
TGM Concentration (ng/m )
2.5
2
1.5
1
0.5
(b)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Fig. 4. Monthly medians of TGM concentration at CAMNet sites. Data from the mid-latitude sites are shown in the top panel, with Point Petre 1998 as a dashed line. Data from Alert are shown in the lower panel. The line with square symbols is the median of the midlatitude monthly median data shown in the top panel.
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
Seasonal cycles in TGM observed elsewhere also show a winter maximum in concentration. For example, measurements of TGM in Scandinavia (Iverfeldt, 1991) found median concentrations in winter to be 33% higher than in summer. In contrast, the seasonal cycle at Alert, also shown in Fig. 4, is distinctly different from that observed at the mid-latitude sites. Annual hourly mean TGM concentrations for each site are shown in Fig. 5. All sites showed some evidence of a diel cycle in concentration. The mean amplitude of the diel cycle ranged among the sites from 3 to 13 percent of site mean concentration. Seven of the 10 sites experienced a cycle of maximum concentrations near solar noon (1000–1400 local standard time) and minimum concentrations in the early morning hours (0300– 0700). The diel cycle observed at seven CAMNet sites contrast with the diel cycle observed at several European sites (Lee et al., 1998; Schmolke et al., 1999). TGM concentrations at the European stations peaked at night. This nighttime peak was attributed to soil mercury emissions building up under the nocturnal inversion. In contrast, the diel cycle at the CAMNet sites must be attributed to nighttime depletion of TGM in the lowermost atmosphere. Overnight a shallow TGMdepleted layer is formed underneath the nocturnal inversion layer. Shortly after sunrise there is a rapid increase in near surface TGM concentration as the nocturnal inversion breaks down and undepleted air is mixed down to the surface. The continued increase in TGM concentration through the morning and into the early afternoon is likely caused by surface emission of TGM, which has been observed to increase with increasing solar radiation (Poissant and Casimir, 1998). The three sites where different diel cycles were observed were Alert, St. Andrews and Reifel Island.
TGM concentration at Alert varied by only 3% of mean concentration, with a slight peak near solar noon and a broad minimum between 1700 and 0100. Diurnal variability is quite limited at Alert since the site experiences months of continuous darkness, followed by months of continuous daylight. Nonetheless, during the boreal spring and summer there are significant daily variations in solar radiation as the solar zenith angle varies by approximately 151 each day. The resulting variations in insolation may be sufficient to drive a daily cycle of deposition and re-volatilization and also to affect the rate of the photochemical reactions that have been observed to deplete TGM at arctic sites (Lindberg et al., 2000). Reifel Island experienced maximum concentrations in the early morning and minimum concentrations in the evening. Reifel Island provides an example of how local wind systems may cause diurnal TGM variability. During daytime hours, the onshore sea-breeze advected low-TGM air from the Strait of Georgia past the analyzer, while at nighttime the land breeze advected TGM-rich urban air from Greater Vancouver past the site. The relationship between wind direction and TGM concentration at Reifel Island is shown in Fig. 2. Fig. 6 demonstrates the diurnal shift in wind direction during the summer months when the sea breeze is most prominent. Finally, at St. Andrews TGM concentrations peaked around 1800 and were minimum around 0600. This diel cycle is similar to the other sites, except that peak concentrations occurred significantly later at St. Andrews. This apparently delayed peak arises in part because during winter there is actually a mid-day minimum at St. Andrews, therefore on an annual average basis mid-day concentrations are depressed relative to concentrations later in the afternoon. The diel cycle was seasonally modulated at all sites. At all nine mid-latitude sites, the diel cycle was strongest
2
TGM Concentration (ng/m3)
1009
Point Petre Reifel Island St. Anicet Esther Egbert Mingan Alert Burnt Island St. Andrews Kejimkujik
1.8
1.6 1.6
1.4
1 3 5 7 9 11 13 15 17 19 21 23 Hour Fig. 5. Diel cycle of TGM concentrations at CAMNet sites based on annual average concentrations for each hour.
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
1010
TGM Concentration (ng/m3)
% variation from seasonal mean
Fig. 6. Wind rose at Reifel Island during July through September for (a) daytime hours (1300–2000) and (b) nighttime hours (0100– 0800).
6 4 2 0 -2 Winter Summer
-4
Spring Fall
-6 1
3
5
7
9
11
13 15 Hour
17
19
21
23
Fig. 7. Seasonal modulation of diel TGM cycle at Esther expressed as percent deviation from seasonal mean concentration for each hour.
June–September, while at Alert the diel cycle was strongest in April and May. Fig. 7 shows the seasonal variation of the diel cycle at Esther, a fairly typical case. At Esther, the amplitude of the diurnal cycle during summer was approximately 10% of the mean TGM concentration, while in winter the cycle was largely absent. Other sites showed an even more dramatic seasonal effect. For example, at Mingan the amplitude of the diurnal cycle changed from near zero during winter to 20% of mean concentration during summer. There are several possible reasons for a stronger diel cycle in summer including: stronger diurnal variations in insolation and temperature, stronger daily cycle of mixed layer development, greater role of uptake/emission by vegetation, increased volatilization of mercury from nearby water bodies, and chemical reactions enhanced by higher temperatures or greater solar radiation. The annual and seasonal average diel cycles tend to understate the strength of the diurnal variability by averaging together many cycles of differing phase and amplitude. For instance, the three Atlantic coastal sites (MIN, KEJ, STA) each had one or more months in
2 1.6 1.2 0.8 0.4 0 203
208
213 Julian Day
218
Fig. 8. Time series of TGM concentration at Mingan from 22 July, 1997 to 10 August 1997 (Julian day 203–222). Vertical grid lines are at 00 h.
winter when mean TGM concentrations were lowest near solar noon (perhaps due photochemical oxidation of TGM in a process similar to the arctic springtime depletions), producing a diel cycle opposite in phase to the summer cycle. Even during the summer months when the diel cycle is strongest, most sites had periods when the diel cycle was nearly absent and other periods when its amplitude was a large fraction of mean concentration. This is illustrated in Fig. 8, a time series plot of TGM concentration at Mingan during a typical 20-day period in summer 1997. On 12 of the 20 days there were strong diel cycles. These diel cycles consisted of nocturnal depletion of TGM, by an average of 0.8 ng m 3, or approximately 50% of mean TGM concentration. On several other days in the period the diel cycle was absent. The strong nocturnal depletion of TGM that occurred at Mingan and several other sites indicates a significant removal of atmospheric mercury over night. For example, on the 12 nights described above TGM concentration was reduced by an average of 0.8 ng m 3 over a 10-h period. If this depletion was confined to a 20-m layer under a strong nocturnal inversion, the inferred deposition flux is 1.6 ng m 2 h 1. A similar calculation was reported in Poissant (2000), however,
M. Kellerhals et al. / Atmospheric Environment 37 (2003) 1003–1011
that study assumed a very deep nocturnal inversion layer and hence calculated a much higher deposition rate. Existing measurements of TGM fluxes over natural surfaces in Canada indicate that emissions are more prevalent than uptake (Poissant and Casimir, 1998; Beauchamp et al., 2000). Nonetheless, depositional fluxes of the same order of magnitude as this inferred flux have been observed (Beauchamp et al., 2000). Further measurements of TGM fluxes as well as vertical profiles of TGM are needed to better define the significance of this nocturnal removal of mercury.
4. Conclusions Measurements of TGM were taken at 10 rural sites across Canada. Median TGM concentrations were found to be slightly higher at four sites located in proximity to anthropogenic sources. The only geographic difference in median TGM concentrations was a tendency for lower TGM concentrations in Atlantic Canada. TGM concentrations at the mid-latitude sites exhibited a slight seasonal pattern with higher median concentrations in late winter and lower concentrations in late summer. A diel cycle in TGM concentration was observed at all sites. The most common pattern was maximum concentration near solar noon and minimum concentration immediately before sunrise. The diel cycle was seasonally modulated at all sites, reaching maximum amplitude between June and September at the mid-latitude sites. The nocturnal decreases in TGM observed at most of the sites in summer may indicate a significant removal of mercury from the atmosphere.
References Beauchamp, S., Tordon, R., Phinney, L., Pinette, A., Rencz, A., Dalziel, J., Wong, H.T.K., 2000. Air-surface exchange of mercury over natural and impacted surfaces in Atlantic Canada. Proceedings International Conference on Heavy Metals in the Environment, Ann Arbor, MI, USA, 6–10 August 2000. Ebinghaus, R., Jennings, S.G., Schroeder, W.H., Berg, T., Donagh, T., Guentzel, J., Kenny, C., Kock, H.H., Kvietkus, K., Landing, W., Muhleck, . T., Munthe, J., Prestbo, E., Schneeberger, D., Slemr, F., Sommar, J., Urba, A., Wallschl.ager, D., Xiao, Z., 1999. International field intercomparison measurements of atmospheric mercury species at Mace Head, Ireland. Atmospheric Environment 33, 3063–3073. Iverfeldt, A., 1991. Occurrence and turnover of atmospheric mercury over the Nordic countries. Water, Air, and Soil Pollution 56, 251–265.
1011
Lee, D.S., Dollard, G.J., Pepler, S., 1998. Gas-phase mercury in the atmosphere of the United Kingdom. Atmospheric Environment 32, 855–864. Lindberg, S.E., Brooks, S., Lin, C.-J., Meyers, T., Chambers, L., 2000. The Barrow Arctic Mercury Study (BAMS): Recent measurements of the production of reactive gaseous mercury during mercury depletion events at Point Barrow, Alaska. Proceedings International Conference on Heavy Metals in the Environment, Ann Arbor, MI, USA, 6–10 August 2000. Lockhart, W.L., Wilkinson, P., Billeck, B.N., Hunt, R.V., Wagemann, R., Bruskill, G.J., 1995. Current and historical inputs of mercury to high latitude lakes in Canada and to Hudson Bay. Water, Air, and Soil Pollution 80, 603–610. Ott, W.R., 1995. Environmental Statistics and Data Analysis. CRC Press, Boca Raton. Poissant, L., 1997. Field observations of total gaseous mercury behavior: interactions with ozone concentration and water vapour mixing ratio at a rural site. Water, Air, and Soil Pollution 97, 341–353. Poissant, L., 2000. Total gaseous mercury in Qu!ebec (Canada) in 1998. Science of the Total Environment 259, 191–201. Poissant, L., Casimir, A., 1998. Water-air and soil-air exchange rate of total gaseous mercury measured at background sites. Atmospheric Environment 32, 883–893. Poissant, L., Casimir, A., 1999. Audit Protocol for Total Gaseous Mercury Measurements, Environment Canada, Montreal. Schmolke, S.R, Schroeder, W.H., Kock, H.H., Schneeberger, D., Munthe, J., Ebinghaus, R., 1999. Simultaneous measurements of total gaseous mercury at four sites on a 800 km transect: spatial distribution and short-time variability of total gaseous mercury over central Europe. Atmospheric Environment 33, 1725–1733. Schroeder, W.H., Anlauf, K.G., Barrie, L.A., Lu, J.Y., Steffen, A., Schneeberger, D.R., Berg, T., 1998. Arctic springtime depletion of mercury. Nature 394, 331. Slemr, F., Scheel, H.E., 1998. Trends in atmospheric mercury concentrations at the summit of the Wank Mountain, southern Germany. Atmospheric Environment 32, 845–853. Slemr, F., Schuster, G., Seiler, W., 1985. Distribution, speciation, and budget of atmospheric mercury. Journal of Atmospheric Chemistry 3, 407–434. Slemr, F., Seiler, W., Schuster, G., 1981. Latitudinal distribution of mercury over the Atlantic Ocean. Journal of Geophysical Research 86, C2, 1159–1166. Steffen, A., Schroeder, W.H., 1999. Standard Operating Procedures Manual Procedure for Total Gaseous Mercury Measurements—Canadian Atmospheric Mercury Measurement Network (CAMNet), Environment Canada, Toronto. US Environmental Protection Agency(US EPA), 1997. Mercury study report to congress, Vol. IV: an assessment of exposure to mercury in the United States. EPA 452/R97-006.