Grantee Research Project Results
Final Report: Inflow, Chemistry and Deposition of Mercury to the West Coast of the United States
EPA Grant Number: R829797Title: Inflow, Chemistry and Deposition of Mercury to the West Coast of the United States
Investigators: Jaffe, Daniel , Prestbo, Eric
Institution: University of Washington
EPA Project Officer: Chung, Serena
Project Period: June 1, 2002 through June 1, 2005
Project Amount: $756,774
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text | Recipients Lists
Research Category: Heavy Metal Contamination of Soil/Water , Air Quality and Air Toxics , Safer Chemicals , Air
Objective:
The primary objectives of this project were to quantify the flux of mercury to the U.S. from global sources, characterize the seasonal patterns and chemical speciation of this flux and to examine the background deposition of mercury along the west coast of the U.S. In particular we wanted to evaluate the significance of large mercury sources in Asia on transport and deposition of mercury in the U.S. We learned in our previous work that long-range transport of pollutants from Asia to the U.S. occurs on a fairly regular basis.
Summary/Accomplishments (Outputs/Outcomes):
Over the three years of this project, instruments were purchased and configured for remote field measurements of mercury (elemental, particulate and reactive). These instruments were deployed at three ground sites and
- Cheeka Peak, a marine boundary layer site along the western tip of Washington state;
- Mt. Bachelor, a high elevation (9000’), free tropospheric site in central Oregon; and
- Cape Hedo, Okinawa, Japan, a remote site which receives substantial influence from Chinese pollution sources.
During the final year of the project, we also used a small, twin engine aircraft to measure Hg0 at altitudes up to 6000 meters asl (results discussed below). Our previous work on mercury identified significant diurnal patterns of elemental mercury in the marine boundary layer and rapid removal during some time periods. Measurements at Cheeka Peak during 2003 provided important information on global background mercury concentrations and processing within the marine boundary layer, however, they did not reveal strong signals of long-range transport from Asia. For this reason, our mercury instrument was moved to the Mt. Bachelor site in March 2004, which receives much stronger signals of long-range transport. In the first year of measurement we detected several episodes of long-range transport of carbon monoxide, aerosols and mercury at Mt. Bachelor.
In Fall of 2003 a decision was made to modify our work plan so as to incorporate measurements of mercury at the Japanese station on Okinawa. This gave us direct information on the sources, transport and speciation of mercury coming out of China (this modification to our work plan was approved by EPA). A full set of mercury instruments (elemental, particulate and reactive) were installed at Okinawa in March 2004 and remained there for approximately two months. In the first two weeks of the campaign we detected several episodes of transport of pollutants from China, which brought high levels of mercury to the station.
Both at Mt. Bachelor and Okinawa, the co-location of mercury instrumentation with carbon monoxide, ozone and aerosol instruments added significantly to the value of our mercury data. The addition data allowed us to characterize the general state of the atmosphere as well as to carry out quantitative budget and flux calculations. During the second year of the project, we made mercury measurements at two sites:
- Mt. Bachelor (March 2004 to present), and;
- Cape Hedo, Okinawa, Japan (March-May 2004).
Our 2004 observations led to several important results, including a pronounced diurnal cycle of reactive gaseous mercury (RGM) at Okinawa, the consistency of the Hg/CO ratio during Asian plumes at both Mt. Bachelor and Okinawa, and comparisons with the GEOS-Chem global model in collaboration with Daniel Jacob at Harvard. Figure 1, below, shows the data from one long-range transport event as observed at Mt. Bachelor. Figure 1. Observations of CO, O3, aerosol scatter and Hg0 as observed at Mt. Bachelor in Central Oregon in April 2004 (Jaffe et al., 2005). The Asian source was confirmed by back-trajectories as well as the Hg/CO ratio.
Figure 1. Observations of CO, O3, aerosol scatter and Hg0 as observed at Mt. Bachelor in Central Oregon in April 2004 (Jaffe et al., 2005). The Asian source was confirmed by back-trajectories as well as the Hg/CO ratio.
In addition to these observations, during 2004 we worked to develop a reliable method for field calibrations of RGM. The system is based on an HgCl2 permeation tube. We were able to develop a steady source of HgCl2, however collection efficiency of the Tekran denuder system appeared to be somewhat low, leading us to continue our investigations of this challenge.
During the third year of the project we continued our measurements of elemental Hg at Mt. Bachelor. We also added in measurements of speciated mercury at this site. RGM was measured with a Tekran 1103, and particulate mercury (PHg) with a Tekran 1135. In collaboration with Jerry Keeler's group at Michigan, we also evaluated the feasibility of measuring Hg0 from a small, twin-engine aircraft. The first test flight was conducted in the fall of 2005, and since the results from this test flight were generally positive, we made plans to conduct a full suite of science flights with the Tekran instrument on-board in the spring of 2006.
The Mt. Bachelor RGM measurements gave us some very interesting data. While the RGM concentrations are usually low, in dry, subsiding air masses the concentrations can reach quite high levels. PHg levels were nearly always low. The high RGM periods were also associated with low Hg(0) on a nearly 1:1 basis. The figure below shows the relationship between RGM and Hg0 during these episodes. This suggests conversion of Hg(0) to RGM, which is a key process in the global cycling of mercury. Figure 2. Inverse relationship between GEM and RGM as seen at Mt. Bachelor during late spring-summer of 2005. The two points that are circled occurred with fog, suggesting loss of RGM by heterogeneous processes, whereas all other high RGM periods occurred in dry, subsiding air (Swartzendruber et al., 2006).
Figure 2. Inverse relationship between GEM and RGM as seen at Mt. Bachelor during late spring-summer of 2005. The two points that are circled occurred with fog, suggesting loss of RGM by heterogeneous processes, whereas all other high RGM periods occurred in dry, subsiding air (Swartzendruber et al., 2006).
Since this oxidation is a key process in the global mercury cycle, all global models must include this process; however, there are significant uncertainties associated with the oxidants, oxidation rates, products and removal of the RGM. Through collaboration with the GEOS-Chem modeling group (at Harvard and UW), we have compared our Mt. Bachelor data to their state-of-the-art global mercury model. The results are shown in the figure below.
Figure 3. Comparison of the GEOS-Chem modeled RGM with observations from MBO for 2005.
The model does a reasonable job of capturing the mean concentration and the diurnal cycle; however, it does not capture the high RGM peaks seen on some days. One of our key goals in 2006 was to identify the cause of these high RGM peaks and understand why the model does not capture them. This is a key task for understanding the global mercury cycle.
Because of the importance of Hg speciation, it is important that the measurements are accurate, and that any biases or artifacts due to changing atmospheric conditions or analytical procedures are identified. While a few intercomparison campaigns and quality assurance measurements have been made there has been no robust field calibration technique for RGM. Without such a system, the current atmospheric mercury speciation method remains operationally defined. To remedy this situation, work was begun at Frontier Geosciences to develop a set of RGM spiking/calibration methods. One method involves spiking KCl coated denuders with known quantities of RGM in a liquid form (as HgCl2). Spiked denuders are shipped into the field, loaded and desorbed into a speciation system, ensuring the system is making accurate RGM measurements. Denuders have been spiked at concentrations ranging from 24.2 pg to 96.9 pg, with 90.5% to 94.9% recoveries. Preliminary analysis on the effects from denuder temperature and holding time study showed no appreciable loss of RGM on denuders stored for two days at 21 degrees C. A second method involves injecting a liquid-based RGM standard onto a quartz cup suspended in the inlet of a sampler; both cup and inlet are then heated. Following injection, the liquid rapidly volatilizes leaving HgCl2 to vaporize into the denuder. This dynamic method best simulates conditions during normal sampling operations. Spikes ranging from 24.2 pg to 96.9 pg have been produced with recoveries of 78.1% to 88.0%. The delivery technique and proper heating of the inlet and quartz cup has proven critical towards reaching recoveries in this range. Frontier continues to develop this method and will hopefully develop a field usable unit in the near future.
During the spring of 2006 we conducted 8 vertical profiles using our small twin-engine Beechcraft Duchess. These flights took place in conjunction with the NASA led INTEX experiments in the North Pacific/Western North America region and were specifically intended to examine the importance of Asian pollutants on background air quality. Our aircraft was outfitted with instruments for continuous measurements of CO, O3, aerosol scatter and Hg0. The figure below shows the results from the 8 vertical profiles.
Figure 4. Vertical profile of O3, aerosol scattering (σsp), CO and Hg0 from 8 flights during April-May 2006 off the coast of Washington state.
Conclusions:
These data are now being analyzed with respect to long-range transport of Asian pollution, comparison with the GEOS-CHEM global chemical transport model and the relationship between Hg0 and possible oxidation processes in the upper atmosphere. We expect to submit a manuscript to the Journal of Geophysical Research within the next 6 months.
Journal Articles on this Report : 7 Displayed | Download in RIS Format
Other project views: | All 34 publications | 7 publications in selected types | All 7 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Jaffe D, Landis MS, Weiss-Penzias P, Prestbo E. Comment on “Mercury concentrations in coastal California precipitation: Evidence of local and trans-Pacific fluxes of mercury to North America” by D. J. Steding and A. R. Flegal. Journal of Geophysical Research 2003;108(D17):4553, doi:10.1029/2003JD003504. |
R829797 (2003) R829797 (Final) |
not available |
|
Jaffe D, Prestbo E, Swartzendruber P, Weiss-Penzias P, Kato S, Takami A, Hatakeyama S, Kajii Y. Export of atmospheric mercury from Asia. Atmospheric Environment 2005;39(17):3029-3038. |
R829797 (2004) R829797 (2005) R829797 (Final) |
Exit Exit |
|
Keating T, West J, Jaffe D. Air quality impacts of intercontinental transport. EM: The Magazine for Environmental Managers, October 2005, pp. 28-30. |
R829797 (2005) R829797 (Final) |
not available |
|
Selin NE, Jacob D, Park R, Yantosca RM, Strode S, Jaeglé L, Holand C, Jaffe DA. Chemical cycling and deposition of atmospheric constraints from observations. Journal of Geophysical Research 2007;112:D02308. doi: 10.1029/2006JD007450. |
R829797 (Final) |
not available |
|
Swartzendruber P, Jaffe DA, Prestbo EM, Weiss-Penzias P, Selin NE, Park R, Jacob DJ, Strode s, Jaeglé L. Observations of reactive gaseous mercury in the free troposphere at the Mount Bachelor Observatory. Journal of Geophysical Research 2006;111:D24301, doi: 10.1029/2006JD007415. |
R829797 (2005) R829797 (Final) |
Exit |
|
Weiss-Penzias P, Jaffe DA, McClintick A, Prestbo EM, Landis MS. Gaseous elemental mercury in the marine boundary layer: Evidence for rapid removal in anthropogenic pollution. Environmental Science & Technology 2003;37(17):3755-3763. |
R829797 (2003) R829797 (Final) |
|
|
Weiss-Penzias P, Jaffe DA, Swartzendruber P, Dennison JB, Chand D, Hafner W, Prestbo E. Observations of Asian air pollution in the free troposphere at Mount Bachelor Observatory during the spring of 2004. Journal of Geophysical Research 2006;111:D10304, doi:10.1029/2005JD006522. |
R829797 (2004) R829797 (2005) R829797 (Final) |
Exit |
Supplemental Keywords:
Air, ambient air, tropospheric pollution, chemical transport, toxics, air toxics, metals, heavy metals, mercury, environmental chemistry, atmospheric chemistry, Pacific Northwest, EPA Region X, long-range transport,, Scientific Discipline, Water, Environmental Chemistry, Ecology and Ecosystems, Mercury, fate and transport, mercury measurement, mercury formation, chemical speciation, marine boundary layer, formation, mercury detection, West CoastProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.