NADP: Keeping You Connected, Issue 5

NADP 2014 Mercury Maps Now Available

The National Atmospheric Deposition Program (NADP) produces two different types of annual maps for mercury across the United States: precipitation-weighted mean (PWM) concentration of total mercury and total mercury deposition. What is the difference between these two maps? And what do they tell us?

Figure 1: Click to enlarge

The PWM total mercury concentration map shows the mass of mercury suspended in a specific amount of precipitation, while the total mercury deposition map shows the flow of mercury to the surface of the earth from precipitation (see Figure 1).

Figure 2 shows the PWM total mercury concentration (in nanograms per liter of precipitation units) for 2014. This is a typical pattern of total mercury concentration across the United States, with very high concentrations in the West and along the Gulf Coast (red colors). Moderate concentrations dominate the Plains and Midwest (beige color), and the lowest concentrations are found in the Northeast U.S. (green colors), extending into Maritime Canada, and in the Pacific Northwest. Concentrations across Alaska are similar to the low concentration regions (<7.0 ng/L). NADP data show that this pattern persists from year to year, but it is somewhat unexpected, given that coal-fired power plants dominate the Midwest and Northeast U.S. (an important source of mercury to the atmosphere). The overall pattern is partly explained by low precipitation rates in the West (mercury in low precipitation amounts leads to high concentrations). High concentrations along the Gulf may be due to deep thunderstorm convection and capturing of mercury from higher regions of the atmosphere. Lower concentrations in the Northeast, Pacific Northwest, and Alaska are assumed to result from similar mercury loading but in more precipitation events (i.e. lower concentrations). NADP concentration maps are shown as PWM concentrations, meaning the concentration of high volume precipitation events are weighted more heavily in the average, and low precipitation volume events are only lightly weighted. The PWM result is an average concentration representing a more typical concentration across all precipitation events.

Figure 3 is the deposition of mercury flowing to the surface in micrograms per square meter over the year. The total mercury deposition map is produced by multiplying the values in the concentration map by the annual precipitation across North America. Figure 3 shows the typical deposition pattern across North America, with high deposition along the Gulf Coast resulting from high mercury concentrations and high precipitation rates. Moderate deposition across the Midwest and Plains is due to moderate mercury concentrations and moderate precipitation. Low deposition in the Northeast and Northwest is driven by lower concentrations and high precipitation in the Northwest and somewhat higher precipitation in the Northeast. The lower elevation West experiences low deposition rates due to lower precipitation rates, but higher predicted deposition rates in the mountain regions, where winter time precipitation (snow) is very high.

Figure 2. Click to enlarge	Figure 3. Click to enlarge

New USGS-led Study: High Mercury Wet Deposition at a “Clean Air” Site in Puerto Rico

An exciting new study, led by researchers with the U.S. Geological Survey (USGS), has just been released entitled “High Mercury Wet Deposition at a “Clean Air” Site in Puerto Rico.” Lead author Dr. James Shanley, along with other USGS scientists and NADP collaborators, documents new observations of mercury wet deposition in clean air over the island of Puerto Rico. Their study is novel in several respects. Perhaps most importantly, it provides a long-term observational record (~7 years) of mercury concentrations and wet deposition in the tropics, where few observations exist.

The authors report very high wet deposition fluxes, well in excess of those reported at any of the U.S. or Canada Mercury Deposition Network (MDN) sites. Annual fluxes average 27.9 micrograms of mercury per square meter. This flux is significantly higher than that found along the U.S. Gulf Coast (the area with the highest mercury measurements in the U.S.). These high fluxes in Puerto Rico are driven by very high precipitation rates (112 inches per year), which is twice the average annual rainfall along the Gulf Coast. On the other hand, the mercury concentrations observed in this relatively clean atmosphere are about the same concentration as those found elsewhere in the MDN. The precipitation weighted mean concentration was 9.8 nanogram of mercury per liter of precipitation.

Figure 4. Click to enlarge

The authors point to a particularly interesting finding of increasing mercury concentrations with increasing cloud height (or cloud tops, see Figure 4). In other words, the rainfall contained more mercury as the thunderstorm heights increase. This suggests that much of the mercury is captured from the upper atmosphere. The authors suggest that the “primary mercury source is the global pool.” This follows several earlier studies that have shown a large amount of oxidized mercury in the upper troposphere, which can be moved to the surface with deep thunderstorm-driven convection and rain droplet capturing of the mercury aloft.

The authors also suggest that other areas in the tropics could have high deposition rates too, based on their similar precipitation regimes and access to the same high altitude global pool of mercury.

The full article in Environmental Science and Technology (2015) can be found here: doi:10.1021/acs.est.5b02430

Thirty Years of NADP Monitoring in a Puerto Rico Rain Forest

Molly Woloszyn, NADP Outreach Coordinator

Figure 5. Click to enlarge

The National Atmospheric Deposition Program (NADP) first established a monitoring site in Puerto Rico (PR20) in February 1985 as part of the National Trends Network (NTN). In 2014, the PR20 NTN site was joined by two other NADP networks: the Ammonia Monitoring Network (AMoN) and the Mercury Deposition Network (MDN). PR20 is sponsored by the U.S. Forest Service (NTN and AMoN) and the U.S. Geological Survey (MDN). Alonso Ramirez has been the PR20 site supervisor since 2001 and John Bithorn (Figure 5) is a field technician with the El Verde Field Station (University of Puerto Rico) and has served as the primary PR20 site operator since 1994. Ramirez took a few minutes to talk with NADP about his experience and time as an NADP site supervisor.

Figure 6. Click to enlarge

NADP: Can you describe where PR20 is located?
Ramirez: PR20 is located in Puerto Rico, the only location within the United States, other than Hawaii, where tropical rain forest ecosystems are available for scientific inquiry. The site is in the Luquillo Experimental Forest (LEF), a Biosphere Reserve that coincides with the boundaries of El Yunque National Forest in the Luquillo Mountains (Figure 6). The site is at El Verde Field Station, situated at 350 meters (1,148 feet) elevation on the northwestern slope of the LEF, which has been an important research location for tropical ecology since the 1960s. In 1988, the National Science Foundation established the Luquillo Long-Term Ecological Research (LUQ-LTER) program and El Verde became the principal research site for the program. Today, the station is used by numerous faculty, technical staff, and students from the University of Puerto Rico, USDA Forest Service, and other academic institutions in Puerto Rico, mainland United States, and other countries.

Figure 7. Click to enlarge

NADP: How isolated is the station? How long does it take you to get to the station from your office?
Ramirez: Since we are a small island, the smallest of the Greater Antilles, everything is close. The university main campus is located at 40 minutes’ drive from the El Verde Field Station. The NADP site is located at the station, a few meters behind the main facilities. This makes monitoring easier.

NADP: Do you have any fun or interesting stories from your time as an NADP site supervisor?
Ramirez: Working in a rainforest is a major challenge. We climb the tower under high humidity, strong solar radiation, or major downpours. Bees like to build nests in our electric boxes and sometime we even got some honey after removing them. We also have to secure the tower every year at the beginning of the hurricane season, make sure that no dead trees or branches will fall on guide wires, and be prepared to repair it if a storm hits the area. Of course, almost all storms change their course at the last minute and we haven’t had much of a problem. No complaints here.

Figure 8. Click to enlarge

NADP: I hear the site is up on a tower and receives a lot of precipitation, can you tell me more about that? Ramirez: The site is located at the top of a 20-foot tower (Figure 8), to be able to sample rain above the forest canopy. We also receive a lot of rainfall and the sampling containers are often full to the rim (see box below  for more details on rainfall at PR20).

NADP: What’s the best part of your job as a site supervisor? What about the most difficult part? Ramirez: Best part is to be able to participate in a large and important network gathering data that is used by a large diversity of researchers. The most challenging part is to maintain the Tuesday sampling, even during holidays, or tropical storms.

NADP: In addition to supervising PR20, can you describe other components of your job or the station? Ramirez: PR20 is located at El Verde Field Station, which is a very active research station. The main research focus is on tropical forest ecology and how tropical forests respond to natural disturbances (e.g., hurricanes) and anthropogenic disturbances in the form of land use legacies.

Interestingly, the forest recovers from hurricane disturbances in a matter of years. For example, the last major hurricane was Georges in 1998. The forest is now completely recovered. In contrast, land use legacies are long lasting. Part of the forest around El Verde was coffee plantation until the 1920s when the land was protected. Almost 100 years since then and we can still measure those legacies in forest plant composition and soil chemistry.

NADP Litterfall Mercury Monitoring Initiative

When the autumn leaves start to fall at NADP sites near forest landscapes in the eastern U.S., site sponsors and operators take advantage of the NADP Litterfall Mercury Monitoring Initiative (Litterfall Initiative) to approximate a large part of the annual mercury dry deposition at the site. The Litterfall Initiative was started by NADP in 2012 to complement the Mercury Deposition Network (MDN) monitoring for mercury in precipitation and the Atmospheric Mercury Network (AMNet) monitoring for mercury in air. In combination with data from the MDN and AMNet, litterfall data can be used to examine ranges of mercury dry deposition, to estimate combined wet and dry mercury deposition, and to evaluate mercury models.

Figure 9. Click to enlarge

Annually, litterfall that consists of leaves and needles, reproductive structures such as flowers and seeds, and woody material such as twigs and bark fall from the forest canopy to the forest floor. In predominately deciduous forest types, approximately three fourths of the annual litterfall occurs in the autumn. Scientific studies form the premise for the Litterfall Initiative – that mercury in forest canopy material is nearly all atmospheric in origin and the mercury in autumn litterfall represents much of the annual dry deposition of atmospheric mercury into the forest canopy. Litterfall is a dominant pathway for mercury dry deposition to the forest floor, where it then becomes part of an active soil cycle.

Figure 10. Click to enlarge

The Litterfall Initiative was established like all NADP networks. A “12-Point” Plan describing the objectives, benefits, protocols, quality assurance, funding, and operation of the Litterfall Initiative was reviewed and refined by an NADP working group during a series of meetings and approved by the NADP executive committee. Some elements of the plan were based on an initial 3-year study at 23 MDN sites in 15 states by Risch et. al (2012). The Litterfall Initiative in 2015 is collecting samples in the fourth year of a 5-year transition program operated by the U.S. Geological Survey. The transition program supports evaluation and improvements typical for a new network, while providing annual litterfall mercury data. For an NADP site to participate in the Litterfall Initiative, it is necessary to have a nearby forest suitable as a representative study plot for collecting annual litterfall samples. Similar to other networks, site sponsors pay an annual fee to the NADP Program Office that supports the costs of sampling supplies, sample analysis, and reporting.

Figure 11. Click to enlarge

Site operators deploy eight passive collectors in the forest study plot at the beginning of autumn leaf drop (figures 9 and 10). They retrieve the litterfall from the collectors every 4 weeks until the end of leaf drop. Samples are bagged and shipped to the U.S. Geological Survey where the mass of material in the collectors is dried and weighed at the laboratory to determine the annual litterfall catch. The dried litterfall is analyzed for total mercury and methylmercury concentrations. Annual litterfall mercury deposition is computed as a product of the litterfall catch and mercury concentrations and reported in units of micrograms per square meter per year, the same as annual mercury wet deposition from an MDN site. Published data from the initial study by Risch et. al and provisional results from the first 3 years of the Litterfall Initiative indicate annual litterfall mercury deposition typically is at least equal to annual mercury wet deposition at most sites. A scientific paper is in preparation to present data from the first 3 years of the Litterfall Initiative. During 2012-2015, 22 sites from 12 states and Puerto Rico have participated (figure 11).

For more information about participating in the Litterfall Initiative in 2016, contact Martin Risch at the U.S. Geological Survey, 317-600-2763, mrrisch@usgs.gov. Additional information, including the 12-Point Plan and the research paper by Risch et. al are available at http://nadp.sws.uiuc.edu/newissues/litterfall/.

Quantification of Methyl Mercury in Precipitation

Organic mercury constitutes a small portion of the total mercury in precipitation. Methyl mercury is a generic term for organic mercury compounds including: mono-methyl mercury (CH₃Hg, also abbreviated MMHg), dimethyl mercury [(CH₃)₂Hg], and ethyl methyl mercury [(CH₃)(CH₃CH₂)Hg]. MMHg comprises the greatest fraction of organic mercury in precipitation. For purposes of the NADP, the term “methyl mercury” refers strictly to MMHg.

Methyl mercury is a neurotoxin. It absorbs easily in the tissues of aquatic organisms, but is difficult to eliminate. This bioaccumulation of methyl mercury is of particular concern with large, predatory fish (e.g., swordfish, tuna, large-mouth bass and walleye) and with shellfish, all of which can be part of the human diet and the diet of wildlife that consume fish. Quantification of methyl mercury in precipitation helps assess the role of atmospheric deposition as a pathway for the movement of methyl mercury in the environment.

Figure 12. Click to enlarge

From the start of NADP’s Mercury Deposition Network (MDN) in 1996, analysis for methyl mercury has been available as an option to monitoring sites. Analysis for methyl mercury in MDN samples is performed either as a split sample or a composite sample. For a split sample, an aliquot from a single MDN sample is analyzed. For a composite sample, a percentage of each MDN sample is taken over a period of four consecutive weeks. The resulting composite sample is then analyzed for methyl mercury. These sample types are identified in the NADP database.

Information regarding the methyl mercury database is available from the NADP Program Office. Methyl mercury concentration values less than the reporting limit (RL) are identified as “<RL.” These concentration values are censored as they are of greater uncertainty than concentrations above the RL. Censored data result from the sample having a mass of methyl mercury less than the minimum for quantification, the sample being of low volume, or both. A separate (uncensored) dataset containing all methyl mercury concentration values, regardless of the reporting limit, is available by request. Users are cautioned against the use of the uncensored dataset due to the uncertainty associated with concentration values below the RL.

Figure 12 indicates the percent of total mercury as methyl mercury in 2014 for each of the sites that monitored for methyl mercury. Of these sites, WA18 is the only site that measured methyl mercury as a split sample. For each of the other sites, a composite sample was analyzed. Numbers at the top of the figure indicate the number of samples analyzed in 2014, and the number of those samples that have a concentration above the reporting limit. Red squares indicate the precipitation-weighted mean methyl mercury concentrations (secondary y axis). Concentrations below the reporting limits are processed as half the reporting limit when calculating the precipitation-weighted mean concentration.

The following references are recommended for additional information on methyl mercury and methyl mercury analysis.

1. Bloom, N. S., and C. J. Watras. "Observations of methylmercury in precipitation." Science of the Total Environment 87 (1989): 199-207.
2. Hall, B. D., H. Manolopoulos, J. P. Hurley, J. J. Schauer, VL St Louis, D. Kenski, J. Graydon, C. L. Babiarz, L. B. Cleckner, and G. J. Keeler. "Methyl and total mercury in precipitation in the Great Lakes region." Atmospheric Environment 39, no. 39 (2005): 7557-7569.
3. Parker, J.L. and N.S. Bloom. “Preservation and storage techniques for low-level aqueous mercury speciation.” Science of the Total Environment, 337 (2005): 253-263.
4. U.S. Environmental Protection, Method 1630: Methyl Mercury in Water by Distillation, Aqueous Ethylation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrophotometry. http://water.epa.gov/scitech/methods/cwa/metals/mercury/upload/2007_07_10_methods_method_mercury_1630.pdf

Recent Publications

A listing of recent journal publications that have used NADP data. Publications are separated by network.

National Trends Network (NTN)

Delavau, C., Chun, K. P., Stadnyk, T., Birks, S. J., & Welker, J. M., 2015. North American precipitation isotope (d18O) zones revealed in time series modeling across Canada and northern United States. Water Resources Research, 51(2).
doi:10.1002/2014WR015687

Lee, H.-M.; Paulot, F.; Henze, D. K.; Travis, K.; Jacob, D. J.; Pardo, L. H.; Schichtel, B. A., 2015. Sources of nitrogen deposition in Federal Class I areas in the US. Atmospheric Chemistry & Physics Discussions. Vol. 15 Issue 17.
doi:10.5194/acpd-15-23089-2015

Prenni, A. J., Day, D. E., Evanoski-Cole, A. R., Sive, B. C., Hecobian, A., Zhou, Y., Gebhart, K. A., Hand, J. L., Sullivan, A. P., Li, Y., Schurman, M. I., Desyaterik, Y., Malm, W. C., Schichtel, B. A., and Collett Jr., J. L., 2015. Oil and gas impacts on air quality in federal lands in the Bakken region: an overview of the Bakken Air Quality Study and first results. Atmospheric Chemistry & Physics Discussions. Vol. 15 Issue 20.
doi:10.5194/acpd-15-28749-2015

Rose, L. A., Elliott, E. M., & Adams, M. B., 2015. Triple Nitrate Isotopes Indicate Differing Nitrate Source Contributions to Streams Across a Nitrogen Saturation Gradient. Ecosystems, 18(7).
doi:10.1007/s10021-015-9891-8

Sullivan, T.J., 2015. Air Pollutant Deposition and Its Effects on Natural Resources in New York State. Cornell University Press. 307 pp.
http://www.cornellpress.cornell.edu/book/?GCOI=80140100176140 .

Mercury Deposition Network (MDN)

Chumchal, M. M., amp; Drenner, R. W., 2015. An environmental problem hidden in plain sight? Small Human‐made ponds, emergent insects, and mercury contamination of biota in the Great Plains. Environmental Toxicology and Chemistry, 34(6), 1197-1205.
doi:10.1002/etc.2954

Shanley, J. B., Engle, M. A., Scholl, M., Krabbenhoft, D. P., Brunette, R., Olson, M. L., & Conroy, M. E., 2015. High mercury wet deposition at a “clean air” site in Puerto Rico. Environmental Science amp; Technology, 49(20), 12474-12482.
doi:10.1021/acs.est.5b02430

Wisniewski, E.A., 2015. The Mercury and Autism Debate: What has Shaped the Public’s Perception? A Dissertation Submitted to the School of Graduate Studies and Research, Indiana University of Pennsylvania August 2015.
http://hdl.handle.net/2069/2392

Ammonia Monitoring Network (AMoN)

Puchalski, M. A., Rogers, C. M., Baumgardner, R., Mishoe, K. P., Price, G., Smith, M. J., Watkins, N. & Lehmann, C. M., 2015. A statistical comparison of active and passive ammonia measurements collected at Clean Air Status and Trends Network (CASTNET) sites. Environmental Science: Processes & Impacts, 17(2), 358-369.
doi:10.1039/C4EM00531G