NADP: Keeping You Connected, Issue 4

Critical Loads of Atmospheric Deposition Maps

Jason Lynch – Ecologist, U.S. Environmental Protection Agency

What are critical loads? Air pollution emitted from a variety of sources is deposited from the air into ecosystems. These pollutants may cause ecological changes, such as long-term acidification of soils or surface waters, soil nutrient imbalances affecting plant growth, and loss of biodiversity. The term “critical load” is used to describe the threshold of air pollution deposition that causes harm to sensitive resources in an ecosystem. A critical load is technically defined as “the quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment are not expected to occur according to present knowledge” (Nilsson and Grennfelt, 1988). Critical loads are typically expressed in terms of kilograms per hectare per year (kg/ha/yr) of wet or total (wet + dry) deposition. Critical loads can be developed for a variety of ecosystem responses, including shifts in microscopic aquatic species, increases in invasive grass species, changes in soil chemistry affecting tree growth, and lake and stream acidification to levels that can no longer support fish.

Figure 1. Click to enlarge

Starting in 2010, the National Atmospheric Deposition Program (NADP)’s Critical Loads of Atmospheric Deposition (CLAD) Science Committee began gathering and synthesizing empirical and calculated critical loads data and information from dozens of regional- and national-scale projects (see Blett et al. 2014). CLAD members submitted data to this cooperative effort as a productive and meaningful way to share information to improve methods for estimating, calculating, mapping, interpreting, and refining critical loads. This critical load data formed the basis for the National Critical Load Database (NCLD), which is publically available on the NADP CLAD page. A collection of critical load maps for various empirical and calculated critical loads for the continental U.S. have also been developed and will be available through the NADP CLAD webpage soon (Figure 1). The intended use of these maps is for scientific, policy-related, educational purposes, and to illustrate the different critical loads that are included in the NLCD.

The purpose of the CLAD Science Committee is to discuss current and emerging issues regarding the science and use of critical loads for effects of atmospheric deposition on ecosystems in the United States.

What about Dry Deposition?

Pam Padgett, Research Plant Biologist, U.S. Forest Service

The original concept in 1978 behind the NADP collector was that there are two types of deposition: wet and dry. The Aerochem collector was designed to keep the “dry” bucket open for collection of “dry deposition” until it rained. Precipitation triggered the motor box to close the dry bucket, opening the “wet” bucket for collection of the precipitation sample. This simplification of dry deposition quickly became outdated as the interaction of chemistry, physics, and biology, which drive dry deposition processes, became better understood.

Typically, nitrogen deposition in precipitation is easily measured as dissolved nitrate and ammonium. However, the understanding of wet deposition of nitrogen species is evolving as more measurements of organic nitrogen are becoming available. Measuring dry deposition is considerably more difficult as the air-surface exchange processes are more challenging to characterize. For example, ammonia can be emitted or deposited depending on the relative concentrations in the atmosphere and at the exchange surface. After nearly 40 years of research and debate, the understanding of dry deposition processes has greatly improved. However, there are no standard methods for routine direct measurement of dry deposition suitable for a national monitoring network.

Figure 2. Click to enlarge

And yet, getting reliable estimates of dry deposition is critical to understanding ecological responses to air pollution, particularly in the western part of the country where dry deposition dominates (Figure 2). Because of the weather patterns of hot dry summers with little or no rain during peak air pollution production, dry deposition may account for as much as 90% of the total nitrogen deposition in southern California and throughout the Southwest. Dry deposition contributes as much as 75% of the total nitrogen deposition in the Rocky and the Sierra Nevada Mountains and in the intermountain regions where agriculture is concentrated or cities have expanded. But even in the wetter climate of the Southeast, dry deposition may contribute as much as 50% of the total nitrogen. NADP is the most widely recognized source of reliable wet deposition data; however when used alone it significantly underestimates total deposition and may give a skewed perspective of atmospheric deposition issues faced by regulators and land managers.

Monitoring, methods, and modeling.

Figure 3. Click to enlarge

To fill the void in direct dry deposition measurement methods, efforts have focused on hybrid measurement/modeling methods such as the EPA Clean Air Status and Trends Network (CASTNET), a national dry deposition monitoring network that uses filter-packs to collect atmospheric concentration data for key pollutants and then estimates dry deposition rates using the Multi-Layer Model. Other national and global scale deposition modeling efforts such as the Community Multi-scale Air Quality (CMAQ) model combine multiple chemical, weather, and physical models to provide insight into quantitative speciation of nitrogen deposition, among many other pollutants. One of the biggest missing components in monitoring and modeling dry nitrogen deposition has been the lack of data on free ammonia concentrations in air. The NADP, in partnership with the EPA and the National Park Service, undertook an effort to evaluate and ultimately adopt a passive monitoring network for ammonia, known as AMoN, that now provides much needed data for estimating dry deposition of this major nitrogen pollutant (Figure 3). Also within NADP, the Total Deposition Science Committee (TDEP) has recently begun developing ways to integrate measurements of wet deposition and ambient air concentrations with modeled dry deposition rates to provide better insight into the distribution and effects of total deposition on ecosystems. Figure 2 is one of the many data products that have resulted from this effort.

Many state and federal agencies know that dry deposition has profound effects on ecosystems, both those identical to wet deposition and effects that are unique to dry deposition. There are still significant gaps in our understanding of dry deposition, both in measurements and in effects. If deposition of individual molecules is complicated, understanding deposition of particles or aerosols is even more difficult. But the genius of the NADP program is that it is always looking for ways to improve measurements and to find new ways to provide data. Most importantly, anyone with an idea is welcome to participate.

Milky Rain in the Pacific Northwest

Christopher Lehmann and Brian Kerschner, NADP Central Analytical Laboratory (NADP/CAL)

Earlier this year, locations in eastern Washington State and northeast Oregon experienced an unusual “milky rain” event, starting on Friday, February 6, 2015. As reported in several news outlets, periods of light rain left a white or grey residue on vehicles and other surfaces. The National Weather Service (NWS)’s office in Spokane, WA noted that “muddy rain” events are often seen during the summer months, but these events rarely occur in winter. Further evaluation indicated that the origin was either from a wind storm carrying dust from alkali beds near Summer Lake, OR, or from regions in northwest Nevada experiencing a multi-year drought. A more detailed description of the event by the NWS office in Spokane is available online.

Figure 4. Click to enlarge

The NADP’s National Trends Network (NTN) operates numerous sites in the Pacific Northwest. Data for samples collected in the period February 2 – 11, 2015 are plotted in the figures, both as maps (Figure 4) and as plots (Figure 5), comparing data for the milky rain period with all samples collected from 2012 – 2014. Some sample comments from site operators indicated that dirt and debris were observed in the sample collection bucket. The plotted chemistry species concentrations include the sum of cations (calcium, magnesium, potassium, and sodium), sulfate ion, sample acidity (as pH), and the difference between measured anions and cations. These data are available for download on the NADP/NTN website.

Figure 5. Click to enlarge

The NADP/NTN stations at Craters of the Moon National Monument (ID03) and Reynolds Creek (Owyhee County, ID11), Idaho were in the region where milky rain was observed, and sample concentrations for the period had notably high chemical concentrations. The sum of cations at these stations for the milky rain period were quite elevated over the 3-year average (830% and 765%, respectively, Figure 4a). The concentration of sulfate ion was also elevated at these stations at 10-times the 3-year average (1,253% and 784%, respectively, Figure 4b), as was chloride ion (674% and 653%, not shown). The plots show that the concentrations of cations and sulfate ions at ID03 and ID11 are much higher than the typical range for these stations, and other sites in the region also experienced unusually high concentrations of these species (Figure 5). Likewise, the rainwater acidity (pH) of samples in the region was unusually high, due to the elevated concentrations of cations.

The NADP/NTN data also indicate that there are other ion species in the rainwater samples from this period that were not measured at the Central Analytical Laboratory (CAL). In water samples, the concentration of negatively-charged ions (anions) such as sulfate must balance the concentration of positively-charged ions (cations). Data from the period indicate anomalous ion balances, with the concentration of cations far exceeding the concentration of anions (Figure 5d). These ions likely include fluoride, organic acids, and other naturally-occurring species.

So what was the cause of the “milky rain”? The NADP/NTN data provide some, but not all the answers. The rainwater had high concentrations of chemical species characteristic of wind-blown dust (calcium, magnesium, potassium, sodium, chloride and sulfate). However, data indicate there are additional chemical species in the rainwater that could provide further answers. The CAL has saved the samples from this period for further analysis.

NADP Data Highlighted in Air and Waste Management Association’s Magazine

The July 2015 issue of the Air and Waste Management Association’s EM magazine featured a series of invited articles on “Policy-Relevant Science for NO_x and SO_x Secondary Standards” with two articles featuring NADP data. The article “Impact of Sulfur Dioxide (SO₂) and Nitrogen Oxide (NO_x) Emissions Reductions on Acidic Deposition in the United States” by NADP authors Christopher Lehmann, Brian Kerschner and David Gay summarizes the regulations limiting sulfur and nitrogen air emissions since the 1970 Clean Air Act and their subsequent effect on the wet deposition of sulfate and nitrate ions. Emissions of sulfur dioxide have decreased from over 25 teragrams per year (Tg/yr) in 1970 to less than 5 Tg/yr in 2014. Nitrogen oxides emissions have decreased from 24 Tg/yr to 11 Tg/yr over the same period.

Figure 6. Click to enlarge

Both sulfate and nitrate wet deposition have decreased dramatically over time in response to emissions reductions, as shown by data from the NADP’s National Trends Network (NADP/NTN, Figure 6). In the continental US, reductions in sulfate wet deposition have outpaced nitrate deposition; nitrate now contributes more to the acidification of precipitation than sulfate in over half of the US, principally in areas west of the Mississippi River. The article highlights the work of the NADP’s Total Deposition Science Committee (TDEP) to understand and integrate modeled estimates of wet and dry deposition of sulfur and nitrogen species to better quantify the total deposition of these pollutants. Such data is critical for ecological assessments, including the evaluation of critical loads and the development of National Ambient Air Quality Standards (NAAQS).

Figure 7. Click to enlarge

The second article “Reactive Nitrogen Monitoring Gaps: Issues, Activities, and Needs” by Bret Schichtel at the National Park Service and John Walker at U.S. EPA also illustrates the impact of emissions regulations on the wet deposition of pollutants, focusing on nitrogen species. The authors note that historic legislation in the US has focused on limiting inorganic nitrogen emissions (i.e., nitrogen oxides), and current monitoring networks (including the NADP/NTN) have emphasized measurement of inorganic species. The authors show ion concentration trends in the NADP/NTN data set, illustrating the relative contribution of ammonium and nitrate deposition to the deposition of inorganic nitrogen. Ammonium ion deposition has grown in relative importance to inorganic nitrogen deposition over time, and it now represents almost 60% of the inorganic nitrogen wet deposition (Figure 7). Air quality modeling and projected emissions indicate that the increasing trends in nitrogen deposition will continue into the future.

While monitoring of “reactive nitrogen” (including ammonia gas) has now been incorporated into the NADP as the Ammonia Monitoring Network (AMoN), routine measurements of other reactive nitrogen species, including organic nitrogen, in the national-scale NADP/NTN and Clean Air Status and Trends Network (CASTNET) programs are lacking. The authors highlight studies in Rocky Mountain and Grand Tetons national parks which indicate that reduced nitrogen species (ammonia and organic nitrogen) account for over half of the measured nitrogen deposition. The article stresses the need for improved monitoring of these components to reduce the uncertainty in reactive nitrogen deposition budgets for the development of NAAQS and to protect vulnerable ecosystems.

Reprints of these articles are available upon request from the NADP Program Office (nadp@isws.illinois.edu). For more information, see:

Lehmann, C.M.B., B.M. Kerschner, and D.A. Gay, 2015. Impact of Sulfur Dioxide (SO2) and Nitrogen Oxide (NOx) Emissions Reductions on Acidic Deposition in the United States. AWMA-EM, July 2015, pp 6-11.

Schichtel, B.A., and Walker, J.T. Reactive Nitrogen Monitoring Gaps: Issues, Activities, and Needs. AWMA-EM, July 2015, pp 12-19.

Acid Rain 2015 - Join Us in Rochester!

On October 19-23, 2015 in Rochester, NY, the International Acid Rain Conference will return to the United States for the first time since 1975. This 9th Acid Rain Conference will highlight the latest information and research on acid rain – environmental effects, the status of recovery across affected regions, and new policies that will define the future. The theme of Acid Rain 2015 is “Successes Achieved and Challenges Ahead”.

Conference Program

Breakout sessions include topics like emissions of air pollutants, clean air policies, linkages of acid rain to climate change and the carbon cycle, emerging issues in atmospheric deposition – and many, many more. Check out the Program page to find out more information.

This conference truly will be an international gathering, with 58% of the 275 abstracts coming from outside of the United States. Abstracts were submitted from five continents and 35 different countries (see Figure 8), with Sweden, Canada, Japan, and China sending the most abstracts behind the U.S.

Figure 8: Abstracts were submitted from all of the countries highlighted in red.

Registration

In Memoriam: Mark E. Peden, 1952-2015

Mark Peden

Mark Peden passed away in Champaign, IL on June 14, 2015 after an extended illness. He will be most remembered within the NADP for his significant contributions to the development of laboratory analytical methods used within the NTN, AIRMoN and the later AMoN atmospheric monitoring networks. Mark served as the Director of the NADP’s Central Analytical Laboratory (CAL) at the Illinois State Water Survey (ISWS) from 1977–1987 and also several years as Acting Chief of ISWS (1991–1994).

Mark received a B.S. in biology from the University of Illinois at Urbana-Champaign and was hired as a research assistant at ISWS in 1974. Mark was a noted expert in methods for the analysis of pollutants in precipitation samples, including sample preparation, preservation, and chromatographic and spectroscopic analysis. Mark contributed to the development of methods for trace metals analysis in precipitation samples, including those currently used in the MDN and related trace metals studies. He made significant contributions to pollutant scavenging studies to assess the impact of air pollution on rainfall patterns, including the notable METROMEX program in the St. Louis region. During his almost 30-year career at ISWS, he authored or co-authored over 30 peer-reviewed publications and related scientific reports, and over 20 scientific presentations. He was awarded the status of Senior Professional Scientist Emeritus at ISWS in 2003.

Mark was a leader in the development of standardized methods for precipitation analysis through ASTM International, receiving an Award of Merit and status of Fellow in 1997. Within ASTM, Mark served as a member and Vice Chairman of committee D22 on Sampling and Analysis of Atmospheres where he received an Award of Appreciation in 1995. He chaired committee D22.06 on Acidic Precipitation where he coordinated efforts to develop standards for wet deposition analysis, receiving the Moyer D. Thomas award in 2009. Mark also served as a member of committees D19 on Water and D34 on Waste Management.

Mark valued the mentoring and development of colleagues and future generations of scientists. He served on the State of Illinois Science Olympiad, receiving a Certificate of Appreciation for his efforts in 2001 and 2002. During his retirement years, Mark served on the Board of Directors for the Orpheum Children’s Science Museum in Champaign, IL serving as President and Head of the Executive Committee.

Mark’s obituary is available online.

Recent Publications

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

National Trends Network (NTN)

Inamdar, S., Dhillon, G., Singh, S., Parr, T., & Qin, Z., 2015. Particulate nitrogen exports in stream runoff exceed dissolved nitrogen forms during large tropical storms in a temperate, headwater, forested watershed. Journal of Geophysical Research: Biogeosciences.
doi:10.1002/2015JG002909

Kim, P. S., Jacob, D. J., Fisher, J. A., Travis, K., Yu, K., Zhu, L., Yantosca, R. M., Sulprizio, M. P., Jimenez, J. L., Campuzano-Jost, P., Froyd, K. D., Liao, J., Hair, J. W., Fenn, M. A., Butler, C. F., Wagner, N. L., Gordon, T. D., Welti, A., Wennberg, P. O., Crounse, J. D., St. Clair, J. M., Teng, A. P., Millet, D. B., Schwarz, J. P., Markovic, M. Z., and Perring, A. E., 2015. Sources, seasonality, and trends of Southeast US aerosol: an integrated analysis of surface, aircraft, and satellite observations with the GEOS-Chem chemical transport model. Atmospheric Chemistry and Physics Discussions, 15(13), 17651-17709.
doi:10.5194/acpd-15-17651-2015

Kennedy, C. D., Buda, A. R., Kleinman, P. J., & DeMoranville, C. J., 2015. Chemical and Isotopic Tracers Illustrate Pathways of Nitrogen Loss in Cranberry Floodwaters. Journal of Environmental Quality, 44(4), 1326-1332.
doi:10.2134/jeq2014.12.0549

Lehmann, C.M.B., B.M. Kerschner, and D.A. Gay, 2015. Impact of Sulfur Dioxide (SO2) and Nitrogen Oxides (NOx) Emissions Reductions on Acidic Deposition in the United States. AWMA-EM, July 2015, pp 6-11.

Saleh, Dina and Joseph Domagalski, 2015. SPARROW Modeling of Nitrogen Sources and Transport in Rivers and Streams of California and Adjacent States, U.S. Journal of the American Water Resources Association (JAWRA), 1-21.
doi:10.1111/1752-1688.12325

Schichtel, B.A., and Walker, J.T., 2015. Reactive Nitrogen Monitoring Gaps: Issues, Activities, and Needs. AWMA-EM, July 2015, pp 12-19.

Ammonia Monitoring Network (AMoN)

Balasubramanian, S., Koloutsou‐Vakakis, S., McFarland, D. M., & Rood, M. J., 2015. Reconsidering Emissions of Ammonia from Chemical Fertilizer Usage in Midwest USA. Journal of Geophysical Research: Atmospheres. 120, 6232–6246.
doi:10.1002/2015JD023219

Mercury Deposition Network (MDN)

Bettez, Neil D., Jonathan M. Duncan, Peter M. Groffman, Lawrence E. Band, Jarlath O’Neil-Dunne, Sujay S. Kaushal, Kenneth T. Belt, and Neely Law, 2015. Climate Variation Overwhelms Efforts to Reduce Nitrogen Delivery to Coastal Waters. Ecosystems: 1-13.
doi:10.1007/s10021-015-9902-9

Gustin, M. S., Amos, H. M., Huang, J., Miller, M. B., & Heidecorn, K., 2015. Measuring and modeling mercury in the atmosphere: a critical review. Atmospheric Chemistry and Physics, 15(10), 5697-5713.
doi:10.5194/acp-15-5697-2015

Williams, M.L., MacCoy, D.E., and Maret, T.R., 2015. Evaluation of mercury in rainbow trout collected from Duck Valley Indian Reservation reservoirs, southwestern Idaho and northern Nevada, 2007, 2009, and 2013: U.S. Geological Survey Scientific Investigations Report 2015–5079, 18 p.
doi:10.3133/sir20155079

Atmospheric Mercury Network (AMNet)

Bettez, Neil D., Jonathan M. Duncan, Peter M. Groffman, Lawrence E. Band, Jarlath O’Neil-Dunne, Sujay S. Kaushal, Kenneth T. Belt, and Neely Law, 2015. Climate Variation Overwhelms Efforts to Reduce Nitrogen Delivery to Coastal Waters. Ecosystems: 1-13.
doi:10.1007/s10021-015-9902-9

Gustin, M. S., Amos, H. M., Huang, J., Miller, M. B., & Heidecorn, K., 2015. Measuring and modeling mercury in the atmosphere: a critical review. Atmospheric Chemistry and Physics, 15(10), 5697-5713.
doi:10.5194/acp-15-5697-2015