National Atmospheric Deposition Program Sites Research Articles

Calcium and ammonium now control the pH of wet and bulk deposition in Ohio, U.S.

As precipitation acidity in the United States has decreased dramatically over the past forty years, new questions have emerged about the role of base cations and nitrogen (N) species in regulating precipitation pH. To address these questions, we studied precipitation chemistry in Ohio, a state with relatively high N deposition and a mix of urban, industrial, and agricultural land use. We used long-term data from the National Atmospheric Deposition Program (NADP) to assess temporal changes in the drivers of wet deposition pH in Ohio. We supplemented our analysis with short-term data on spatial patterns of bulk deposition pH collected along an agricultural-to-urban transect in central Ohio. From 1979 to 2019, wet deposition pH at the Ohio NADP sites increased from 4.22 ± 0.05 to 5.33 ± 0.21, and the cation-to-anion ratio increased from 0.92 ± 0.02 to 1.35 ± 0.06, due to a dramatic decline in measured anions. As correlations between wet deposition pH and the anions SO42− and NO3− weakened, correlations between wet deposition pH and the cations NH4+ and Ca2+ strengthened, suggesting that these cations are playing a larger role in controlling precipitation pH today. In recent years (2021–2022), NH4+ was significantly correlated with bulk deposition pH in all land uses except the urban site, with the strongest correlation at the rural/agricultural site, where the pH was 5.96 ± 0.49. Similarly, Ca2+ was strongly and significantly correlated with bulk deposition pH across all land uses. For Ca2+, the urban site was distinct from the other sites due to the high pH (6.58 ± 0.44) and Ca2+ concentrations at that site, likely driven by high levels of particulate matter deposition. Our results indicate that land use plays a role in the strength and dominant process of acid neutralization, with Ca-rich dust more important in urban areas and reduced N more important in agricultural regions. As precipitation pH increases and land use changes across the country, base cations and reduced N will play an increasingly important role in regulating precipitation pH in the U.S., with important implications for ecosystems continuing to recover from acid rain.

Assessing the roles emission sources and atmospheric processes play in simulating <i>δ</i><sup>15</sup>N of atmospheric NO<sub><i>x</i></sub> and NO<sub>3</sub><sup>−</sup> using CMAQ (version 5.2.1) and SMOKE (version 4.6)

Abstract. Nitrogen oxides (NOx= nitric oxide (NO) + nitrogen dioxide (NO2)) are important trace gases that affect atmospheric chemistry, air quality, and climate. Contemporary development of NOx emissions inventories is limited by the understanding of the roles of vegetation (net NOx source or net sink), vehicle emissions from gasoline- and diesel-powered vehicles, the application of NOx emission control technologies, and accurate verification techniques. The nitrogen stable isotope composition (δ15N) of NOx is an effective tool to evaluate the accuracy of the NOx emission inventories, which are based on different assumptions. In this study, we traced the changes in δ15N values of NOx along the “journey” of atmospheric NOx, driven by atmospheric processes after different sources emit NOx into the atmosphere. The 15N was incorporated into the emission input dataset, generated from the US EPA trace gas emission model SMOKE (Sparse Matrix Operator Kernel Emissions). Then the 15N-incorporated emission input dataset was used to run the CMAQ (Community Multiscale Air Quality) modeling system. By enhancing NOx deposition, we simulated the expected δ15N of NO3-, assuming no isotope fractionation during chemical conversion or deposition. The simulated spatiotemporal patterns in NOx isotopic composition for both SMOKE outputs (simulations under the “emission only” scenario) and CMAQ outputs (simulations under the “emission + transport + enhanced NOx loss” scenario) were compared with corresponding measurements in West Lafayette, Indiana, USA. The simulations under the emission + transport + enhanced NOx loss scenario were also compared to δ15N of NO3- at NADP (National Atmospheric Deposition Program) sites. The results indicate the potential underestimation of emissions from soil, livestock waste, off-road vehicles, and natural-gas power plants and the potential overestimation of emissions from on-road vehicles and coal-fired power plants, if only considering the difference in NOx isotopic composition for different emission sources. After considering the mixing, dispersion, transport, and deposition of NOx emission from different sources, the estimation of atmospheric δ15N(NOx) shows better agreement (by ∼ 3 ‰) with observations.

Linking improvements in sulfur dioxide emissions to decreasing sulfate wet deposition by combining satellite and surface observations with trajectory analysis

Sulfur dioxide (SO2), a criteria pollutant, and sulfate (SO42−) deposition are major environmental concerns in the eastern U.S. and both have been on the decline for two decades. In this study, we use satellite column SO2 data from the Ozone Monitoring Instrument (OMI), and SO42− wet deposition data from the NADP (National Atmospheric Deposition Program) to investigate the temporal and spatial relationship between trends in SO2 emissions and the downward sulfate wet deposition over the eastern U.S. from 2005 to 2015. To establish the relationship between SO2 emission sources and receptor sites, we conducted a Potential Source Contribution Function (PSCF) analysis using HYSPLIT back trajectories for five selected Air Quality System (AQS) sites - (Hackney, OH, Akron, OH, South Fayette, PA, Wilmington, DE, and Beltsville, MD) - in close proximity to NADP sites with large downward SO42− trends since 2005. Back trajectories were run for three summers (JJA) and three winters (DJF) and used to generate seasonal climatology PSCFs for each site. The OMI SO2 and interpolated NADP sulfate deposition trends were normalized and overlapped with the PSCF, to identify the areas that had the highest contribution to the observed drop. The results suggest that emission reductions along the Ohio River Valley have led to decreases in sulfate deposition in eastern OH and western PA (Hackney, Akron and South Fayette). Farther to the east, emission reductions in southeast PA resulted in improvements in sulfate deposition at Wilmington, DE, while for Beltsville, reductions in both the Ohio River Valley and nearby favorably impacted sulfate deposition. For Beltsville, sources closer than 300 km from the site contribute roughly 56% observed deposition trends in winter, and 82% in summer, reflecting seasonal changes in transport pattern as well as faster oxidation and washout of sulfur in summer. This suggests that emissions and wet deposition are linked through not only the location of sources relative to the observing sites, but also to photochemistry and the weather patterns characteristic to the region, as evidenced by a west to east shift in the contribution between winter and summer. The method developed here is applicable to other regions with significant trends such as China and India, and can be used to estimate the potential benefits of emission reduction in those areas.

On-road emissions of ammonia: An underappreciated source of atmospheric nitrogen deposition

We provide updated spatial distribution and inventory data for on-road NH3 emissions for the continental United States (U.S.) On-road NH3 emissions were determined from on-road CO2 emissions data and empirical NH3:CO2 vehicle emissions ratios. Emissions of NH3 from on-road sources in urbanized regions are typically 0.1–1.3tkm−2yr−1 while NH3 emissions in agricultural regions generally range from 0.4–5.5tkm−2yr−1, with a few hotspots as high as 5.5–11.2tkm−2yr−1. Counties with higher vehicle NH3 emissions than from agriculture include 40% of the U.S. population. The amount of wet inorganic N deposition as NH4+ from the National Atmospheric Deposition Program (NADP) network ranged from 37 to 83% with a mean of 58.7%. Only 4% of the NADP sites across the U.S. had <45% of the N deposition as NH4+ based on data from 2014 to 2016, illustrating the near-universal elevated proportions of NH4+ in deposition across the U.S. Case studies of on-road NH3 emissions in relation to N deposition include four urban sites in Oregon and Washington where the average NH4-N:NO3-N ratio in bulk deposition was 2.3. At urban sites in the greater Los Angeles Basin, bulk deposition of NH4-N and NO3-N were equivalent, while NH4-N:NO3-N in throughfall under shrubs ranged from 0.6 to 1.7. The NH4-N:NO3-N ratio at 7–10 sites in the Lake Tahoe Basin averaged 1.4 and 1.6 in bulk deposition and throughfall, and deposition of NH4-N was strongly correlated with summertime NH3 concentrations. On-road emissions of NH3 should not be ignored as an important source of atmospheric NH3, as a major contributor to particulate air pollution, and as a driver of N deposition in urban and urban-affected regions.

A Great Lakes Atmospheric Mercury Monitoring network: Evaluation and design

As many as 51 mercury (Hg) wet-deposition-monitoring sites from 4 networks were operated in 8 USA states and Ontario, Canada in the North American Great Lakes Region from 1996 to 2010. By 2013, 20 of those sites were no longer in operation and approximately half the geographic area of the Region was represented by a single Hg-monitoring site. In response, a Great Lakes Atmospheric Mercury Monitoring (GLAMM) network is needed as a framework for regional collaboration in Hg-deposition monitoring. The purpose of the GLAMM network is to detect changes in regional atmospheric Hg deposition related to changes in Hg emissions. An optimized design for the network was determined to be a minimum of 21 sites in a representative and approximately uniform geographic distribution. A majority of the active and historic Hg-monitoring sites in the Great Lakes Region are part of the National Atmospheric Deposition Program (NADP) Mercury Deposition Network (MDN) in North America and the GLAMM network is planned to be part of the MDN.To determine an optimized network design, active and historic Hg-monitoring sites in the Great Lakes Region were evaluated with a rating system of 21 factors that included characteristics of the monitoring locations and interpretations of Hg data. Monitoring sites were rated according to the number of Hg emissions sources and annual Hg emissions in a geographic polygon centered on each site. Hg-monitoring data from the sites were analyzed for long-term averages in weekly Hg concentrations in precipitation and weekly Hg-wet deposition, and on significant temporal trends in Hg concentrations and Hg deposition. A cluster analysis method was used to group sites with similar variability in their Hg data in order to identify sites that were unique for explaining Hg data variability in the Region. The network design included locations in protected natural areas, urban areas, Great Lakes watersheds, and in proximity to areas with a high density of annual Hg emissions and areas with high average weekly Hg wet deposition. In a statistical analysis, relatively strong, positive correlations in the wet deposition of Hg and sulfate were shown for co-located NADP Hg-monitoring and acid-rain monitoring sites in the Region. This finding indicated that efficiency in regional Hg monitoring can be improved by adding new Hg monitoring to existing NADP acid-rain monitoring sites.Implementation of the GLAMM network design will require Hg-wet-deposition monitoring to be: (a) continued at 12 MDN sites active in 2013 and (b) restarted or added at 9 NADP sites where it is absent in 2013. Ongoing discussions between the states in the Great Lakes Region, the Lake Michigan Air Directors Consortium (a regional planning entity), the NADP, the U.S. Environmental Protection Agency, and the U.S. Geological Survey are needed for coordinating the GLAMM network.

Quantifying particulate matter deposition in Niwot Ridge, Colorado: Collection of dry deposition using marble inserts and particle imaging using the FlowCAM

Atmospheric wet and dry deposition are important sources of carbon for remote alpine lakes and soils. The carbon inputs from dry deposition in alpine National Atmospheric Deposition Program (NADP) collectors, including aeolian dust and biological material, are not well constrained due to difficulties in retaining particulate matter in the collectors. Here, we developed and tested a marble insert for dry deposition collection at the Niwot Ridge Long Term Ecological Research Station (NWT LTER) Soddie site (3345 m) between 24 May and 8 November 2011. We conducted laboratory tests of the insert's effect on particulate matter (PM) mass and non-purgeable organic carbon (DOC) and found that the insert did not significantly change either measurement. Thus, the insert may enable dry deposition collection of PM and DOC at NADP sites. We then developed a method for enumerating the collected wet and dry deposition with the Flow Cytometer and Microscope (FlowCAM), a dynamic-image particle analysis tool. The FlowCAM has the potential to establish morphology, which affects particle settling and retention, through particle diameter and aspect ratio. Particle images were used to track the abundance of pollen grains over time. Qualitative image examination revealed that most particles were biological in nature, such as intact algal cells and pollen. Dry deposition loading to the Soddie site as determined by FlowCAM measurements was highly variable, ranging from 100 to >230 g ha−1 d−1 in June–August 2011 and peaking in late June. No significant difference in diameter or aspect ratio was found between wet and dry deposition, suggesting fundamental similarities between those deposition types. Although FlowCAM statistics and identification of particle types proved insightful, our total-particle enumeration method had a high variance and underestimated the total number of particles when compared to imaging of relatively large volumes (60–125 mL) from a single sample. We recommend use of the FlowCAM, especially for subclasses of particles, but in light of uncertainty in particle counts, believe that it should be paired with traditional methods such as microscopy in this stage of the technique's development. Analysis of well-mixed samples produced lower variability than settling methods used for algae samples. Use of the marble inserts in the dry deposition collector in the NADP context is recommended, and the implications of various particle counting and identification methods are explored.

Wet Deposition of Fission-Product Isotopes to North America from the Fukushima Dai-ichi Incident, March 2011

Using the infrastructure of the National Atmospheric Deposition Program (NADP), numerous measurements of radionuclide wet deposition over North America were made for 167 NADP sites before and after the Fukushima Dai-ichi Nuclear Power Station incident of March 12, 2011. For the period from March 8 through April 5, 2011, wet-only precipitation samples were collected by NADP and analyzed for fission-product isotopes within whole-water and filterable solid samples by the United States Geological Survey using gamma spectrometry. Variable amounts of (131)I, (134)Cs, or (137)Cs were measured at approximately 21% of sampled NADP sites distributed widely across the contiguous United States and Alaska. Calculated 1- to 2-week individual radionuclide deposition fluxes ranged from 0.47 to 5100 Becquerels per square meter during the sampling period. Wet deposition activity was small compared to measured activity already present in U.S. soil. NADP networks responded to this complex disaster, and provided scientifically valid measurements that are comparable and complementary to other networks in North America and Europe.

Atmospheric deposition of mercury and major ions to the Pensacola (Florida) watershed: spatial, seasonal, and inter-annual variability

Abstract. Atmospheric deposition was measured at three sites in the Pensacola Bay watershed, Florida, between November 2004 and December 2007. Mercury deposition in the Pensacola Bay watershed was similar to that from nearby Mercury Deposition Network sites along the Northern Gulf of Mexico coast. Mercury deposition during the summer months is higher than other months due to higher concentrations in rainfall throughout the region. Deposition of constituents like H+, sulfate, nitrate, ammonium, chloride and sodium, were much higher in Pensacola Bay that at National Atmospheric Deposition Program (NADP) sites. Chloride and sodium deposition are higher because Pensacola Bay sites are closer to the Gulf of Mexico which is a source of sea salt aerosols. Acid rain constituents, H+, sulfate, nitrate and ammonium are most likely higher at Pensacola Bay sites because these sites are much closer to emission sources of these constituents than NADP sites, particularly two Florida NADP sites, FL14 and FL23, which are located in rural counties far from major industrial activities.

Oxygen isotope ratios in Holocene carbonates across a climatic gradient, eastern Washington State, USA: Evidence for seasonal effects on pedogenic mineral isotopic composition

Oxygen isotope ratios of pedogenic carbonates were analyzed along a climatic gradient in the Palouse region of eastern Washington State to evaluate changes in their isotopic composition with changing climate. A modern analog approach was used to assess the relationship between the istotope ratios in the pedogenic carbonates with those in soil water (measured in June when we anticipated carbonate formation might occur because of low soil water content), and meteoric water from two National Atmospheric Deposition Program (NADP) sites adjacent to our sequence. Mean annual meteoric water δ18O values (wrt VSMOW) from the two NADP sites varied slightly with elevation, —13.9‰ at the Starkey site (elevation 1253 m) and —13.0‰ at the Palouse site, (elevation 766 m) but showed distinct seasonal trends with weighted means ranging from —11.9‰ during autumn to —15.0‰ during the winter. Carbonate δ18O (wrt VPDB) varied significantly with MAP, ranging from —14.1‰ at the most arid site to —11.4‰ at the wettest site but displayed a trend opposite to that of soil water δ18O (—4 to —12‰ VSMOW for the 70 cm depth). Neither measured soil water δ18O values nor mean meteoric water δ18O values generated realistic temperatures of formation for the observed trend in carbonate δ18O values. When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ18O in equilibrium with carbonate δ 18O was similar to winter precipitation δ18O at the most arid sites but became increasingly similar to mean annual meteoric δ 18O as MAP increased. We suggest that the unexpected trend in carbonate δ18O is largely due to the diminishing influence of winter precipitation/snowmelt on soil moisture as MAP increases across the climatic gradient and that seasonal effects of meteoric water may need to be considered in pedogenic mineral isotopic equilibrium in temperate regions.

Changes in base cation deposition across New York State and adjacent New England following implementation of the 1990 Clean Air Act amendments

To reduce atmospheric deposition, in 1990 Congress passed amendments to the Clean Air Act requiring electric utility power plants to decrease emissions of sulfur dioxide and nitrogen oxides, with Phase I beginning in 1995. We analyzed precipitation volume, wet deposition, and concentration of the sum of base cations measured at 12 National Atmospheric Deposition Program sites in Massachusetts, New Hampshire, New York, and Vermont. We compared five-year means prior to and following passage of the amendments and for five years after the implementation of Phase I. Whereas only one of the monitoring stations recorded a decline in base cation deposition, three sites out of the 12 showed a decline in base cation concentration. None of the sites exhibited a significant change in precipitation volume. Continued deposition of base cations may help to reduce the detrimental effects of acidic deposition.

Isotopic (?18O) characteristics of weekly precipitation collected across the USA: an initial analysis with application to water source studies

A portion of the precipitation samples collected and stored by the National Atmospheric Deposition Program (NADP) are shown to be useful for analysis of isotopes in precipitation. The potential problems with evaporation are small based on deuterium excess analyses and comparisons with the Global Meteroic Water Line. Presented here are the δ 18 O values of precipitation collected from nine NADP sites during 1989, 1990 and 1991. The trends in the isotopic (δ 18 O) characteristics of recent precipitation are in agreement with findings from previous International Atomic Energy Agency (IAEA) sites in the USA. The findings are also in agreement with several major isotope-environment relationships, further supporting the use of these samples for a modern global data base on the isotopes in precipitation being developed by IAEA, called GNIP (Global Network for Isotopes in Precipitation) and for use by research groups in the hydrological modelling, palaeoclimate and ecological communities. As expected, the average δ 18 O values of precipitation that is derived from the Gulf of Mexico (-3‰) and from the Pacific North-west are isotopically distinct (-7‰). In addition, using the NADP network, isotopic depletion in the δ 18 O values of precipitation in the range of 8‰ was observed from coastal to inland locations either in the Pacific North-west or along the east side of the Rocky Mountains, from Texas to Eastern Montana. In central USA, especially at high elevation, there is a strong seasonal variation in the δ 18 O values of precipitation, differing by almost 25‰ between January and August, whereas at coastal locations the seasonal variation in the δ 18 O values of precipitation was minimal. Comparisons between the average δ 18 O values of precipitation from several NADP sites and those of the IAEA collected in the 1960s and 1970s are exceptionally comparable, differing in some cases by only 0.1‰, further substantiating the quality and utility of precipitation from this network. The δ 18 O-temperature relationships for two of our sites had slopes of 0.55 (Colorado) and 0-46 (Nebraska) that are within the range of those reported earlier (0.29-0.7), although they are slightly lower, possibly owing to the large temperature variation between winter and summer in central USA. Further, the seasonal variation in the δ 18 O values of precipitation are used in a pair of ecological studies to articulate water sources of different plant life forms in riparian and prairie ecosystems, with grasses typically using summer rain to a much greater extent than woody species, such as willow or saltbush.

Emissions of alkaline elements calcium, magnesium, potassium, and sodium from open sources in the contiguous United States

Models of dust emissions by wind erosion (including winds associated with regional activity as well as dust devils) and vehicular disturbances of unpaved roads were developed, calibrated,and used to estimate alkaline dust emissions from elemental soil and road composition data. Emissions from tillage of soils were estimated from the work of previous researchers. The area of maximum dust production by all of those sources is the area of the old “Dust Bowl” of the 1930s (the panhandles of Texas and Oklahoma, eastern New Mexico and Colorado, and western Kansas). The areas of maximum alkaline dust production are the arid southwest, the “Dust Bowl,” and the midwestern‐mideastern states from Iowa to Pennsylvania. Our calculations show that calcium is the dominant alkaline element produced by “open sources” (sources too great in extent to be controlled by enclosure or ducting). Although the largest dust mass source is wind erosion (by winds associated with regional activity and convective activity), the largest producer of the alkaline component is road dust because the abundance of alkaline materials in road coverings (which include crushed limestone) is significantly higher than for soils. Comparing the above estimated sources of alkaline material with inventories of SO2 and NOx emissions by previous investigators gives the rough approximation that alkaline emission rates are of the order of the SO2 + NOx emissions in the western United States and that they are much smaller than SO2 + NOx in the eastern United States. This approximation is substantiated by data on Ca/(SO4 + NO3) for wet deposition for National Atmospheric Deposition Program sites.

Rain Scavenging of Tephra Aerosols from Mount St. Helens 1980 Eruptions

Mount St. Helens had three major eruptions in 1980: 18 May, 25 May, and 12 June. Tephra in the atmosphere from these eruptions was traced by analyzing the particulate matter from weekly rain collections at 62 NADP (National Atmospheric Deposition Program) sites across the United States. The particulate matter was on 0.45 micron pore-sized filters (47 mm diameter) in amounts of 0-214 mg. Identification of tephra in the particulate matter was made by x-ray diffraction identification of feldspar (70% of the tephra) and chemical analyses compared to a standard tephra. Tephra was identified at sites in the path defined by observable amounts on the ground and outside and beyond this path. Dilution of the tephra by other aerosols varied from 20% new Mount St. Helens to about 300% at downwind sites, which were two days and 3000 km from Mount St. Helens. Atmospheric cleanup of the tephra at a given site occurred in 1–2 weeks, sometimes in a few days, as measured by the particulate matter in rain. Particulate matter captured in rains before the eruptions was mainly alpha quartz, feldspar, illite, kaolinite and organics with an average flux for the United States of 112 kg ha−1 per week.