Atmospheric deposition of nitrogen (N) and sulfur (S)
compounds from human activity has greatly declined in the United States (US)
over the past several decades in response to emission controls set by the
Clean Air Act. While many observational studies have investigated spatial
and temporal trends of atmospheric deposition, modeling assessments can
provide useful information over areas with sparse measurements, although
they usually have larger horizontal resolutions and are limited by input data
availability. In this analysis, we evaluate wet, dry, and total N and S
deposition from multiyear simulations within the contiguous US (CONUS).
Community Multiscale Air Quality (CMAQ) model estimates from the EPA's (Environmental Protection Agency) Air QUAlity TimE Series (EQUATES) project contain important model updates to
atmospheric deposition algorithms compared to previous model data, including
the new Surface Tiled Aerosol and Gaseous Exchange (STAGE) bidirectional
deposition model which contains land-use-specific resistance
parameterization and land-use-specific deposition estimates needed to
estimate the differential impacts of N deposition to different land use
types. First, we evaluate model estimates of wet deposition and ambient
concentrations, finding underestimates of SO
Human activity doubled the amount of reactive nitrogen (
Deposition monitoring and assessment played a decisive role informing the
United States (US) Clean Air Act Amendments (CAAA) of 1990 (United
States Congress, 1990). Wet deposition, sampled weekly in rain or snow, has been
measured by the National Atmospheric Deposition Program (NADP) National
Trends Network (NTN) since 1978. Dry-deposition measurements are inferred by
combining weekly measured concentrations from the Clean Air Status and
Trends Network (CASTNET) and modeled deposition velocities from the
multilayer model (MLM) (Meyers et al., 1998). Dry-deposition
modeling is still uncertain, particularly for land use and dry-deposition
schemes in models and emission data. Despite providing critical deposition
information, the limited number of NADP and CASTNET sites in essential
locations, such as areas with complex terrain, near urban centers, at high
elevation, or in forest ecosystems, restrict a thorough understanding on the
amount and consequences of deposition. For instance, strong concentration
gradients in N deposition have been documented over urban areas such as
Boston, Massachusetts (Rao et al., 2013), and near Baltimore, Maryland (Bettez
et al., 2013), and along coastlines like the Chesapeake Bay (Loughner
et al., 2016). Additionally, networks are unable to capture the full budget
of
Chemical transport models (CTMs) can be used to study deposition
relationships and trends for locations without measurements and compounds
that are challenging to quantify (e.g., organic N). However, to provide
reliable estimates, CTMs estimates must first be evaluated with measurement
data. Various studies have compared N and S wet deposition and
concentrations estimated by the Community Multiscale Air Quality (CMAQ)
model (
In this study, we assess deposition from the EPA's (Environmental Protection Agency) Air QUAlity TimE Series
(EQUATES) project (
The long-term EQUATES simulations from 2002–2017 were run using the CMAQ model version 5.3.2. Emissions were from different national emission inventories (NEIs) or used a base year NEI and estimated emissions for other years by creating scaling factors based on activity data and emission control information (Xing et al., 2013; Foley et al., 2020). A summary of recent CMAQ version 5.3 model updates and their impact on modeled concentrations is provided in Appel et al. (2021). The Weather Research and Forecasting version 4.1.1 (WRFv4.1.1) model was used for meteorology. Lateral boundary conditions for the 12 km grid spacing CONUS domain used in this study were provided by a 108 km grid spacing Northern Hemispheric simulation. Hemispheric and North American emission inventories were specifically prepared for EQUATES to ensure consistent input data and methods across all years. The Surface Tiled Aerosol and Gaseous Deposition (STAGE) option in CMAQv5.3.2 (Galmarini et al., 2021) was used to estimate atmospheric dry-deposition rates. The STAGE dry-deposition option generally performs similar to M3Dry (the other dry-deposition option in CMAQv5.3.2) when compared to network observations of ambient gaseous and aerosol pollutant concentrations (Appel et al., 2021) while providing additional land-use-specific deposition data useful for assessments of ecosystem exposure (Hood et al., 2021).
Locations of the 200 National Atmospheric Deposition Program (NADP) National Trends Network (NTN, white circles) and 75 Clean Air Status and Trends Network (CASTNET, triangles) sites examined in this study. NADP NTN sites shown in black circles did not meet completeness criteria thresholds and therefore were not included in this analysis. Color-coded US climate regions shown in this map are referred to throughout this analysis. The black-bordered white circles indicate NADP NTN sites that meet annual completeness criteria for 13 years of the time series and are examined in the model evaluation presented in Sect. 3.1.
We assess CMAQ's ability to reproduce annual accumulated wet deposition and ambient concentrations of N and S species measured by the NADP NTN and EPA's CASTNET networks (Fig. 1). Since anthropogenic sources dominate deposition and their area of influence is largely regional (Paulot et al., 2013), we evaluate the agreement over CONUS climate regions from 2002–2017 following the NOAA definition defined by Karl and Koss (1984), although these groupings do not imply the regions are isolated from each other. The nine climate regions include the Northwest, West, Northern Rockies, Southwest, South, Ohio Valley, Southeast, and Northeast as shown in Fig. 1.
The NADP's NTN (
The modeled wet-deposition fields are adjusted to account for input biases
and uncertainty in the chemical and physical processes governing deposition.
No corrections are applied to dry deposition due to limited dry-deposition
measurements. Since model performance is improved for annual instead of
seasonal values (Table S3; refer to Zhang et al., 2019, for detailed seasonal model evaluation), we apply a measurement–model
fusion technique previously described by Zhang
et al. (2019) to adjust the modeled annual wet-deposition fields of
inorganic N (NO
After adjusting simulated wet deposition by precipitation, an additional
bias adjustment (EQUATES
Taylor plot comparing annual accumulated wet deposition (kg ha
EQUATES model performance metrics of annual (2002–2017) accumulated
wet deposition of NH
Scatterplots of annual accumulated bias-adjusted modeled and NTN
observed wet deposition (kg ha
In this section, we investigate the EQUATES model performance, estimating wet
deposition and ambient concentrations of N and S compounds throughout the
US. The impact of the precipitation and bias correction technique on
reproducing 2002–2017 accumulated measured wet deposition of NH
Evaluation of bias-adjusted SO
Figure 3 provides an overall comparison between the bias-corrected EQUATES
wet deposition and NTN observations, along with an evaluation of simulated
and observed precipitation. Table 2 provides summary statistics of wet
deposition and precipitation for each climate region. A slight negative bias
of modeled precipitation and bias-corrected NH
The 16-year annual accumulated NH
Scatterplots comparing 2002–2017 annual average concentrations
(
To indirectly evaluate the ability of the EQUATES simulations to reproduce
dry deposition, we compare the model-simulated annual average concentrations
of SO
Evaluation of EQUATES modeled average concentrations (
Figure 5 shows the spatial distribution and overall trend of modeled total deposition of N (i.e., the sum of oxidized and reduced N) and S between 2002 and 2017. For the trend analysis shown here and throughout, we calculate a linear least-squares regression with significance examined at the 95 % confidence level using a Wald test, with any insignificant trends set to zero. The eastern US has higher total N deposition amounts than the western US in both 2002 and 2017, particularly in parts of the Ohio Valley and Upper Midwest, regions associated with large N emissions from agriculture (Dammers et al., 2019) and energy consumption (Kim et al., 2006). The major N deposition region shifts from the eastern to the central US between 2002 and 2017, with highest deposition amounts found in parts of Iowa, North Carolina, and Indiana, a pattern consistent with N inventories compiled by Sabo et al. (2019) indicating an increase in agricultural fertilizer and livestock waste in the Midwest between 2002 and 2012. Increases in agricultural fertilizer have previously been linked to cropland expansion in the Great Plains (Lark et al., 2015; Wright et al., 2017) and increasing demand for domestically sourced biofuels (Donner and Kucharik, 2008). Urban regions in the central and eastern US indicate a substantial amount of N deposition compared to nearby rural areas (Fig. S8), consistent with previous findings that bulk N deposition in urban areas is twice as much as rural and remote sites (Decina et al., 2019).
Similar to total N deposition, total S deposition has a noticeable spatial
gradient in the east compared to the west, particularly in the beginning of
the time series, following larger changes in emissions in these regions
(Aas et al., 2019; Holland et al., 1999). In 2002, total S deposition
across the Southeast, Ohio Valley, and Northeast is broadly greater than
10 kg S ha
Spatial distribution of total N
Figure 6 summarizes average total N and S trends across the climate regions
and CONUS based on three different time bins: 2002–2017, 2002–2009, and
2010–2017. While there are significant regional deposition composition
changes from 2002–2017, smaller changes in the overall deposition trends are
observed for the CONUS. From 2002 to 2017, the largest average trend in
decreasing total N deposition (
Comparison of mean total N
To elucidate the chemical drivers responsible for the varying changes in
total N deposition across the US, we examine trends in total oxidized and
reduced N in Fig. 7. Overall, most regions show larger decreasing trends
in oxidized N than increasing trends in reduced N. Regions with
insignificant total trends (see grey areas in Fig. 5) indicate similar
magnitudes of oxidized and reduced N trends. The largest decreasing trends
in total oxidized N are found in the eastern US and along the Pacific coast.
The large declines in total oxidized N in the Upper Midwest, Ohio Valley,
Northeast, and Southeast showcase both the coordinated application of
emission controls in major contributing regions and increasingly lower
amounts of NO
Maps of 2002–2017 trends of total oxidized
Figure 8 shows trends of the wet and dry components of oxidized and reduced
N and S deposition. Average dry deposition of oxidized N is found to decline
faster than average wet deposition of oxidized N for all climate regions, as
oxidized N is more efficiently removed by dry deposition due to high
deposition velocities (Zhang et al., 2012). Larger amounts of
dry vs. wet deposition across the eastern US in 2002 (Fig. S9a and d)
and more comparable amounts in 2017 (Fig. S9b and e) suggest a shift over
time in the contributions of oxidized wet and dry deposition, although the
portioning is highly dependent upon annual rainfall and changes to climate.
The Southeast, Northeast, and Ohio Valley have the steepest simulated
average dry and wet oxidized N deposition trends (approximately
Trends of reduced N are increasing and overall smaller in magnitude compared
to the oxidized N trends (Fig. 7b). Regions of elevated wet and dry
reduced N deposition have expanded and increased in magnitude across the
CONUS (Fig. S11) compared to oxidized N, also observed in the NTN NH
Decreasing trends in wet and dry S deposition are found across the CONUS,
although the amount varies by climate region (Figs. 8c, S13). Similar to
the oxidized N trends, the Northeast, Southeast, and Ohio Valley experience
the largest decreasing trend of approximately
Annual average trend (2002–2017, kg ha
Average total (wet
The average N and S deposition budgets across all CONUS climate regions
generally decrease from 2002 to 2017 (Fig. 9). The average total N budget
for the CONUS decreases from 7.8 to 6.3 kg N ha
Changes in the percent of total N deposited as reduced nitrogen from 2002 to 2017 throughout the NOAA climate regions and CONUS.
The relative proportion of reduced N to the total budget has shifted over
the time period of 2002 to 2017 (Fig. 10). On average across the CONUS,
oxidized N deposition dominates the total budget until 2016, when reduced N
comprises
The total S budget also decreases across the CONUS on average, falling from
5.3 in 2002 to 1.8 kg S ha
In this study, we examine CMAQ model simulations from the EPA's Air QUAlity
TimE Series (EQUATES) project to investigate spatial and temporal trends of
nitrogen (N) and sulfur (S) deposition over a period with substantial
emission reductions implemented to meet requirements set by the Clean Air
Act Amendments (CAAA). We assess changes in modeled dry deposition and
precipitation- and bias-adjusted wet deposition across nine
climatologically consistent regions within the US. EQUATES CMAQ simulations
included important science updates to previous modeling studies, such as a
revised model for bidirectional air–surface exchange of NH
The modeled total N and S deposition amounts and trends are larger across
the eastern than the western US, particularly in the Northeast, Southeast,
Ohio Valley, and Upper Midwest climate regions. While the average CONUS
trend indicates a decrease of
The average total N deposition budget over the CONUS has decreased from 7.8
in 2002 to 6.3 kg N ha
Our analysis, in addition to the analyses of many others (Zhang et al.,
2018; Li et al., 2016; Du et al., 2014), highlights the increasing
contribution of reduced compounds to the N budget in all climate regions,
particularly the Northern Rockies and Upper Midwest. Both the model and
observations indicate statistically significant increasing trends in
NH
Data are available for download at
The supplement related to this article is available online at:
KMF and GP were the EQUATES project leads and responsible for project administration, methodology, validation, formal analysis, data curation, and visualization. RG conducted the meteorological runs, and KWA and CH performed the model simulations. RG, CH, KWA, and JOB provided the methodology, formal analysis, and validation. SEB performed the data analysis and created all figures and tables. SEB wrote the paper with comments from all authors.
The contact author has declared that none of the authors has any competing interests.
The views expressed in this paper are those of the authors and do not necessarily represent the views or policies of the US Environmental Protection Agency. Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
The authors would like to thank Chris Allen, Michael Aldridge, Megan Beardsley, James Beidler, David Choi, Alison Eyth, Caroline Farkas, Janice Godfrey, Barron Henderson, Shannon Koplitz, Rich Mason, Rohit Mathur, Chris Misenis, Norm Possiel, Havala Pye, Lara Reynolds, Matthew Roark, Sarah Roberts, Donna Schwede, Karl Seltzer, Darrell Sonntag, Kevin Talgo, Claudia Toro, and Jeff Vukovich for their contribution to the development of emission and meteorology inputs for the EQUATES simulations used in this study. This research was supported in part by an appointment to the Research Participation Program at the US EPA Office of Research and Development (ORD), administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the US Department of Energy and the US EPA.
This paper was edited by Qiang Zhang and reviewed by Ruth Heindel and Yuqiang Zhang.