Long-term regional trends of nitrogen and sulfur deposition in the United States from 2002 to 2017
Authors: Benish Sarah E. , Bash Jesse O. , Foley Kristen M. , Appel K. Wyat , Hogrefe Christian , Gilliam Robert , Pouliot George 
Published: September 2022

Abstract. 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 SO4, NO3, and NH4
wet deposition compared to National Atmospheric Deposition Program
observations and underestimates of NH4 and SO4 and overestimates
of SO2 and TNO3 (HNO3+NO3) compared to the Clean Air
Status and Trends Network (CASTNET) ambient concentrations. Second, a
measurement–model fusion approach employing a precipitation and bias correction to wet-deposition estimates is found to reduce model bias and
improve correlations compared to the unadjusted model values. Model
agreement of wet deposition is poor over parts of the West and Northern
Rockies, due to errors in precipitation estimates caused by complex terrain
and uncertainty in emissions at the relatively coarse 12 km grid resolution
used in this study. Next, we assess modeled N and S deposition trends across
climatologically consistent regions in the CONUS. Total deposition of N and
S in the eastern US is larger than the western US with a steeper decreasing
trend from 2002–2017; i.e., total N declined at a rate of approximately
−0.30 kg N ha−1 yr−1 in the Northeast and Southeast and by −0.02 kg N ha−1 yr−1 in
the Northwest and Southwest. Widespread increases in reduced N deposition
across the Upper Midwest, Northern Rockies, and West indicate evolving
atmospheric composition due to increased precipitation amounts over some
areas, growing agricultural emissions, and regional NOx/SOx
emission reductions shifting gas–aerosol partitioning; these increases in
reduced N deposition are generally masked by the larger decreasing oxidized
N trend. We find larger average declining trends of total N and S deposition
between 2002–2009 than 2010–2017, suggesting a slowdown of the rate of
decline likely in response to smaller emission reductions. Finally, we
document changes in the modeled total N and S deposition budgets. The
average annual total N deposition budget over the CONUS decreases from 7.8 in 2002 to 6.3 kg N ha−1 yr−1 in 2017 due to declines in oxidized N
deposition from NOx emission controls. Across the CONUS during the
2002–2017 time period, the average contribution of dry deposition to the
total N deposition budget drops from 60 % to 52 %, whereas wet
deposition dominates the S budget rising from 45 % to 68 %. Our analysis
extends upon the literature documenting the growing contribution of reduced
N to the total deposition budget, particularly in the Upper Midwest and
Northern Rockies, and documents a slowdown of the declining oxidized N
deposition trend, which may have consequences on vegetation diversity and
productivity.