Introduction
The introduction of reactive nitrogen to the environment by anthropogenic
activities (e.g., from fossil fuel combustion and the production of
fertilizers for agriculture) has drastically altered the global nitrogen
cycle with consequences throughout the Earth system (Galloway et al., 2004).
Reactive nitrogen dominates the chemical production of tropospheric ozone
and contributes to inorganic aerosol formation, with implications for air
quality and climate. Deposition of nitrogen from the atmosphere has been
linked to eutrophication and acidification (Bouwman et
al., 2002), reductions in biodiversity
(Bobbink
et al., 2010; Hernández et al., 2016; Isbell et al., 2013; De Schrijver
et al., 2011), and climate impacts through coupling with the carbon cycle
and greenhouse gas emissions
(Liu and
Greaver, 2009; Reay et al., 2008; Templer et al., 2012). Despite its global
importance, observational constraints on nitrogen deposition are lacking in
many parts of the world due to poor or nonexistent measurement coverage
(Vet et al.,
2014).
Atmospheric transport is a dominant process for distributing reactive
nitrogen around the world (Galloway et al., 2008). Some
forms of reactive nitrogen can be transported over distances greater than
1000 km (Neuman et
al., 2006; Sanderson et al., 2008), depositing across national boundaries
and continents. For example, the US is estimated to export 30–40 % of
its reactive nitrogen emissions
(Dentener
et al., 2006; Holland et al., 2005; Zhang et al., 2012), while transport
from China accounts for up to 66–92 % of total nitrogen deposition to
parts of the northwestern Pacific Ocean
(Zhao et
al., 2015). Fertilization of the open ocean due to atmospheric transport and
deposition of anthropogenic nitrogen may be a considerable factor in marine
productivity (Duce et al., 2008), prompting
important questions about the fate and impact of deposition far downwind of
sources where observations are limited.
Reactive nitrogen oxides (NOy≡ NO + NO2+ HNO3
+ HONO + organic nitrate molecules + aerosol nitrate) contribute about
half of the total nitrogen deposited worldwide
(Dentener et al.,
2006). NOy deposition was estimated to be around 45–50 Tg N yr-1
in the late 1990s and early 2000s, representing a three- to fourfold increase since
the preindustrial era
(Dentener
et al., 2006; Kanakidou et al., 2016; Lamarque et al., 2013). A substantial
range exists in the trajectory of global NOy deposition beyond the year
2000, depending on the emission scenario. Galloway et al. (2004)
projected that NOy deposition could increase by > 70 % by
2050, while Dentener et al. (2006) projected
changes between -25 to +50 % by 2030 for maximum feasible reduction
and “pessimistic” scenarios, respectively. More recent multimodel
projections by Lamarque et al. (2013) estimate NOy
deposition would change by -16 to +5 % for 2030 and by -48 to
-25 % for 2100, depending on the representative concentration pathway
(RCP) scenario. This range in projections highlights the need for global
observational constraints on contemporary changes in nitrogen oxide
emissions.
Sources of NOx (≡ NO + NO2), whose oxidation is
responsible for the formation of other reactive nitrogen oxides, include
fossil fuel combustion, biomass burning, lightning, and biogenic emission
from soil. The magnitude and spatial distribution of NOx emissions have
changed considerably over the past several decades, corresponding to
patterns of human development and emission control measures. Tropospheric
NO2 columns derived from satellite remote sensing observations have
been used extensively to constrain regional and global NOx emissions
(Streets et al.,
2013). This top-down approach complements bottom-up inventories that are
assembled using regionally specific emission factors and fuel combustion
data for various source categories. In particular, satellite NO2
observations can provide insight into otherwise poorly constrained sources
(Beirle
et al., 2010; Jaegle et al., 2005; Richter et al., 2004; Vinken et al.,
2014), produce coherent long-term trends
(van
der A et al., 2008; Lu et al., 2015; Stavrakou et al., 2008; Zhang et al.,
2007), document interannual variability
(Castellanos
and Boersma, 2012; Konovalov et al., 2010; Russell et al., 2012), offer
information to evaluate and improve bottom-up inventories on the global
scale
(Martin,
2003; Miyazaki et al., 2017), and provide timely emission updates
(Lamsal
et al., 2011; Mijling et al., 2013; Souri et al., 2016).
Satellite observations of NO2 began with GOME (1995–2003), were followed by
SCIAMACHY (2002–2011), and continue today with OMI (2004–), GOME-2 (2007–), and TROPOMI (2017–), resulting in a record of global atmospheric
NO2 abundance over the past 20 years. These observations have been used
previously to estimate the deposition of nitrogen species, either in
combination with chemical transport modeling or with empirical approaches
(Cheng
et al., 2013; Jia et al., 2016; Lu et al., 2013; Nowlan et al., 2014). For
example, Nowlan et al. (2014) demonstrated how satellite-inferred surface
concentrations of NO2 can be combined with modeling to produce
spatially continuous estimates of NO2 dry deposition fluxes. They found
that dry deposition of NO2 contributes as much as 85 % of total
NOy deposition in urban areas, but represents only 3 % of global
NOx emitted. The remaining 97 % of global NOy deposition is made
up of both wet and dry deposition of other reactive nitrogen oxide compounds
that are not directly observed by satellite-based instruments.
In this study, we expand on the approach of Nowlan et al. (2014) by using
the satellite observations of NO2 columns to constrain total NOy
deposition, including other oxidized nitrogen species and wet deposition
which contribute substantially to NOy deposition. We accomplish this by
constraining surface NOx emissions using the satellite observations of
NO2 and simulating subsequent NOy deposition with a global
chemical transport model. Given the effective mass balance between NOx
emissions and deposition of reactive nitrogen oxides, observational
constraints on NOx emissions provide a powerful top-down constraint on
deposition (which to our knowledge has not yet been exploited in this way).
We leverage the long-term coverage of GOME, SCIAMACHY, and GOME-2
observations to produce a globally consistent and continuous record of
NOy deposition from 1996 to 2014. We highlight long-term trends in
satellite-constrained NOy deposition around the world and discuss
changes in regional export of NOx. Our satellite-constrained estimates
of NOy deposition are evaluated using measured wet nitrate
(NO3-) deposition from a variety of sources worldwide. We also
explore the sensitivity of the NOy deposition estimates to
uncertainties in NH3 emissions and discuss other considerations.
Methods
Satellite-based constraints on NOy deposition
The application of satellite NO2 column observations to constrain
NOy deposition requires a method to propagate the observational
information across the entire NOy system containing species with
lifetimes of days or longer. The short NOx lifetime of hours during
daytime satellite observations implies that a direct assimilation for
NO2 column abundance would rapidly lose the observational information
as the assimilation returns to its unperturbed state well before the next
satellite observation days later. We therefore apply satellite NO2
observations to constrain NOx emissions in a simulation of NOy
deposition so the information propagates across the entire NOy system.
We calculate top-down surface NOx emissions from 1996 to 2014 using
observations from GOME (1995–2003), SCIAMACHY (2002–2011) and GOME-2 (2007–). The similar overpass time of these three instruments (from about 09:30 to 10:30 LT, local time) facilitates their combination to provide
consistent long-term coverage
(Geddes
et al., 2015; Hilboll et al., 2013; Konovalov et al., 2010; van der A et al.,
2008). We achieve consistency across all three instruments despite their
varying pixel sizes (320 km × 40 km, 60 km × 30 km, and 80 km × 40 km for
GOME, SCIAMACHY, and GOME-2, respectively) by gridding the daily observations
from each to a regular coarse grid of 2∘ × 2.5∘ latitude
by longitude. Tropospheric NO2 vertical column densities are provided
by KNMI at http://www.temis.nl/airpollution/. In all cases, NO2 column
densities are retrieved by differential optical absorption spectroscopy
using backscattered radiance in the 425–450 nm wavelength range according to
the retrieval algorithm documented in Boersma et al. (2004). The error in individual satellite-derived
tropospheric NO2 column retrievals is estimated to be around 35–60 % for polluted
scenes and greater than 100 % for clean regions (Boersma et al., 2004).
Boersma et al. (2016)
describe well the value of accounting for vertically resolved instrument
sensitivity. We use the averaging kernels provided with the data to replace
a priori NO2 vertical profiles with those from GEOS-Chem model
following Lamsal et al. (2010). We use daily
nadir observations with a cloud radiance fraction of less than 0.5. We
minimize errors associated with wintertime retrievals by using a solar
zenith angle cut-off of 50∘.
We use the GEOS-Chem chemical transport model (www.geos-chem.org) v9-02 to conduct the inversion of satellite
observations of NO2 and constrain global NOy deposition. The
simulation is described in Appendix A. Briefly, GEOS-Chem is driven by
assimilated meteorology from the NASA Global Modeling and Assimilation
Office and simulates detailed HOx–NOx–VOC–aerosol (VOC: volatile organic compound) chemistry
(Bey et al.,
2001; Park et al., 2004). Removal occurs through wet deposition
(Amos et al., 2012;
Liu et al., 2001) and dry deposition based on the widely used
resistance-in-series formulation (Wesely, 1989).
Anthropogenic, biogenic, soil, lightning, and biomass burning emissions are
included (see Appendix A). In the case of NOx, the bottom-up emissions
are used as prior estimates that we then overwrite with the top-down
emissions.
We adopt a finite-difference mass-balance inversion
(Cooper et al., 2017; Lamsal et al., 2011) for
computational expedience given the 19-year period of interest. In two
initial simulations, a perturbation (30 %) to the a priori emissions, E, in
a grid cell is used to find the relationship between the a priori NO2
column, Ω, and the change in the column resulting from that
perturbation:
ΔEE=β×ΔΩΩ.
The factor β in Eq. (1) accounts for nonlinear feedbacks between
a change in NOx emissions and NOx chemistry in a grid cell leading
to grid cell NO2 column abundance.
We then use monthly-mean gridded satellite observations, Ωsat,
in combination with monthly β values for each grid box to derive new
gridded annual emissions, Etopdown, from the prior emissions estimates,
Eprior, by rewriting Equation 1 as:
Etopdown=Eprior⋅1+βΩsat-ΩpriorΩprior.
We use GOME observations for years 1996 to 2002, SCIAMACHY observations for
years 2003 to 2006, and GOME-2 observations for years 2007 to 2014. In all
cases, monthly mean simulated NO2 columns are calculated using days
with coincident satellite observations. The simulated NO2 vertical
column is output daily for late morning. We calculate scaling factors for
every month with available satellite observations, then calculate an annual
mean scaling factor that is used to infer annual mean top-down emissions.
Our top-down emissions retain the same seasonality as the prior emissions to
mitigate concerns about seasonally missing data (such as from snow or
monsoonal clouds). The top-down emissions are then used in a final
simulation to model NOy deposition. For locations without any satellite
observations, the a priori emissions are used. The resultant simulation of
NOy deposition is thus constrained by, and consistent with, the
satellite NO2 observations (similar in essence to an assimilation
system). We note that uncertainty in tropospheric NO2 from lightning will
propagate into the inversion
(Travis et al., 2016),
but there is no evidence of a significant trend in lightning NOx over
the long term (Murray et al., 2012). A constant bias is unlikely to affect
the trend analyses presented here.
Global top-down NOx emissions calculated using the finite mass-balance inversion approach with observations from GOME, SCIAMACHY, and
GOME-2.
Year
Global NOx
emissions
(Tg N yr-1)*
1996
60.1
1997
58.4
1998
59.2
1999
59.6
2000
53.4
2001
52.3
2002
55.1
2003
50.1
2004
51.5
2005
51.2
2006
50.0
2007
54.7
2008
56.1
2009
55.9
2010
57.5
2011
58.9
2012
59.3
2013
58.5
2014
54.0
Mean
55.6
* Includes anthropogenic HNO3 flux of 2.3 ± 0.1 Tg N yr-1.
Table 1 shows the annual global top-down NOx emissions we obtain from our
calculations. We derive global mean satellite-constrained NOx emissions
from 1996 to 2014 of 55.6 ± 3.4 Tg N yr-1. Our top-down global
NOx emissions for 2001 of 52.3 Tg N are consistent with the mean
± standard deviation from over 20 models used in the Coordinated Model
Studies Activities of the Task Force on Hemispheric Transport of Air
Pollution (HTAP) for the same year of 46.6 ± 7.8 Tg N (Vet et al.,
2014).
Measurements of wet deposition
We use a variety of regional and global measurements of wet nitrate
(NO3-) deposition to evaluate our satellite-constrained simulation
from 1996 to 2014.
To evaluate overall global performance we use data compiled by the World
Data Centre for Precipitation Chemistry for two time periods: 2000–2002 and
2005–2007 (http://www.wdcpc.org/). The use of these data ensures
optimal and consistent quality standards across all stations, allowing for
evaluation of global performance with careful consideration of sampling
protocols and data completeness (Vet et al., 2014).
To evaluate the long-term means and trends from 1996 to 2014, we obtain
observations of wet NO3- deposition from North America, Europe,
and East Asia where continuous measurements are available throughout most of
our study period. Observations come from the National Atmospheric Deposition
Program in the United States (http://nadp.sws.uiuc.edu/,
available 1996–2014), from the Canadian Air and Precipitation Monitoring
Network in Canada (http://www.ec.gc.ca/rs-mn/, available
1996–2011), from the European Monitoring and Evaluation Programme in Europe
(http://www.emep.int/, available 1996–2014) and from the Acid
Deposition Monitoring Network in East Asia (http://www.eanet.asia, available 2000–2014). In the US and Canada, wet
deposition is measured by wet-only samplers that are triggered at the onset
of precipitation. Measurements in Europe are made by bulk- and wet-only
sampling methods, and we used both in this analysis. Measurements across East
Asia are reported as wet-only, although at some stations this may not be
accomplished by strictly wet-only samplers (http://www.eanet.asia/product/manual/prev/techwet.pdf).
For our trend analysis, we only included stations which had quality controlled
annual data for at least 15 of the 19 years in our study. This left 128
stations across the United States, 14 stations in Canada, 18 stations across
Europe, and 14 stations across East Asia. For comparison with the GEOS-Chem
model, if multiple stations are available within a single grid box we grid
all measurements of annual wet deposition to the model horizontal
resolution.
Long-term (1996–2014) mean NOy deposition derived from the
GEOS-Chem simulation, constrained by satellite observations of NO2
columns from the GOME, SCIAMACHY, and GOME-2 instruments (a). Mean ratio
of simulated dry NOy deposition to total NOy deposition (b).
Satellite-constrained estimates of NOy deposition
Here we summarize the overall patterns in long-term mean deposition
resulting from our satellite-constrained simulation, followed by a
discussion of the long-term trends, changes in regional export, and the
sensitivity of the simulated NOy deposition to potential uncertainties
in NH3 emissions and other considerations.
Long-term mean NOy deposition
Figure 1 (top) shows our satellite-constrained long-term mean NOy
deposition from 1996 to 2014. We find that 32.2 Tg N yr-1 is deposited
on average over the continents (57 % of the total), and 23.8 Tg N yr-1 is deposited on average over the oceans (43 % of the total).
This is similar to the estimate by Galloway et al. (2004) that 46 % of
modern day NOy deposition occurs over the oceans. Critical nitrogen
deposition loads for various natural freshwater and terrestrial ecosystems
lie in the range of 5–20 kg N ha-1 yr-1, depending on the
ecosystem, soil conditions, and land history (World Health
Organization, 2000). We estimate that mean deposition of oxidized nitrogen
alone exceeds 5 kg N ha-1 yr-1 over a land area of approximately
12.7 × 106 km2 (or ∼ 8 % total land area).
In the Northern Hemisphere, high NOy deposition tends to be associated
with regions that have high anthropogenic NOx sources. We find that mean
NOy deposition in the eastern United States exceeds 10 kg N ha-1 yr-1 (maximum = 11.4 kg N ha-1 yr-1) with elevated
deposition extending into southeastern Canada and hundreds of kilometers
into the Atlantic Ocean. This is similar to the multimodel ensemble results
from ACCMIP and HTAP, predicting between 5 and 15 kg N ha-1 yr-1 in
this region (Lamarque et al., 2013; Vet et al., 2014). A prior GEOS-Chem
analysis over North America for the years 2006–2008 also predicted NOy
deposition exceeding 10 kg N ha-1 yr-1 in the eastern US (Zhang et
al., 2012). Elsewhere in North America, we find high NOy deposition
along the western coast of California (up to 6 kg N ha-1 yr-1) and in
the vicinity of Mexico City (up to 10 kg N ha-1 yr-1).
We find mean NOy deposition is also elevated throughout Europe, with a
maximum of 8.5 kg N ha-1 yr-1 located in northern Italy near the
Po Valley region. Again, our long-term estimate in this region is similar to
the ACCMIP and HTAP ensemble means, predicting NOy deposition in the
range of 5–10 kg N ha-1 yr-1 (Lamarque et al., 2013; Vet et
al.,
2014). The elevated deposition here is also spatially consistent with the
results from Holland et al. (2005). We find high deposition extending into
western Russia with a hotspot in the vicinity of Moscow approaching 5 kg N ha-1 yr1. Our observation-constrained estimate also has
isolated regions of high deposition in the Middle East (around 4–5 kg N ha-1 yr-1 in the vicinity of Tehran and around the Persian Gulf).
We find that the highest mean deposition in the world occurs in China,
exceeding 10 kg N ha-1 yr-1 in many regions. High deposition
extends into the midlatitude western Pacific Ocean off the coast of East
Asia. NOy deposition in the ACCMIP and HTAP ensemble means also exceeds
10 kg N ha-1 yr-1 throughout eastern China. We find the highest
long-term mean deposition (with a maximum close to 20 kg N ha-1 yr-1) occurs in the south, around the Pearl River Delta and in the
vicinity of Guangzhou, although deposition is also high in the regions just
west of Beijing and Shanghai.
In the Southern Hemisphere, high NOy deposition is associated with
biomass burning and soil NOx sources, in addition to anthropogenic
sources. For example, we find NOy deposition is between 3 and 5 kg N ha-1 yr-1 in central and southern Brazil as well as in the tropical
rainforests and moist savannahs of Africa. Our estimates in these biomass-burning- and soil-NOx-dominated regions are also generally consistent
with the ACCMIP and HTAP ensemble estimates (2–5 kg N ha-1 yr-1).
We find NOy deposition up to 10 kg N ha-1 yr-1 in the
vicinity of São Paulo and Rio de Janeiro, and in the vicinity of
Johannesburg and the industrialized Mpumalanga Highveld of South Africa (all
dominated by anthropogenic NOx emissions). Our constrained simulation
also identifies hotspots of deposition in the vicinity of Melbourne and
Sydney, Australia (∼ 4 kg N ha-1 yr-1).
Annual wet NO3- deposition from measurements available
through the World Data Centre for Precipitation Chemistry and from the
GEOS-Chem simulation constrained with satellite observations of NO2.
Two time periods are represented: 2000–2002 and 2005–2007.
Figure 1 (bottom) shows the simulated long-term ratio of dry NOy
deposition to total (wet + dry) NOy deposition. Globally, dry and wet
deposition contribute roughly equally to total NOy deposition (52
and 48 %, respectively). Dry deposition usually accounts for more than
50 % of the total over the continents and directly offshore, whereas wet
deposition dominates over the remote oceans. In the generally arid regions
of the world (e.g., the southwestern US, the Sahara Desert, the Arabian
Peninsula, and the Gobi Desert) dry deposition accounts for ∼ 85 % or more of the total deposition. Elsewhere, dry deposition fractions
tend to be highest (> 60 %) nearest to major surface NOx
sources (e.g., the eastern US, western Europe, and near other major urban centers
around the world in addition to the soil- and biomass-burning-dominated
source regions in South America and Africa). HNO3 typically makes the
dominant contribution to dry NOy deposition, although NO2 and
HNO3 can make almost equal contributions in certain high-NOx
environments. Isoprene nitrates and peroxyacetyl nitrates comprise
∼ 10–30 % of dry NOy deposition in some densely
forested and high-latitude environments, respectively.
We evaluate our estimates of NOy deposition with measured wet
NO3- from several sources. Figure 2 shows measurements of annual
wet NO3- deposition from the World Data Centre for Precipitation
Chemistry, available for two time periods: 2000–2002 (N= 470) and
2005–2007 (N= 484). In both we see the patterns of elevated deposition in
eastern North America, western Europe, and parts of South and East Asia,
with lower deposition in western North America, across high latitudes in the
Northern Hemisphere, and in the available observations in Africa. High
deposition in the Southern Hemisphere is observed between São Paulo and Rio
de Janeiro, and just southeast of Johannesburg. Figure 2 also shows the wet
NO3- deposition from our constrained simulation during the same
two time periods (2000–2002 and 2005–2007), which exhibits similar patterns
to those found in total NOy deposition (Fig. 1).
Scatter plot of the satellite-constrained simulated wet
NO3- deposition vs. measurements available through the World Data
Centre for Precipitation Chemistry for specific subsets of the data. The red
lines show the result of a reduced major axis linear regression. In the
right column, the blue line shows the fit across all data and the red line
shows the fit excluding the two circled data points that are discussed in
the text (reported statistics refer to the red line fit).
We find a high degree of consistency between our estimate and the
observations for both 2000–2002 (N= 306 model–data pairs) and
2005–2007 (N= 310 model–data pairs); the normalized mean bias (NMB) is
-14 and -16 %, respectively. The vast majority of pairs (> 80 %) agree to within 50 % of each other. Figure 3 shows scatter plots
for specific subsets of the global data. The agreement for both time periods
is strongest over North America (r= 0.92 for both 2000–2002 and
2005–2007, NMB =+1.0 and -5.0 %, respectively). Robust model
agreement with wet nitrate deposition observations over densely monitored
North America is characteristic of other global studies (Dentener et al.,
2006; Lamarque et al., 2013; Vet et al., 2014). Our agreement is also good in
Europe (r= 0.69 and 0.66, and NMB = -31.0 and -29.8 %
, respectively). The weaker correlation and low bias in this region is
likewise characteristic of global studies, although our spatial correlation
(r= 0.66–0.69) is on the high end of previously reported multimodel
ensembles (r∼ 0.4–0.6, Dentener et al., 2006; Lamarque et al.,
2013; Vet et al., 2014). The negative bias over Europe compared to North
America has previously been attributed to poor modeling of precipitation
and/or spatial representativeness of the measurements compared to model
resolution. Throughout the rest of the world (encompassing observations
mostly over Asia, but also over eastern Russia, and some locations in the
Southern Hemisphere) the combined spatial coverage of the observations is
very low (N= 53). Normalized mean bias in these estimates is also high
compared to North America (NMB = -19.5 and -17.8 % for 2000–2002 and
2005–2007, respectively), and our spatial correlation with the measurements
is weak (r= 0.35 and 0.42, respectively). We find that our poor agreement
here is disproportionately driven by the two observations that also have the
highest measured deposition in the world: near Port Blair on the South
Andaman Island in the Bay of Bengal, and in the Arunachal Pradesh state in
northeastern India. Agreement is considerably better with the rest of the
data (r= 0.78 and 0.72, NMB =+0.01 and -0.01 % for 2000–2002
and 2005–2007, respectively). Excluding these two points substantially
improves the global agreement as well (from r= 0.57 to 0.75 and r= 0.59 to 0.75, respectively). Site representativeness, precipitation errors,
or uncertainty in our satellite-constrained NOx emissions may explain
the discrepancy at these two specific sites.
Long-term (1996–2014) wet NO3- deposition from available
regional network measurements (a: NADP and CAPMON; b: EMEP;
c: EANet) and from the GEOS-Chem simulation, constrained with satellite
observations of NO2.
In addition to global data for 2000–2002 and 2005–2007 from the World Data
Centre for Precipitation Chemistry, we also evaluate our estimates of
NOy deposition over the long term (1996–2014) using continuous
observations provided by regional networks. Figure 4 shows measured wet
NO3- deposition over North America, Europe, and East Asia for
locations where at least 15 years of quality-controlled annual data are
available. These long-term mean observations demonstrate many of the same
spatial patterns as the time slices from 2000–2002 and 2005–2007. In North
America, a relatively smooth gradient is observed from low deposition in the
west to high deposition at sites in the east. In Europe, the highest
measured long-term mean wet NO3- deposition occurs at a coastal
site in southern Norway, at a site just east of Copenhagen, and at locations
in northern Italy and in Switzerland. At higher-latitude sites, deposition
is lower. Across the eastern Asia network, the measurements show the highest
deposition at sites in Southeast Asia (e.g., at a location between Kuala
Lumpur and Singapore, and another in the vicinity of Jakarta) and in Japan.
The lowest long-term mean deposition occurs at high-latitude sites along the
border of Russia and Mongolia, while moderate to high deposition is measured
on the coast of eastern China.
In general, our satellite-constrained estimate reflects the spatial
variability that is seen in the measurements. Globally, the correlation
between measured NO3- deposition and our estimated wet
NO3- deposition is excellent (r= 0.83, NMB = -7.7 %, N= 136 gridded model–data pairs). The vast majority of pairs
(> 85 %) agree to within 50 % of each other. For the individual regions,
normalized mean bias in our estimate is the smallest over North America (NMB = +2.4 %), and higher over Europe and East Asia (NMB = -32 and
-25 %, respectively). The spatial correlation over each region is strong (r= 0.89, r= 0.87, and r= 0.69 for North America, Europe, and East
Asia, respectively), but sample sizes over Europe (N= 16) and East Asia (N= 11) are small so we emphasize caution in the interpretation of the
statistics for these two regions. The lack of continuous measurement
coverage even in parts of the world with routine network observations
highlights the imperative of using other novel observational constraints on
deposition (such as the global satellite observations of NO2 used
here).
Long-term trend (1996–2014) in the satellite-constrained
simulation of NOy deposition. (a) Annual mean; (b) December–January–February; (c) March–April–May;
(d) June–July–August; (e) September–October–November. Diagonal hatching represents trend significance
(p < 0.01). Hatching from top-left to bottom-right indicates a
decreasing trend; hatching from bottom-left to top-right indicates an
increasing trend.
Trends in global NOy deposition from 1996 to 2014
Our long-term satellite-constrained estimate of NOy deposition
facilitates a unique and up-to-date investigation of the changes in NOy
deposition around the world. We calculate linear trends in annual NOy
deposition using the nonparametric Sen's method (Sen, 1968)
and test for significance with the nonparametric Mann–Kendall method
(Kendall, 1975; Mann, 1945). We treat increasing or
decreasing trends as significant if p < 0.01. Given that this is a
test for linear trends, regions where shorter-term trends in deposition may
have changed signs over the period of study could result in erroneous or
insignificant trends. Below we discuss particular regions where this is the
case.
Figure 5 shows the long-term annual and seasonal trends calculated from our
satellite-constrained estimate of total NOy deposition across 1996–2014
(hatching indicates statistical significance). Figure 6 highlights
time series of total NOy deposition over three specific regions covering
parts of North America, western Europe, and East Asia (as outlined in dashed
boxes in Fig. 5a).
Time series of annually integrated dry and wet NOy deposition over
specific regions (North America, Europe, and East Asia) as defined by the
dashed rectangles in Fig. 5.
Substantial decreases are seen throughout North America, extending over the
Atlantic Ocean to remote regions. The time series for this region (Fig. 6,
left) shows that NOy deposition decreased by almost 40 % from 6.4 Tg N yr-1 in 1996–1998 to 3.9 Tg N yr-1 in 2012–2014. The steepest
local decline in the world appears over the Ohio River Valley area, with a
maximum near Pittsburgh where NOy deposition decreased by -0.6 kg N ha-1 yr-2. NOy deposition near Pittsburgh decreased from
consistently exceeding 15 kg N ha-1 yr-1 during 1996–2000, to
below 6 kg N ha-1 yr-1 by 2014. The strong decrease in the
northeast is consistent with other long-term observational studies for the
US
(Sickles
and Shadwick, 2007, 2015). Studies of US NOx emissions derived from
satellite observations have also highlighted the remarkable success of
emission controls
(Duncan et al., 2013;
Russell et al., 2012). Our constrained estimate has the steepest declines
during the summer (Fig. 5; June, July, and August, JJA), restricted tightly to the source regions.
This also agrees with long-term observations showing the strongest
reductions in the summer (Sickles and Shadwick, 2015), consistent with the
shorter lifetime of NOx and efficient dry deposition of NOy over
the forested eastern US. We find significant decreases far downwind over the
Atlantic Ocean during the other months, when NOy can be transported
farther. The steep change in NOy deposition in the eastern US over the
last 20 years may have important consequences on tree mortality rates in the
region, which have been demonstrated to be very sensitive to NO3-
deposition in the range of 5–15 kg N ha-1 yr-1
(Dietze and Moorcroft, 2011). The steeply decreasing trends
across the US in our satellite-derived NOy also support the increasing
dominance of reduced nitrogen in total nitrogen deposition, evidenced by
observations (Li et al., 2016) and model
predictions (Ellis et al., 2013).
We find a small but statistically significant positive trend in NOy
deposition (+0.06 kg N ha-1 yr-2) in northern Alberta, Canada,
dominated by the trend in JJA. The region is downwind of development in the
Canadian oil sands, which has seen notable changes in NO2 column
abundance as observed from space (McLinden et al.,
2012). We estimate that deposition of NOy in this area was at a maximum
of 3.4 kg N ha-1 yr-1 in 2011 (up from 1.3 kg N ha-1 yr-1 in 1996–1997), and has since declined to
1.6 kg N ha-1 yr-1 by 2014. Elsewhere in Canada we estimate that NOy deposition
has decreased in the southern and eastern parts of the country, consistent with
observational analyses (Zbieranowski and Aherne, 2011).
Declines in NOy deposition are also found across Europe, but
statistical significance tends to be limited to western continental Europe
and the United Kingdom (while changes in the southern, northern, and eastern
countries tend to be insignificant). According to the time series for this
region (Fig. 6, middle), NOy deposition decreased by about 15 % (from
2.5 Tg N yr-1 in 1996–1998 to 2.1 Tg N yr-1 in 2012–2014). We find
the steepest local trends (-0.1 kg N ha-2 yr-1) in eastern Germany
and the southern UK, where NOy deposition in 2012–2014 decreased by 20 %
compared to 1996–1998. Previous satellite constraints on NOx emissions
established that NOx emissions in France, Germany, Great Britain, and
Poland have declined since 1996 while emissions in Greece, Italy, Spain, and
the Ukraine for example have either stayed constant or increased
(Konovalov et al., 2008). The local variability in
emission trends leads to notable transboundary impacts. For example, our
simulation predicts no net trend in NOy deposition over the Ukraine;
but we find this is a result of opposing trends in dry (increasing) and wet
(decreasing) deposition. This would be explained by increasing local
emissions but decreasing transport from upwind. Similarly, we find
significant increases in dry deposition in parts of western Russia but no
significant trend in wet deposition.
Large increases in NOy deposition are found throughout Asia,
concentrated especially in eastern China and parts of Southeast Asia. Figure 6 shows the time series of wet and dry NOy deposition within the
rectangular region outlined in Fig. 5 that encompasses eastern China and
part of the adjacent ocean. We find that NOy deposition in the region
increased by 65 % from 5.2 Tg N yr-1 in 1996–1998 to 8.6 Tg N yr-1 in 2012–2014. The time series also shows that NOy deposition
decreased after peaking around 9.3 Tg N yr-1 in 2011–2012. We find that
the steepest increasing local trends in the world appear in eastern China,
and in the Pearl River Delta region (up to +0.6 kg N ha-1 yr-2).
In fact, deposition in the Pearl River Delta region is the highest in the
world for most of our record, exceeding 20 kg N ha-1 yr-1 every
year from 2003 to 2014 (almost doubling from just over 11 kg N ha-1 yr-1 in 1996). The trends in deposition over China are largest in
summer when the NOx lifetime is short, with more obvious indications of
increasing NOy transport in the spring months (Fig 5; March–April–May, MAM). The
substantial increase in NOx emissions throughout East Asia has been
inferred from satellite instruments in several previous studies
(Mijling et al., 2013;
Richter et al., 2005). The timing and extent of the reversal in NOy
deposition that we see is also consistent with observed NO2 columns
over eastern China derived from OMI
(de Foy et al., 2016;
Krotkov et al., 2016). The ability of our satellite-constrained NOy
deposition estimate to capture this sudden dramatic decrease over China, in
contrast with previous projections (e.g., the RCP2.6, RCP4.5, and RCP8.5
projections to 2030, Lamaque et al., 2013), emphasizes an attribute of the
satellite constraint.
Small statistically significant decreasing trends are found over the
biomass-burning-dominated source regions of Africa. The decrease in NOy
deposition of about ∼ 3 % yr-1 relative to the
long-term mean in northern Africa is consistent with the most recent GFED4
inventory from 1997 to 2014 (http://www.globalfiredata.org/), which has fire
NOx emissions decreasing at a rate of about 3 % yr-1 in this
region. In contrast, we also estimate a similar decrease in southern Africa
that is not represented in the recent GFED4 emission time series. A reduction
in NO2 column abundance in this region (observed by GOME and SCIAMACHY)
was also reported by van der A (2008). They postulate that this decline
could be a result of deforestation leading to less biomass burning, but
changing NOx emission factors from biomass burning could also
potentially contribute to the trend.
Long-term (1996–2014) trends in wet NO3- deposition from
available regional network measurements (as in Fig. 4) and from the
GEOS-Chem simulation, constrained by satellite observations of NO2.
Closed circles around the measurements indicate significant trends (p < 0.01); hatching indicates statistical significance in the
simulation.
Despite the large regional trends described above, we find that global
deposition changed very little between 1996–1998 (56.1 Tg N yr-1) and
2012–2014 (58.5 Tg N yr-1) due to the opposing changes in different
regions. Total NOy deposition was lowest in 2006 (50.5 Tg N), and
peaked at 60.8 Tg N in 2012. Since then, it appears that global NOy
deposition may be on the decline. Future observations in the coming years
will be needed to establish whether this most recent decline is robust or
temporary.
Figure 7 shows the calculated long-term trends in the measured wet
NO3- deposition for locations across North America, Europe, and
East Asia where at least 15 years of quality-controlled annual data are
available (the coverage of these observation is the same as in Fig. 4).
The observations over North America show the gradient in the trend from
negligible in the west to steeply and significantly decreasing in the
northeastern US and southeastern Canada. The steepest observed statistically
significant trend (-0.18 kg N ha-1 yr-2) occurs east of Detroit in
southwestern Ontario, Canada. In Europe, only one of the gridded
observations has a statistically significant trend (-0.07 kg N ha-1 yr-2), located near the border of Denmark and Germany. The other
locations in Europe show statistically insignificant trends over the
long-term. In East Asia, we also we find that most of the stations record
statistically negligible trends over the long term (only 2 of the 11
gridded observations have significant trends). The steepest observed trend
in this region (+0.39 kg N ha-1 yr-2) is found near Kuala Lumpur
and is statistically significant.
We compare the long-term trends in these measurements with our
satellite-constrained trends in wet NO3- deposition. We find a
similar spatial gradient in North America, and the same magnitude of
declines through the northeastern US and southern Ontario (-0.12 to -0.16 kg N ha-1 yr-2). Over Europe, our estimates have low statistical
significance in the trends throughout much of this region, consistent with
the observations. Where we do see statistical significance (the northern United
Kingdom, southern Denmark, and in some central to eastern European countries),
observations are not available over the long-term for evaluation. In East
Asia, our satellite-constrained trend estimates show statistically
significant increases throughout much of the region (in contrast to most of
the available observations). The trend over Kuala Lumpur is significant and
positive (+0.24 kg N ha-1 yr-2), as expected from the available
measurements.
We again emphasize the small sample size in Europe (N= 16) and East Asia
(N= 11). Moreover, in many cases trends in one (or both) datasets are
small and/or insignificant. For these reasons, we focus on comparing the
confidence intervals of the measured and satellite-constrained trends. We
find that for 129 of the 136 gridded pairs (> 90 % of the
data), the 95 % confidence intervals overlap; of the pairs for which the
intervals do not overlap, 3 (out of 109) occur in North America, with 1 (out of
16) in Europe and 3 (out of 11) in East Asia. For a large majority of the
data in all three regions we therefore conclude that the satellite-derived
trends are not significantly different from the trends inferred with
ground-based measurements. Continued long-term measurements with better
spatial coverage are imperative to better evaluate long-term estimates of
global NOy deposition, especially throughout Europe and East Asia (but
also in other parts of the world where long-term coverage is not available
at all).
Changes in continental export of NOy
NOy deposition is a transboundary, and even intercontinental, issue
(HTAP, 2010). In a multimodel study, Sanderson et al. (2008)
found that between 3 and 10 % of NOx emissions from Europe, North
America, South Asia, and East Asia are ultimately deposited over foreign
regions. Long-range transport events of NO2 alone can be systematically
detected by satellite observations (Zien et al.,
2014). Here we extend such studies using our satellite-constrained long-term
estimates of annual NOy deposition to evaluate how the amount of
NOx exported from specific regions (i.e., the net balance between
emissions and deposition over a land area) has changed over the last 2 decades.
Decreases in NOy export over the Atlantic Ocean from North America and
increases in export over the western Pacific Ocean from East Asia are
evident in Fig. 5. We find that net export of NOx from North America
via atmospheric transport has decreased by more than 40 % (from 2.5 in 1996, to 1.4 Tg N yr-1 in 2014). In contrast, we find that
export of NOx from Asia increased by 40 %, from 3.3 Tg N in 1996 to a
maximum of 4.7 Tg N in 2011, with a subsequent decrease to 3.8 Tg N by 2014.
As a result of these opposing trends, total deposition to the global oceans
has changed remarkably little (25.0 Tg N yr-1 in 1996–1998 compared to
24.4 Tg N yr-1 in 2012–2014), but has experienced substantial regional
redistribution.
NOy export from North America has received considerable attention.
Urban plumes from the eastern US that are transported across the North
Atlantic for several days could still contain 20–50 ppb of reactive nitrogen
oxides (Neuman et al., 2006). A recent detailed GEOS-Chem study of nitrogen
deposition over the US estimated that net annual export of NOx was
around 38 % of NOx emissions (or 2.5 Tg N) for 2006–2008 (Zhang et al., 2012). We estimate a similar fraction of export from the continental US
using our observationally constrained simulation (34 % ± 2 % from
1996–2014), with a small decreasing trend from 35 % in 1996–1998 to 32 %
in 2012–2014. As a result of declining emissions we find that absolute
export from the continental US decreased by 50 % from 2.9 in 1996 to
1.5 Tg N in 2014. We find declines in NOy deposition across the
Atlantic Ocean, with small though statistically significant declines as far
downwind as southern Greenland. The decreases downwind of the continent are
clearest and most significant in the winter, spring, and fall (Fig. 5b, c,
and e), while the trends are more local in the summer (when the NOx
lifetime is short and when midlatitude wind speeds are weaker).
We similarly calculate the net imbalance between NOx emissions and
NOy deposition over western European countries and find a decrease of
almost 40 %, from 2.2 to 1.3 Tg N. We calculate mean export of
NOx emissions from western European countries to be 45 % ± 4 %, with a notable decreasing trend from 50 % in 1996–1998 to 40 % in
2012–2014. As a result, the decrease in net export is steeper than the
decrease in emissions from the region. As alluded to in Sect. 3.2, the
decrease in NOx export from some western European countries has likely
compensated for increases in emissions in some of the central to eastern
European countries, as well as in western Russia, where we find dry deposition has
significantly increased, but wet deposition has decreased or shows no
significant net trend.
Reactive nitrogen transport from Asia has previously been shown to
contribute to Ox production across the midlatitude Pacific, reaching as
far as the western coast of North America
(Walker
et al., 2010; Zhang et al., 2008), and major NOx transport events from
China can be indirectly observed by NO2 columns
(Lee et al., 2014). Our satellite-constrained
estimate predicts that export from China alone more than tripled from 1.0 Tg N in 1996 to a maximum of 3.5 Tg N in 2011, then decreased to 2.5 Tg N by
2014. We estimate that an average of 24 % ± 4 % emissions from
China are exported, varying over time from as little as 15 % of emissions
in 1998 to a maximum of 31 % of emissions in 2011 (with an overall
increasing trend). Zhao et al. (2017) used a higher resolution
(0.5∘ × 0.667∘) GEOS-Chem simulation and estimated that
36 % of China's NOx emissions over 2008–2012 are exported. We calculate an
export fraction of around 27 % for the same time period. The discrepancy
between the two estimates may be attributed to the coarser horizontal
resolution of our simulation (2∘ × 2.5∘), pointing to
important resolution-dependent effects in global simulations of deposition.
Other factors may include the use of different NOx emissions (our
satellite-constrained emissions indicate rapid change over this period of
time), and the treatment of adjacent oceans.
Zhao et al. (2015) used GEOS-Chem to explore nitrogen deposition to the
northwestern Pacific Ocean off the coast of China from 2008–2010. They
estimated total (wet + dry) NOy deposition of 6.9 and 3.1 kg N ha-1 yr-1 to the Yellow Sea and the South
China Sea, respectively. Our simulation predicts that NOy deposition to
the same regions of the Yellow Sea increased from 5.1 in 1996 to 9.5 kg N ha-1 yr-1 by 2012 and to the South
China Sea from 2.8 in 1996 to 4.3 kg N ha-1 yr-1 in 2011. Subsequent declines in the following years will hopefully
have encouraging implications for nitrogen availability and the incidence of
algal blooms in these regions (Hu et al., 2010).
Export of
pollution from China has been shown to influence deposition over Japan in
particular (e.g., Lin et al.,
2008). Using observations of wet nitrate deposition, Morino et al. (2011) report increases throughout Japan from 1989–2008,
and attribute this trend largely to transport from China. Likewise,
integrated NOy deposition over Japan increased (p < 0.01) in
our satellite-constrained estimate. In fact, we find that Japan transitioned
from a net “exporter” of NOy over 1996–2006 (emissions exceeded local
deposition by up to 24 %) to a net “importer” of NOy over 2007–2014
(local deposition exceeded emissions by up to 20 %). The increase in
deposition was dominated by statistically significant increases in wet
deposition in some parts of the country. We find the increase over Japan is
most uniform during the spring (Fig. 5, MAM), consistent with transport from
China being pronounced during the spring season (Tanimoto et
al., 2005). Nevertheless, the impacts of local NOx controls can also be
important. Dry deposition dominates the decline in annual NOy
deposition just west of Tokyo. Declines are seen throughout the southern
part of the country during both the summer and fall seasons (Fig. 5, JJA and
SON, September, October, and November). These results demonstrate the indirect relationship between local
emissions and local deposition of NOy for regions influenced by
atmospheric transport and also show how long-term trends can depend
strongly on the season and process (wet or dry deposition).
Sensitivity of NOy deposition to NH3 emissions
The transport and ultimate deposition of oxidized nitrogen may be tightly
coupled with the reduced nitrogen (NHx= NH3 + NH4+)
and sulfate systems, due to the formation of NH4NO3 aerosol that
becomes favorable once all H2SO4 has been neutralized (i.e., if
there is “excess” NH3). Examples of the resulting nonlinearity
between PM2.5 concentrations and precursor emissions have been noted in
the literature
(Banzhaf
et al., 2013; Derwent et al., 2009; Fowler et al., 2005). The formation of
NH4NO3 aerosol at the expense of HNO3 with changing excess
ammonia could therefore conceivably change the atmospheric lifetime of
NOy at the surface; accumulation-mode aerosol may have a dry deposition
lifetime of days, whereas HNO3 tends to have a dry deposition lifetime
of shorter than a day. As a result, the predicted footprint of source
impacts is sensitive to NH3 emissions
(Lee
et al., 2016).
Contemporary emissions of NH3 are highly uncertain
(Reis et al., 2009), so we perform a sensitivity
experiment by perturbing (increasing) all anthropogenic and natural NH3
emissions in the model by 25 % for the year 2012. Predicted NOy
deposition from this simulation is compared to the predicted NOy
deposition in the 2012 simulation where NH3 emissions were not
perturbed. Since we have not altered the emissions of oxidized nitrogen,
simple mass balance dictates that increases in deposition over some regions
will be countered by decreases elsewhere. Our perturbation is therefore to
be interpreted as an experiment that tests how accurately the spatial
pattern in NOy deposition at our model resolution can be predicted,
given some uncertainty in NH3 emissions. Given the horizontal
resolution of our simulation (2.5∘ × 2.0∘), we
acknowledge that our estimates of the sensitivity of NOy deposition to
perturbations in NH3 emissions may underestimate the importance of
those interactions on finer spatial scales.
Figure 8 shows the results of this experiment. The sensitivity of NOy
deposition to an increase in NH3 emissions is positive or negative
depending on the region, while net deposition over the global domain does
not change (to within 1–2 %). Over the continents, the sensitivity in
total (wet + dry) NOy deposition to the 25 % perturbation in
NH3 emissions tends to be less than ±5 %, with a few
exceptions. We find differences in NOy deposition on the order of
10 % over parts of high-latitude Russia, northwest and central Africa,
eastern China, southern South America, and Australia. However, with the
exception of China, these are also regions where deposition is relatively
low. We conclude that for most regions of interest, our
satellite-constrained estimates of NOy deposition over the continents
and their trends will not be severely impacted by uncertainty in the
NH3 inventories.
Sensitivity of simulated NOy deposition in 2012 to a 25 %
perturbation (increase) in ammonia emissions in all grid boxes (shown
separately for total deposition, dry deposition, and wet deposition).
Notably, the difference exceeds +50 % over Myanmar, suggesting that
simulated NOy deposition over this country is extremely sensitive to
changes in NH3 emissions. It is clear from Fig. 8 that this results
from a high sensitivity in dry deposition (middle panel) instead of wet
deposition (bottom panel). Myanmar has some of the lowest estimated NH3
emissions in all of South and East Asia (at least an order of magnitude
lower than surrounding India, China, and Thailand), so this sensitivity
reflects changes in the upwind emissions and subsequent transport of
NOy. We find the opposite sensitivity in nearby Cambodia, where the
sensitivity of dry NOy deposition to a 25 % perturbation is NH3
emissions is -50 %.
Over the oceans, the sensitivity of NOy deposition to the 25 %
increase in NH3 emissions is generally low (< ± 5 %),
with the expected exceptions in areas that are directly offshore from major
continental source regions. In the North Atlantic Ocean east of Canada and
Greenland, the North Pacific Ocean off the coasts of China, Japan, and
the South China Sea, the sensitivity of NOy deposition is between
5 and 20 %. Our predicted decrease in dry NOy deposition to the Yellow
Sea given an increase in NH3 emissions is consistent with previous
adjoint analyses showing increased NOy dry deposition in this region
with a decrease in Asian NH3 emissions (Zhao et al., 2015). Likewise,
the sensitivity of deposition to the Mediterranean Sea is between 10 and 20 %.
The differences in NOy deposition over the oceans results from
sensitivity in both dry and wet deposition (although in the case of the
Mediterranean it is dominated by dry deposition). We conclude that although
changes (or uncertainties) in NH3 emissions can impact the distance of
transport and deposition to oceans downwind of the major NOx sources,
the absolute magnitude of deposition is low where the sensitivity of
NOy deposition to NH3 is relatively high.
Other considerations
A number of other uncertainties are important in an inversion of satellite
NO2 columns to calculate surface NOx emissions and simulate
long-term NOy deposition. These can depend on, for example, the choice
of inversion approach, errors in the satellite retrieval, and uncertainties
in model processes (e.g., emissions, boundary layer mixing, chemical NOx
sinks, meteorology, and dry deposition).
Cooper et al. (2017) found that the finite mass-balance inversion approach
used here can be improved upon by using an iterative method that performs
with similar accuracy as a four-dimensional variational data assimilation.
Multiconstituent data assimilation also shows considerable promise for
constraints on surface NOx emissions (Myazaki et al., 2017). Satellite
retrieval algorithms continue to develop with advances that will improve the
accuracy of future estimates of satellite-constrained NOy deposition.
Uncertainties in model processes are also of interest. For example,
uncertainties in the chemical sink of NOx alone (e.g., the rate of
HNO3 formation, heterogeneous loss of N2O5 onto aerosol) can
have a substantial impact on top-down emissions estimates
(Stavrakou et al., 2013), suggesting more
fundamental work in constraining these processes is required. Lin et al. (2010) found that top-down NOx emissions estimates
over East Asia are sensitive to other model uncertainties, including
planetary boundary layer mixing scheme, lightning emissions, diurnal profile
of emissions, and a priori NOx, CO, and VOC emissions. Uncertainties in
model meteorology are also important. For example, the MERRA precipitation
fields used in our study are known to correlate weakly with observational
datasets (Rienecker et al.,
2011), but improvements can be expected from MERRA-2 due to the inclusion of
gauge- and satellite-based precipitation corrections
(Reichle et al., 2017). Finally,
dry deposition schemes are also highly variable among models
(Flechard et al., 2011;
Hardacre et al., 2015), and future work in dry deposition evaluation should
be a priority.
Nonetheless, despite these uncertainties we find a high degree of
consistency between observations and our predictions in the long-term
changes to deposition. Evidence continues to emerge about potential biases
in bottom-up inventories (e.g., Travis et al., 2016), and our observational
constraint on NOx emissions mitigates against such biases. We expect
that continued advancements in inversion approaches, satellite retrieval
algorithms, and fundamental atmospheric chemistry processes will allow for
increasingly accurate satellite-based constraints on deposition.
Conclusions
NOy deposition represents about half of the total reactive nitrogen
deposited to Earth's surface. Even in the US where nitrogen oxide emissions
have decreased substantially, constituents of NOy remain major
contributors to the nitrogen deposited in areas of concern (Lee et al., 2016;
Li et al., 2016). We applied NO2 observations from multiple satellites
over 1996–2014 together with the GEOS-Chem chemical transport model to
estimate long-term changes to reactive nitrogen oxide deposition around the
world. Given the effective global mass balance between NOx emissions
and deposition of reactive nitrogen oxides, we show that satellite
constraints on NOx emissions can provide a powerful top-down constraint
on deposition in order to evaluate long-term changes worldwide. Observations
from the GOME, SCIAMACHY, and GOME-2 satellite instruments have provided
continuous global coverage over the last 20 years, allowing observational
constraints on NOy deposition that enhance the poor spatial coverage of
ground-based deposition measurements.
We find substantial variability in regional trends of NOy deposition.
NOy deposition declined most steeply throughout the northeastern United
States at a rate of up to -0.6 kg N ha-1 yr-2, but has also
decreased significantly throughout most of the country and in southern
Canada. In Europe, statistically significant declines at a rate of up to
-0.1 kg N ha-1 yr-2 are seen over some western countries. NOy deposition has increased substantially throughout East
Asia, exceeding a rate of +0.6 kg N ha-1 yr-2 in some parts.
Since reductions in deposition over some regions were counteracted by
increases in others, global NOy deposition did not change considerably
over the long term. However, we find that global NOy deposition could
now be on the decline overall, since deposition in Asia peaked around
2010–2012. The ability to resolve the striking recent decline in NOy
deposition in China (despite prior projections of increasing NOx
emissions) demonstrates one of the attributes of using a satellite-based
constraint. Future observations will be important in evaluating whether this
trend persists.
We find that changes over the last 2 decades in the export of reactive
nitrogen oxides via atmospheric transport have impacted countries downwind
of source regions. Export from North America has decreased by at least
40 %, while export from Asia has increased by the same relative amount. We
find evidence that decreases in NOx export from some western European
countries have counteracted increases in local emissions from some
eastern and central European countries, resulting in negligible net change in
NOy deposition over the long term. Likewise, Japan is highly sensitive
to changes in export from China, but this depends strongly on the season and
whether wet and dry deposition are both considered. While uncertainty in
NH3 emissions can impact the footprint of NOy export and
deposition, we show that this sensitivity is small in most regions of
concern.
Direct measurements of deposition are sparse, inhibiting evaluation. This is
especially challenging for global simulations, where individual measurements
may not necessarily be regionally representative. Nevertheless, we find that
for the vast majority of locations our satellite-derived trends are largely
consistent with the observed trends. Expanded coverage of ground-based
observations over the long-term is needed to more comprehensively evaluate
long-term estimates of global NOy deposition. This need also motivates
the value of using alternative observational constraints such as the satellite
NO2 columns, as presented here.
Forthcoming satellite observations of NO2 at higher spatial resolution
(e.g., TROPOMI;
Veefkind
et al., 2012) and with diurnally varying observations (e.g., TEMPO;
Zoogman
et al., 2017, Sentinel-4, and GEMS) will offer increasingly robust
constraints on NOx emissions that affect NOy deposition. Satellite
observations of NH3 (e.g., Van Damme
et al., 2014) may offer additional opportunities to constrain the reactive
nitrogen budget. Higher resolution global modeling will also be an important
development to accurately account for nonlinear NO2 losses in global
emission inversions (Valin et al., 2011).
Our satellite-constrained estimates of NOy document interannual changes
over the past 2 decades worldwide. We expect that this information will be
useful in future research into the impacts of nitrogen deposition to
important biodiversity hotspots, in regions dealing with excessive nitrogen
inputs leading to algal blooms, or for estimating the changing impacts of
nitrogen deposition on global carbon uptake.