Executive Summary
Integrated Science Assessment
Oxides of Nitrogen and Sulfur
Ecological Criteria
December 2008 ¦ EPA/600/R-08/082F
U.S. Environmental Protection Agency | Office of Research and Development | National Center for Environmental Assessment
http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=201485
Introduction
his Integrated Science Assessment
(ISA) is a synthesis and evaluation of
M the most policy-relevant science
that forms the scientific foundation for the
review of the secondary (welfare-based)
national ambient air quality standards
(NAAGS) for oxides of nitrogen (NOx) and
oxides of sulfur (SOx). The Clean Air Act defi-
nition of welfare effects includes, but is not
limited to, effects on soils, water, wildlife,
vegetation, visibility, weather, and climate,
as well as effects on man-made materials,
economic values, and personal comfort
and well-being. The current secondary
NAAQS for SOx, set in 1973, is a 3-hour aver-
age of 0.5 ppm1 sulfur dioxide (SO2), not to
be exceeded
more than
once per year.
The secondary
NAAQS for
NOx is identi-
cal to the pri-
mary standard
set in 19711 an
annual average not to exceed 0.053 ppm
nitrogen dioxide (NO2). The current secon-
dary NAAQS were set to protect against di-
rect damage to vegetation by exposure to
gas-phase SOx or NOx.
Scope
7
his ISA is focused on ecological
effects resulting from current deposi-
tion of compounds containing nitro-
gen (N) and sulfur (S). Acidification, N nutri-
ent enrichment and effects of sulfate (SO42
on methylation of mercury (Hg) are high-
lighted in the document. The following fig-
ure illustrates the scope of the document.
1 ppm= 1000 ppb
Atmospheric cycle
of N oxides
See figure 2-16
Ambient Air
Concentration
Sunlight
Dissolution
2H+ +SO42-
~ H++N03
Atmospheric cycle
of S compounds
See figure 2-18
Oxidation
S02	H2SO4
NO*	~HNOa
Wet Deposition
H*. NH4+, NO3-, S042
t/li
VOC
[
Foliar and
nutrient effects
Dry deposition
NOx, NHX, SOx
Deposition
C and N cycle in
terrestrial ecosystem
See figure 3-36
Acidification of water + Eutrophication
Hg cycle in
aquatic ecosystem
See figure 3-59
Ecological
Effect
C, N and P cycle in
aquatic ecosystem
See figure 3-40

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ISA for Oxides of Nitrogen arid Sulfur
Executive Summary
Both N and S contribute to acidification of
ecosystems. This ISA considers several
chemical forms that contribute to acidifying
deposition, including gases and particles
derived from SOx, NOx, and reduced nitro-
gen (NHx).
Deposition of N contributes to N-nutrient en-
richment and eutrophication. An assess-
ment of the complex ecological effects of
atmospheric N deposition requires consid-
eration of many different chemical forms of
reactive N (Nr). For this reason, the ISA in-
cludes evaluation of data on the most
common reduced inorganic forms of N,
ammonia (NHa) and ammonium (NH/); on
oxidized inorganic forms including nitric ox-
ide (NO) and N02> nitrate (N03~), nitric acid
(HNO3), and nitrous oxide (N2O); and on or-
ganic N compounds including peroxyacetyl
nitrate (PAN).
Other welfare effects addressed in the ISA
include effects of SO42" deposition on Hg
methylation, along with evidence related to
direct exposure to gas-phase NOx and SOx.
The key conclusions of the ISA follow.

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ISA for Oxides of Nitrogen arid Sulfur
Executive Summary
Current concentrations and deposition
mbient annual NOx and SOx con-
centrations as reported in
the routine national networks have
decreased substantially owing to controls
enacted since the 1970s. NOx decreased
-35% in the period 1990-2005, to current an-
nual average concentrations of -15 ppb.
Emissions of SOx have been substantially re-
duced in recent years: annual average
ambient SOx concentrations have de-
creased -50% in the period 1990-2005 and
now stand at -4 ppb for both aggregate
annual and 24-hour average concentra-
tions nation-wide.
Emitted NOx, SOx, NHx and other pollutants
can be transported vertically by convection
into the upper part of the mixed layer on
one day, then transported overnight in a
layer of high concentrations. Once pollut-
ants are lofted to the middle and upper tro-
posphere, they typically have a much
longer lifetime and, with the generally
stronger winds at these altitudes, can be
transported long distances from their source
regions. The length scale of this transport is
highly variable owing to differing chemical
and meteorological conditions encoun-
tered along the transport path.

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ISA for Oxides of Nitrogen anci Ann.
Numerical chemical-transport models
(CTMs) are the prime tools for computing
emissions and interactions among pollutants
like NOx, SOx, and NHx, their transport and
transformation including production of sec-
ondary aerosols like ammonium nitrate and
ammonium sulfate, the evolution of particle
size distributions, the resulting atmospheric
concentrations and the deposition of these
pollutants to the surface. CTMs are driven by
calculated emissions for primary species
such as NOx, SOx, NH3, and primary particu-
late matter, and by the meteorological
fields produced by other numerical predic-
tion models. As such, CTMs are the chief
means of relating emitted pollutants with
deposited ones.
The emitted, transported, and transformed
pollutants reach the surface where they can
have ecological effects largely through
deposition. Direct and indirect wet and dry
deposition to specific locations like water-
sheds depend on air pollutant emissions and
concentrations in the airshed above the wa-
tershed, but the shape and areal extent of
the airshed is quite different from that of the
watershed owing to the transport and trans-
formation of emitted pollutants described
above.
Deposition is spatially heterogeneous across
the U.S. In the years 2004-2006, routine na-
tional monitoring networks reported mean S
deposition in the U.S. highest east of the Mis-
sissippi River with the highest reported depo-
sition, 21 kg S/ha/yr, in the Ohio River Valley
where most recording stations reported
three-year averages for this period of more
than 10 kg S/ha/yr. Numerous other stations
in the eastern U.S. reported S deposition
greater than 5 kg S/ha/yr. Data are sparse
for the central U.S. between the 100th me-
ridian and the Mississippi River; but, where
available, deposition values there were
lower than in most of the eastern U.S., rang-
ing from 4 to over 5 kg S/ha/yr. Total S depo-
sition in the U.S. west of the 100th meridian is
lower than in the East or upper Midwest, ow-
ing to lower densities of high-emitting
sources in the West. In the years 2004-2006,
all routine recording stations in the West re-
ported less than 2 kg S/ha/yr and many re-
ported less than 1 kg S/ha/yr. S was primarily
deposited in the form of wet SO42-, followed
by a smaller proportion of dry SO2, and a
much smaller proportion of dry SO42-.
Expanding urbanization, agricultural intensi-
fication, and industrial production during the
previous 100 years have produced a nearly
10-fold increase in total N deposited from
the atmosphere compared to pre-industrial
levels. NOx, chiefly from fossil fuel combus-
tion, often dominates total N pollution in the
U.S. and comprises from 50 to 75% of current
4

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ISA for Oxides of Nitrogen anci Ann.
total N atmospheric deposition. This wet and
dry atmospheric N deposition is spatially
heterogeneous, too, owing to the influence
of meteorology, transport, precipitation pat-
terns and land use.
For 2004-2006, routine national monitoring
networks reported the highest mean N
deposition totals in the U.S. in the Ohio River
Valley, specifically in the states of Indiana
and Ohio, with values greater than 9 kg
N/ha/yr. N deposition was lower in other
parts of the East, including the Southeast
and in northern New England. In the central
U.S., the highest N annual average deposi-
tion totals were on the order of 6 to 7 kg
N/ha/yr. Measured concentrations and in-
ferred deposition totals were dominated by
wet NOs" and NH/ species, followed by
dry HNO3, dry NH/, and dry N03~. NH3 is not
yet measured routinely in any national net-
works; however, smaller-scale intensive
monitoring and numerical air quality model-
ing both indicate that it may account for
more than 80% of the dry reduced N deposi-
tion total.
The thin coverage of monitoring sites in
many locations, especially in the rural West,
means that limited data exist on deposition
totals in a large number of potentially sensi-
tive places. Numerical modeling experi-
ments can help fill in these data gaps and
suggest that local and even regional areas
of high ambient concentration and deposi-
tion exist where measured data are un-
available. Model-predicted values for N
deposition in some regions of the Adiron-
dacks in New York are greater than 20 kg
N/ha/yr; other model estimates as high as
32 kg N/ha/yr have been made for a region
of southern California, where more than half
of that total was predicted to come from
NO and NO2.
The ISA concludes that the national-scale
networks routinely monitoring N deposition
are inadequate to characterize both the full
range of reduced and oxidized forms of N
deposition and the substantial regional het-
erogeneity across the landscape of the U.S.
Although S and N deposition in most areas
of the U.S. occurred as wet deposition,
there were some exceptions, including parts
of California where N deposition was primar-
ily dry.
5

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ISA for Oxides of Nitrogen and Sulfur
Executive Summary
Ecological effects of acidification
~~ W he effects of acidifying deposition
m on ecosystems have been well
§ studied over the past several dec-
ades and vulnerable areas have been iden-
tified in the U.S. The wealth of data has led
to the development of widely used ecologi-
cal models for predicting soil and surface
water acidification. Regional and ecosys-
tem vulnerability to acidification results from
inherent sensitivity and exposure to acidify-
ing deposition.
Sensitivity of terrestrial and aquatic ecosys-
tems to acidification from S and N deposi-
tion is regional and predominantly governed
by surficial geology. Other factors contribut-
ing to the sensitivity of soils and surface wa-
ters to acidifying deposition include topog-
raphy, vegetation, soil chemistry, land use,
and hydrologic flowpath.
most sensitive to terrestrial effects from acidi-
fying deposition include forests in the Adi-
rondack Mountains of New York, the Green
Mountains of Vermont, the White Mountains
of New Hampshire, the Allegheny Plateau of
Pennsylvania, and high-elevation forest
ecosystems in the central and southern Ap-
palachians. While studies show some recov-
ery of surface waters, there are widespread
areas of ongoing depletion of exchange-
able base cations in forest soils in the north-
eastern U.S., despite recent decreases in
acidifying deposition.
In aquatic systems, consistent and coherent
evidence from multiple studies of many
species shows that acidification can cause
the loss of acid-sensitive species, and that
more species are lost with greater acidifica-
tion. These effects are linked to changes in
Soil acidification is a natural process,
but is often accelerated by acidifying
deposition, which can decrease con-
centrations of exchangeable base
cations in soils. Biological effects of
acidification on terrestrial ecosystems
are generally attributable to Al toxic-
ity and decreased ability of plant
roots to take up base cations. Areas
The evidence is sufficient to infer a causal
relationship between acidifying deposition
and effects on:
(1)	biogeochemistry related to terrestrial and
aquatic ecosystems;
(2)	biota in terrestrial and aquatic ecosystems.
6

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ISA for Oxides of Nitrogen and Sulfur
Executive Summary
surface water chemistry, including concen-
trations of SO42", NOs", inorganic Al, and cal-
cium (Ca2+), surface water pH, sum of base
cations, acid neutralizing capacity (ANC),
vertebrates, and fish species richness. These
effects on species richness may also affect
ecosystem services, such as biodiversity and
cultural services such as fishing and tourism.
Although both N and S deposition can
cause terrestrial and aquatic acidification, S
deposition is generally the primary cause of
chronic acidification, with secondary con-
tributions from N deposition. Following de-
creases in S deposition in the 1980s and
1990s, one quarter to one third of the
chronically acidic lakes and streams in the
U.S. were no longer acidic during baseflow
in the year 2000. A number of lakes and
streams, however, remain acidic even
though wet SO42- deposition has decreased
by as much as 30% since 1989. N deposition,
which has also decreased in the years since
1990 in most places in the U.S. with routine
monitoring, is the primary cause of episodic
acidification which, despite its short duration,
has been shown to cause long-term bio-
logical effects.
Many of the surface waters most sensitive to
acidification in the U.S. are found in the
Examples of biogeochemical indicators of
effects from acidifying deposition on ecosystems
Ecosystem
Biogeochemical Indicator
Terrestrial
•	Soil base saturation
•	Inorganic Aluminum concentration
in soil water
•	Soil carbon-to-nitrogen ratio
Aquatic
•	Sulfate
•	Nitrate
•	Base cations
•	Acid neutralizing capacity
•	Surface water inorganic Aluminum
•	pH

Examples of biological indicators of
effects from acidifying deposition on ecosystems
Indicator
Measure
Terrestrial Ecosystems
Red Spruce
Sugar
Maple
•	Percent dieback of canopy
trees
•	Dead basal area, crown
vigor index, fine twig die-
back
Aquatic Ecosystems
Fishes, zoo- • Presence/absence
plankton, crus- • Fish condition factor
taceans, rotifers • Biodiversity	
and base cation surplus. These effects are
also influenced by historical inputs to these
systems. Decreases in ANC and pH and in-
creases in inorganic Al concentration con-
tribute to declines in zooplankton, macroin-
7

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ISA for Oxides of Nitrogen arid Sulfur
Executive Summary
—i	1	1	1	\	1—
-200 -100	0 100 200 300 400 500
ANC(|ieq/L)
Number of fish species per lake vs. acidity status, expressed
as acid neutralizing capacity (ANC), for Adirondack lakes.
Source: Suliivcin et nL 9006b
Adirondack
Mountains.
Northeast, the Southeast, and the moun-
tainous West. In the West, acidic surface
waters are rare and the extent of chronic
surface water acidification that has oc-
curred to date has been limited. However,
episodic acidification does occur. In both
the mountainous West and the Northeast,
the most severe acidification of surface wa-
ters generally occurs during spring snowmelt.
The ISA highlights evidence from two well-
studied areas to provide more detail on
how acidification affects ecosystems: The
Adirondacks (NY) and Shenandoah Na-
tional Park (VA). In the Adirondacks, the cur-
rent rates of N and S deposition exceed the
amount that would allow recovery of the
most acid sensitive lakes. In the Shenan-
doah, past SO# has accumulated in the soil
and is slowly released from the soil into
stream water where it causes acidification,
making parts of this
region sensitive to even
the current lower
deposition loadings.
Numerical models spe-
cifically calibrated to
these locations and
conditions suggest that
the number of acidic
streams will increase
even under current
deposition loads.
Regions of the northern and eastern U.S. that contain appreciable numbers of lakes
and streams sensitive to deleterious effects from acidifying deposition.
Source: Stoddard et ai., 2003
8

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ISA for Oxides of Nitrogen and Sulfur
Executive Summary
Ecological effects of nitrogen
rhere are many well-studied effects
of N deposition on ecosystems and
some vulnerable areas have been
identified in the U.S. However, the full extent
of ecosystem vulnerability is still unknown.
Substantial empirical information from spe-
cific ecosystems and for specific endpoints
is available, but given the complexity of the
N cycle, a broadly applicable and well-
tested predictive model of the ecological
effects of N deposition is not yet available.
Though the sensitivity of ecosystems to N
deposition
deposition across the U.S. varies, a large
body of evidence clearly demonstrates a
relationship between N deposition and a
broad range of ecological effects.
The contribution of N deposition to total N
load varies among ecosystems. Atmos-
pheric N deposition is the main source of
new N to most headwater streams, high
elevation lakes, and low-order streams. At-
mospheric N deposition contributes to the
total N load in terrestrial, wetland, freshwa-
ter, and estuarine ecosystems that receive N
through multiple pathways (i.e. biological N-
fixation, agricultural land runoff and waste
water effluent). There are multiple biogeo-
chemical indicators of N deposition effects.
Examples of biogeochemical indicators of
effects from reactive N deposition on ecosystems
Ecosystem
Biogeochemical Indicator

• NO3 leaching

• Nitrification

• Denitrification
Terrestrial
• N2O emissions
and
• CH4 emissions
Wetland
• Soil C:N ratio

• Foliar / plant tissue [N], C:N,

N:magnesium, N:phosphorus

• Soil water [NO3 ] and [NH4 ]

• Chlorophyll a
Freshwater
• Water [NOs ]
and
• Dissolved inorganic N
Estuarine
• Dissolved oxygen

• N:P
The evidence is sufficient to infer a causal rela-
tionship between N deposition, to which NOx
and NHx contribute, and the alteration of the
following:
(1)	biogeochemical cycling of N and carbon
(C) in terrestrial, wetland, freshwater aquatic,
and coastal marine ecosystems;
(2)	biogenic flux of methane (CH4), and N2O in
terrestrial and wetland ecosystems;
(3)	species richness, species composition, and
biodiversity in terrestrial, wetland, freshwater
aquatic and coastal marine ecosystems.
9

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ISA for Oxides of Nitrogen and Sulfur
Executive Summary
In terrestrial ecosystems, the onset of NC>3~
leaching is one of the best documented
biogeochemical indicators that an ecosys-
tem receives more N than it uses and is able
to retain. N removal by ecosystems is a
valuable ecosystem service regulating wa-
ter quality. When atmospheric deposition of
N impairs the ability of terrestrial and
aquatic ecosystems to retain and remove
N, NOs" leaching occurs and the degrada-
tion of water quality can occur. The onset of
leaching was calculated to occur with
deposition levels between 5.5 and 10 kg
N/ha/yr for sensitive eastern forests. In the
mixed conifer forests of the Sierra Nevada
and San Bernardino mountains, the onset of
increased NC>3~ leaching was calculated to
be 17 kg N/ha/yr. Several studies in the
Rocky Mountains indicate that the capacity
of alpine catchments to retain N was ex-
ceeded at levels greater than 5-10 kg
N/ha/yr.
N deposition alters the biogenic sources and
sinks of two greenhouse gases (GHGs), ChU
and N2O, in terrestrial and wetland ecosys-
tems, resulting in increased emissions to the
atmosphere. Non-flooded upland soil is the
largest biological sink and takes up about
6% of atmospheric CH4. N addition de-
creases CH4 uptake in coniferous and de-
ciduous forests, and N addition increases
CH4 production in wetlands. Soil is the larg-
est source of N2O, accounting for 60% of
global emissions. N deposition increases the
biogenic emission of N2O in coniferous for-
est, deciduous forests, grasslands, and wet-
lands. Although N addition can cause a
general stimulation of biogenic CH4 and
N2O emissions from soils, it is difficult to gen-
eralize a dose-response relationship be-
tween the amount of N addition and the
changes in GHG flux on a large heteroge-
neous landscape. This is because GHG pro-
duction is influenced by multiple environ-
mental factors (e.g., soil, vegetation and
climate), which vary greatly over small spa-
tial and temporal scales.
N is often the most limiting nutrient to growth
in ecosystems. N deposition thus often in-
creases primary productivity, thereby alter-
ing the biogeochemical cycling of C. N
Examples of biological indicators of
effects from N deposition on ecosystems
Ecosystem

Biological Indicators
Terrestrial and
•
Altered community composi-
Wetlands

tion, biodiversity and/or popu-


lation decline. Taxa affected


include: diatoms, lichen, my-


corrhizae, moss, grasses and


other herbaceous plants

•
Plant root: shoot ratio

•
Terrestrial plant bio-


mass/production
Freshwater
•
Phytoplankton bio-
and Estuarine

mass/production

•
Toxic or nuisance algae


blooms

•
Submerged aquatic vegetation

•
Fauna from higher trophic lev-


els
10

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ISA for Oxides of Nitrogen arid Sulfur
Executive Summary
deposition can cause changes in ecosys-
tem C budgets. However, whether N depo-
sition increases or decreases ecosystem C-
sequestration remains unclear. A limited
number of studies suggest that N deposition
may increase C-sequestration in some for-
ests, but has no apparent effect on C-
sequestration in non-forest ecosystems.
In terrestrial ecosystems, N deposition can
accelerate plant growth and change C al-
location patterns (e.g. shoot:roof ratio),
which can increase susceptibility to severe
fires, drought, and wind damage. These ef-
fects have been shown in studies con-
ducted in the western U.S. and Europe. The
alteration of primary productivity can also
alter competitive interactions among plant
species. The increase in growth is greater for
some species than others, leading to possi-
ble shifts in population dynamics, species
composition, community structure, and in
few instances, ecosystem type.
There are numerous sensitive terrestrial biota
and ecosystems that are affected by N
deposition. Acidophytic lichens are among
the most sensitive terrestrial taxa to N depo-
sition, with adverse effects occurring with
exposures as low as 3 kg N/ha/yr in the
Pacific Northwest and southern California.
The onset of declining biodiversity in grass-
lands has been estimated to be 5 kg
N/ha/yr in Minnesota and the European Un-
ion. Altered community composition of al-
pine ecosystems in the Rocky Mountains
and forest encroachment into temperate
grasslands in Southern Canada is estimated
to be 10 kg N/ha/yr.
The productivity of many freshwater ecosys-
tems is N-limited. N deposition can alter
species assemblages and cause eutropni-
cation of aquatic ecosystems to the extent
that N is the growth-limiting nutrient. In the
Rocky Mountains, deposition loads of ap-
proximately 1.5-2 kg N/ha/yr are reported to
alter species composition in the diatom
communities in some freshwater lakes, an
indicator of impaired water quality.
11

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ISA for Oxides of Nitrogen and Sulfur
Executive Summary
In estuarine ecosystems, N from atmospheric
and non-atmospheric sources contributes to
increased phytoplankton and algal produc-
tivity, leading to eutrophication. Estuary eu-
trophication is an ecological problem indi-
cated by water quality deterioration,
resulting in numerous adverse effects includ-
ing hypoxic zones, species mortality, and
harmful algal blooms. The calculated con-
tribution of atmospheric deposition to total
N loads can be as high as 72% in estuaries.
The Chesapeake Bay is an example of a
large, well-studied, and severely eutrophic
estuary that is calculated to receive as
much as 30% of its total N load from the at-
mosphere.
Examples of quantified relationships between
deposition levels and ecological effects
Kg
N/ha/yr
Ecological effect
Altered diatom communities in
high elevation freshwater lakes
~1.5	and elevated N in tree leaf tissue
high elevation forests in the west-
ern U.S.
3.1
Decline of some lichen species in
the western U.S.	
Altered growth and coverage of
alpine plant species in the western
U.S.	
Onset of decline of species rich-
ness in grasslands of the U.S. and
U.K.	
Onset of nitrate leaching in Eastern
forests of the U.S.	
Multiple effects in tundra, bogs
and freshwater lakes in Europe
Multiple effects in arctic, alpine,
5-15	subalpine and scrub habitats in
Europe
5.5 - 10
5-10
Other welfare effects:
Mercury methylation
Hg is highly neurotoxic and once methy-
lated, principally by S-reducing bacteria, it
can be taken up by microorganisms, zoo-
plankton and macroinvertebrates, and
concentrated in higher trophic levels, in-
cluding fish eaten by humans. In 2006, 3,080
fish consumption advisories were issued be-
cause of methylmercury (MeHg), and as of
July 2007, 23 states had issued statewide
advisories. The production of meaningful
amounts of MeHg requires the presence of
S042- and Hg, and where Hg is present, in-
creased availability of SCU2- results in in-
creased
production
of MeHg.
The
amount of
MeHg
produced
varies with
oxygen content, temperature, pH, and sup-
ply of labile organic C. Watersheds with
conditions known to be conducive to Hg
methylation can be found in the northeast-
ern U.S. and southeastern Canada, but bi-
otic Hg accumulation has been widely ob-
served in other regions that have not been
studied as extensively, and where a different
set of conditions may exist.
The evidence is sufficient to
infer a causal relationship be-
tween S deposition and in-
creased Hg methylation in
wetlands and aquatic envi-
ronments.
12

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ISA for Oxides of Nitrogen and Sulfur
Executive Summary
Other welfare effects:
Direct phytotoxic
Acute and chronic exposures to SO2 have
phytotoxic effects on vegetation which in-
clude foliar injury, decreased photosynthe-
sis, and decreased growth. Acute exposures
to NO2, NO, PAN, and HNO3 cause plant
foliar injury and decreased growth. How-
ever, the majority of studies have been per-
formed at concentrations of these gas-
phase species above current ambient con-
ditions observed in the U.S. Consequently,
there is little evidence that current concen-
Conclusion
The main effects of N and S pollution assessed in the ISA are acidification, N en-
richment, and Hg methylation. Acidification of ecosystems is driven primarily by
deposition resulting from SOx, NOx, and NHx pollution. Acidification from the
deposition resulting from current emission levels causes a cascade of effects
that harm susceptible aquatic and terrestrial ecosystems, including slower
growth and injury to forests and localized extinction of fishes and other aquatic
species. In addition to acidification, atmospheric deposition of reactive N re-
sulting from current NOx and NHx emissions along with other non-atmospheric
sources (e.g., fertilizers and wastewater), causes a suite of ecological changes
within sensitive ecosystems. These include increased primary productivity in
most N-limited ecosystems, biodiversity losses, changes in C cycling, and eutro-
phication and harmful algal blooms in freshwater, estuarine, and ocean eco-
systems. In some watersheds, additional SO42" from atmospheric deposition in-
creases Hg methylation rates by increasing both the number and activity of S-
reducing bacteria. Methylmercury is a powerful toxin that can bioaccumulate
to toxic amounts in food webs at higher trophic levels (e.g. bass, perch, otters,
or kingfishers).
13
trations of gas-phase S or N oxides are high
enough to cause phytotoxic effects. One
excep-
tion is
that
some
studies
indicate
that cur-
rent
HNO3 concentrations may be contributing
to the decline in lichen species in the Los
Angeles basin.
The evidence is sufficient to
infer a causal relationship
between exposure to SO2,
NO, NO2, PAN, and HNOs
and injury to vegetation.

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