United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-91/070 Jan. 1992
EPA Project Summary
Tropospheric Nitrogen: The
Influence of Anthropogenic
Sources on Distributions and
Deposition
J.E. Penner, C.S. Atherton, and J.J. Walton
In this report, we describe our simu-
lations of the global cycle of reactive
nitrogen. We use a three-dimensional
chemistry-transport-deposition model.
Our model is a Lagrangian model in
which trace species react and are car-
ried in constant-mass air parcels; the
parcels are advected by winds calcu-
lated by the NCAR Community Climate
Model (CCM), a nine-layer general cir-
culation model. We describe the model,
together with our specification of the
reactive nitrogen sources. Predicted
concentrations and deposition amounts
are compared with measurements. The
report includes regional analysis of
sources and deposition.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to an-
nounce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
Nitrogen oxides are important for glo-
bal photochemistry because they deter-
mine whether O3 is produced or destroyed.
Elevated concentrations of O3 are of con-
cern because O3 affects the growth of plants
and the respiratory systems of man and
other animals, because it absorbs infrared
radiation and contributes to the greenhouse
warming, and because it is a key player in
the photochemistry of hydroxyl, the most
important chemical scavenger in the atmo-
sphere. In regions of high NOX concentra-
tions, a photochemical sequence, initiated
by the reaction of CO with OH, leads to O3
production. At low NOX concentrations, the
reaction of HO2 with NO, is slow. Instead,
HO2 reacts with O3 and the reaction se-
quence initiated by the reaction of CO with
OH leads to O3 destruction. For typical O3
production, this occurs when NOX (NO +
NO2) concentrations are around 20 to 50
ppt. Indeed, fossil fuel emissions of NOX
have increased substantially over the last
two decades. A further increase in these
sources could fundamentally alter the at-
mosphere by turning vast portions from
regions which are typically regions of net
O3 destruction to regions of net O3 produc-
tion.
In addition to its importance for O3 and
the photochemistry of the troposphere, re-
active nitrogen is also important because
NO3-, the end product of the reactive nitro-
gen cycle, is a key component of acid rain
and a key nutrient for both ocean phyto-
plankton and for land biota.
Model Description
The model we use is based on the
GRANTOUR model. The chemistry of re-
active nitrogen in the model has been sim-
plified. We treat reactive nitrogen as NOX
(NO + NO2) and HNO3. The ratio of NO to
NO2 is determined by the photostationary
state such that the reaction of NO with O3
is instantaneously balanced by photolysis
of NO2:
NO + O3 -> NO2 + O2
NO2+hv -» NO + O
Transformations between NOX and
HNO3 follow the reactions:
Printed on Recycled Paper
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NOj + OH -» HN03
HNOa + OH -» H2O + NOX + OX
HNOa+hv -» OH + NO2
Here, we have assumed that NO3 (which is
produced by the reaction HNO3 with OH) is
immediately photolized to produce either
NO or NOj,. The concentration of hydroxyl,
OH, is specified to vary with latitude and
height, according to the predicted con-
centrations in the LLNL two-dimensional
model. We also use photolysis rates from
the LLNL model, but our reaction rate co-
efficients, which vary with temperature and
pressure, have a longitudinal variation, as
well, according to the monthly-averaged
temperatures calculated in the Community
Climate Model (CCM).
Dry deposition velocities are applied to
the concentrations of NO, NO2, and HNO3
in the lowest 100 mb of the atmosphere
according to:
vd(NO) - 0.05 cm/s
vd(NOj,) - 0.25 cm/s
vd(HNO3) - 0.5 cm/s
which are chosen as representative mean
values. Precipitation scavenging of HNO3
is assumed to be proportional to the pre-
cipitation rate in the CCM.
The sources of reactive nitrogen in our
model are specified as emission rates (Mt
N/yr) as follows: fossil-fuel combustion,
22.4; lightning discharges, 3.0; soil micro-
bial activity, 10.0; production in strato-
sphere, 1.0; and biomass burning, 5.8.
Fossil fuel is by far the largest source of
NO,, and its dominant contribution to the
total of all sources is seen in the maximum
contours over North America, Europe, and
Japan and eastern Asia. The biomass burn-
Ing source is a major contributor in the
tropics and Southern Hemisphere, while
soil microbial sources shift in importance
from the Southern Hemisphere in January
to the Northern Hemisphere in July.
Comparison to Measurements
We have attempted to validate our
model by comparison of the predicted con-
centrations and wet deposition values with
measured quantities. The model does well
in predicting the removal of nitrate by pre-
cipitation in the United States, especially in
summer. In Europe, the predicted rainout
is too small, especially in January, perhaps
because the winds in the CCM are too
high and the model placement of precipita-
tion fields is incorrect relative to the ob-
served fields and the emissions. The
comparison of the predicted and observed
wet deposition at remote areas confirms
that, overall, the transport of nitrate to re-
mote regions is relatively well represented.
The predicted values of HNO3 in remote
areas are in reasonable agreement with
the measured concentrations, but those
over continental areas are somewhat high.
In contrast to the comparison of our
predictions with the data of Huebert and
Lazrus, where we tend to be somewhat
high, the predicted concentrations from the
model appear to be low relative to the
surface concentrations of nitrate measured
during the SEAREX program. Savoie mea-
sured nitrate using a high-volume sam-
pling system, and the measurements refer
to total inorganic nitrate (i.e., particulate
nitrate plus gaseous HNO3), but it is be-
lieved that much of the nitrate is present in
the particulate form.
Global Deposition
Wet Deposition
Maps of wet deposition of HNO3 in both
January and July are shown for the all-
sources scenario. The peak continental
wet HNO3 deposition magnitudes are com-
parable for many regions. For example,
South America and southern Africa are
both enveloped by the 3 kg N knv2 contour,
and have peak contours of 15 kg N knrv2.
Likewise, in the Northern Hemisphere the
3 kg N knv2 contour encircles the eastern
U.S., much of Europe, and a region from
east Asia across the north Pacific Ocean.
Peak values are 15,5, and 10 kg N knv1 for
the U.S., Europe, and east Asia, respec-
tively. Additionally, a 1 kg N knv1 contour
covers most of the oceans north of 20° S.
Although the precipitation in the Northern
and Southern Hemispheres contains com-
parable HNO3 levels, different sources are
responsible. All sources include biomass
burning.
Conclusions about wet HNO3 deposi-
tion on a regional basis also hold on a
hemispheric scale because within each
hemisphere most regions have similar fuel
use and vegetation patterns. In the North-
ern Hemisphere, fossil fuel sources ac-
count for roughly 51-53% of wet HNO3
deposition and natural sources for 34-38%.
In the Southern Hemisphere, 60-63% of
the wet HNO3 in precipitation arises from
natural sources.
Dry Deposition
The dry deposition of both HNO3 and
NO, can be important. The dry deposition
of HNO3 is generally comparable to its wet
deposition. Additionally, NOX dry deposi-
tion can account for 18-44% of total depo-
sition for the Northern and Southern
Hemispheres. The deposition amounts are
also functions of season. The relative im-
portance of NOX. dry deposition increases
in the winter, when the conversion of NOX
to HNO3 is slower and more NOX is sent.
These calculations, of course, are based
on a very simple model of dry deposition
and on highly uncertain dry deposition ve-
locities. The results may be subject to
change as more information becomes avail-
able in the future.
Regional Analysis of Sources
and Deposition
All regions have the same order of
magnitude of emissions, although the
sources responsible vary. Thus, the domi-
nant sources for Northern Hemisphere re-
gions (U.S., Canada, Europe, China) are
fossil fuel combustion, followed by soil
emissions. Conversely, for the equatorial
and Southern Hemisphere continents of
South America and Africa, the primary NOX
sources are soil emissions and biomass
burning.
The deposition results also reflect the
findings discussed earlier for the Northern
and Southern Hemispheres. All three types
of deposition (dry NOX, wet HNO3, dry
HNO3) contribute significantly to total depo-
sition. In January, the Northern Hemisphere
regions see relatively strong NO, dry depo-
sition, due to the slow wintertime conver-
sion of NOX to HNO3. Conversely, the
Southern Hemisphere regions of South
America and Africa, see substantial depo-
sition (wet and dry) of HNO3 in this sum-
mer month.
The situation differs slightly in July. In
the Northern Hemisphere, more NOX is
converted to HNO3. Thus, NOX dry deposi-
tion is low, while HNO3 wet and dry deposi-
tion are relatively higher. For the Southern
Hemisphere regions in South America and
Africa in July (winter), more NOX is present.
Consequently, NOX dry deposition is rela-
tively larger (especially for South America,
which is almost entirely contained in the
Southern Hemisphere).
Three other oceanic regions—the south
Atlantic, Indian, and south Pacific Oceans
receive less total nitrogen deposition than
the Northern Hemisphere oceans, and a
relatively smaller fraction is due to anthro-
pogenic nitrogen sources. This is expected
because these three oceans lie in the
Southern Hemisphere, where the largest
nitrogen source is natural: soil emissions.
However, the fraction from anthropogenic
sources is still surprisingly large, ranging
from 19-42%.
Man's activities contribute a significant
fraction of the amount of oceanic nitrogen
deposition. In the Northern Hemisphere,
anthropogenic sources of reactive nitro-
gen from the burning of fossil fuel and
biomass may account for 53-80% of the
nitrogen deposited to oceanic surfaces. In
the Southern Hemisphere, these anthro-
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pogenic sources contribute roughly 19-51%
of the total nitrogen deposition to oceans.
Conclusions
We have shown that we are able to
successfully simulate many of the observed
features of the reactive nitrogen cycle in
the troposphere. Many significant prob-
lems remain to be addressed. In particular,
there is a need to quantify the role of
anthropogenic sources of NOX in the global
budget of O3. The spatial extent of the
contribution of anthropogenic sources of
NOX to surface concentrations is unex-
pected. In the future, we hope to extend
our model in order to quantify the role of
anthropogenic sources of NOX in the global
budget of tropospheric O3.
Acknowledgments
This work was supported by the U.S.
EPA under Interagency Agreement
DW89932676-01-0 and by the Institutional
Research and Development Program of
the Lawrence Livermore National Labora-
tory. Computer time was supplied by the
D.O.E. Office of Health and Energy Re-
search. Lawrence Livermore National Labo-
ratory is operated by the University of
California under contract number W-7405-
Eng-48 with the U.S. Department of En-
ergy.
&U.S. GOVERNMENT PRINTING OFFICE: 1992-€48-080/40127
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J.E. Penner, C.S. Atherton, andJJ. Walton are with Lawrence Livermore National
Laboratory, University of California, Livermore, CA 94550.
Joseph P. Pinto is the EPA Project Officer, (see below).
The complete report, entitled "Tropospheric Nitrogen: The Influence of Anthropogenic
Sources on Distributions and Deposition," (Order No. PB92-126937/AS; Cost: $26.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield,VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
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