&EPA
Offici
1 DC
. .J79
f-001
A Preliminary
Analysis of
Nitrous Oxide (N2O)
Including A
Materials Balance
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This document is available through the National
Technical Information Service (NTIS), Soringfield, VA,
22161, telephone t (703) 557-4650.
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EPA-560/6-79-001
A PRELIMINARY ANALYSIS OF
NITROUS OXIDE(N20)
INCLUDING A MATERIALS BALANCE
JANUARY 1979
IN HOUSE REPORT
US Environmental Protection Agency
Office of Toxic Substances
401 M St Sw
Washington DC 20460
Dr C Richard Cothern
Project Officer
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This report has been reviewed by the Office
of Toxic Substances, EPA, and approved for
publication. Approval does not signify that
the contents necessarily reflect the views
and policies of the Environmental Protection
Agency, nor does mention of trade names or
commercial products constitute endorsement
or recommendation for use.
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TABLE OF CONTENTS
EXECUTIVE SUMMARY
I . INTRODUCTION
II . TROPOSPHERIC NO
X
III . STRATOSPHERIC NO
X
A. INTRODUCTION
B . ATMOSPHERIC CONCENTRATION
AND RESIDENCE TIME
C. SOURCES OF N20
1 . LAND
a . REVIEW
b . ANALYSIS
2 . FRESHWATER & SEDIMENTS
a . REVIEW
b . ANALYSIS
3 . OCEANS
4 . FERTILIZERS
a . REVIEW
b. ANALYSIS
5 . OTHER ANTHROPOGENIC SOURCES . . .
a . REVIEW
b . ANALYSIS
D . SINKS FOR N~ 0
2.
1 . STRATOSPHERIC SINKS
2 . OTHER POSSIBLE SINKS
Page
3
9
15
21
21
27
31
31
31
34
36
36
37
38
43
43
45
45
45
48
49
49
54
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IV.
V.
VI.
GLOBAL MASS/MATERIALS BALANCE OF N20
EFFECT OF N20 ON STRATOSPHERIC OZONE ....
RECOMMENDATIONS
APPENDIX
REFERENCES
Page
56
62
70
74
75
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- 3 -
Executive Summary
The Clean Air Act Amendments of 1977 have directed that
the Environmental Protection Agency study the effects of and
possible control techniques of and other details concerning
chemicals that deplete stratospheric ozone. To understand
how gases reach the ozone layer, their transport through the
troposphere must be determined. The chemical reactions of a
gas in all the atmosphere must be known in order to determine
the possible effects it may have on stratospheric ozone.
Nitrous oxide (N~0), a gas in the family often referred
to as NO , is produced both naturally and anthropogenically
5C
and takes part in the stratospheric chemistry that controls
the ozone level. Nitrous oxide appears not to enter into any
chemical reactions in the troposphere. The chemistry of nitrogen
oxides (NO ) in the troposphere appears to be reasonably well
X,
known although more study is indicated for some particular
reactions.
The nitrogen cycle requires at one point the fixing of
nitrogen from the atmosphere either naturally or industrially-
Then either through microbiological action or combustion,
the gases NO, NO-, N20 or N~ may be released mainly through
the process of denitrification.
Nitrous oxide appears to be uniformly distributed in the
troposphere with a concentration of approximately 310 ppb.
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- 4 -
Its lifetime in the troposphere that best seems to fit the
models is of the order of 100-150 years.
A number of sources of N20 are well known but their
relative contribution has not been well quantified due to
problems of measurement and due to the wide diversity and
variability of global sources. Land sources, particularly
wet, low pH soils appear to be the largest contributers to
atmospheric N20. The rapidly increasing use of fertilizers
represents a source of N?0 that by the end of the century
may contribute as much N-O as do natural sources.
The contribution of nitrous oxide from freshwater sources
such as rivers and lakes appears to be small. The relative
input from oceans is in dispute although it appears that the
smaller estimates are the most likely. Other anthropogenic
sources such as fossil fuel combustion may be significant
sources but more study seems to be indicated for sources
such as fossel fuel plants.
The only well known and proven atmosphere sinks for
nitrous oxide are reactions in the stratosphere. It can
react with 0( D) to form NO which depletes the ozone or it
can be photolyzed to form N2. Other possible sinks have been
suggested but none have been demonstrated as viable alternatives,
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- 5 -
It is important to realize that N-0 is part of a natural
regulatory cycle involving ozone. The balance of ozone levels
depends on the natural creation and destruction of N-O. The
effect of human activity may be to upset this balance. A
rough idea of the relative contributions of various sources to
the atmospheric burden of N20 can be formed from the data presented
in Table I.
Table I
Sources Ranges of Values
(Mt. of N20/yr)+
Marine 5-250
land (soil) 10 - 100
fixed nitrogen including fertilizers* 6-38
man made (direct) 2-6
freshwater 3-40
Total 34 -474 (a)
*Fertilizer, represent approximately 1/2 of fixed nitrogen
+Mt. is a million metric tons and is defined in the Appendix
(a) This range of values corresponds to a residence time of
45 - 3.2 yr (53)
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- 6 -
Sinks
Stratospheric Reactions 14 - 45
Trospospheric Reactions 1-5
Soil 0 - 600
15 - 650(b)
(b) This range corresponds to a residence time between
100 and 2.3 years
As can be seen in Table I the fluxes from sources and
sinks do overlap but the range of possible values is so large
as to prevent any definite conclusions as to whether other
sources (or sinks) exist.
A number of modelers have predicted the possible effects
of an increase in atmospheric nitrous oxide concentration (due
to anthropogenic processes) on ozone concentration in the
stratosphere. See table II for a typical list of reactions
used in such calculations. The models include many
reactions some of whose rate constants are not well known.
Also in some cases the concentrations of the reactants are
not well known. Model calculations before 1978 predicted that
N20 increases would have depleted stratospheric ozone by
2023 roughly 5%. However, the present model calculations
predict that if N20 flux is doubled (achievable in the next
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- 7 -
50 years) the ozone concentration will only be slightly
affected. If N20 flux is quadrupoled ozone concentration
will decrease by 5%. A comparable prediction for the
effect of chlorofluorocarbons (CFC) on ozone concentration
is a depletion of 20% in the next 25 years at present rates
of production.
The problem with respect to CFC's was easy to solve
because (a) sources could be readily identified and quanti-
tatively assessed, (b) the stratospheric reactions were well
understood, and (c) the solution was easy to implement.
None of the above is true for the ozone-nitrous oxide cycle.
It is often argued that if a problem is not a crisis
then its study can be postponed. Such an attitude leads to
poor planning, inadequate study and understanding of the
mechanism involved and sometimes ineffectual control pro-
cedures. The N20 problem could well become such a crisis
and with a moderate effort over the next ten years a
possible crisis may be averted.
To study the effect of N^O on stratospheric ozone requires
researchers from a wide variety of disciplines. If a coordinated
long term and moderate effort to determine, measure and assemble
the diverse pieces of information concerning N20 and its
relationship to stratospheric ozone is not pursued, the likeli-
hood of a crisis appears good.
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- 8 -
A number of recommendations have been made for future
studies. From the present snapshot in time of this rapidly
moving object it can be concluded that the problem could be
a serious one but it is not an immediate crisis.
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- 9 -
I. Introduction
The Clean Air Act Amendments of 1977 directed, among
other things, that the Environmental Protection Agency should
study the state of knowledge and the adequacy of research
efforts to understand the effects of all substances, practices,
processes, and activities which may affect the stratosphere,
especially ozone in the stratosphere. Most of the ozone in the
atmosphere is found in the stratosphere and protects the earth
by absorbing much of the incoming solar radiation in the wave
length range 290 to 320 nm that contribute to DUV (biologically
damaging ultraviolet radiation sometimes referred to as BUV
or erythemal).
The balance of ozone concentration is affected by the
amount of various gases entering or formed in the stratosphere.
The most important of these gases are the oxides of hydrogen
(HO ), the oxides of chlorine (CIO ) and the oxides of nitrogen
j£ H
(NO ). The compounds NO include mainly NO, NO-, N-O,- N00 and
2C X 4* £* 31 ^
HNO., which are produced directly by fossil fuel combustion,
high flying aircraft or by the interaction of microorganisms
in soils, oceans, freshwaters and as a result of the use of
fertilizers. NO is sometimes referred to as odd nitrogen.
X
Other mechanisms may also deplete the ozone layer. For
example, metals on fine aerosol particles, traces of which may
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- 10 -
or may not be catalytically active are a potential ozone depletor.
Research is actively going on in this area. Other possible
ozone depletors include among others, most of the nonfluorinated
halomethanes.
Models of chemical-kinetic reactions and transport for
chemicals which directly or indirectly interact with ozone have
been developed which consist of a set of mathematical equations.
Most current models involve about 100 chemical interactions,
many interrelated with each other (see for example table II and
reference 2). Due to limitations of computers, most of the
models are only one dimensional (altitude). One development
from modeling is the realization that the major chemical,
catalytic cycles cannot be studied separately. The models do
include the parameters of strength of sources, rate coefficients
of chemical and photochemical reactions, and mixing coefficients
as a function of altitude. They also include absorption
coefficients for solar radiation and quantum yields (fraction
of light that leads to decomposition) with respect to wave
length. Even with all this sophistication the picture is not
complete. What is needed is a 3-D model that includes chemical
kinetic reactions and more precise and complete measurements of
the parameters of the model.
Gases such as N20, F-ll, F-12 and others can absorb thermal
radiation in the infrared region of the electromagnetic spectrum
and this may lead to a greenhouse effect such as that for C02.
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- 11 -
Table II
CHEMICAL EQUATIONS - THERMAL REACTIONS
(Reference 67)
0 + 02 + M = 03 + M
03 + NO = N02 + 02
N20 + 0(1D) = N2 +
N + 0 + NO + 0
OD) + H20 = 20H
0, + OH = H09 + 09
J £ £
03 + H02 = OH + 202
H + 02 + M = H02 + M
= 20
0 + N02 = NO + 02
N20 + 0(1D) = 2ND
N + NO = N2 + 0
+0(1D) + CH4 = OH + 2H02 + CO
0 + OH = 02 + H
0 + H02 = OH + 02
03 + H = OH + 02
H0 + OH = H0 +
OH + N02 + M = HN03 + M
H0 + OH = H + H0
OH + HNO., = H20 + N03
0(1D)
M =
M
N
NO + H02 = N02 + OH
OH + OH = H20 + 0
N02 + °3 = N03 + °2
OH + OH + M = H202 + M
+0 + CH4 = OH + 2H02 + CO
O^D) + M = 0 + M
Cl + OC10 = 2C10
Cl + N02 + M = C1N02 + M
NO + 0 + M = N02 + M
H2 + 0(1D) = OH + H
N + 03 = NO + 02
OH
H2°2
+2H0
= OH
H°
CO + OH = H + CO,
Cl +
+
= CIO +
CO
C1 + CH4 = HC1 + 2H02 + CO
Cl + C1N02 = 2C1 + N02
CIO + 0 = Cl + 0.
NO + CIO = N02 + Cl
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- 12 -
CIO + 03 = Cl + 02 + 02
CIO + N02 + M = C10N02 + M
CIO + CIO = 2C1 + 02
OH + HC1 = H2 + Cl
0 + OC10 = CIO + 02
N + OC10 = NO + CIO
Cl + OH = HC1 + 0
Cl + HN03 = HC1 + NO.,
Cl +
0(D) = 2C1
= HC1 + H0
CH3C1 + OH = Cl + H20 + H02
N02 + 0 + M = N03 + M
N2°5
0(1D)
+ M =
= 2N0
M
CIO + 03 = OC10 + 02
CIO + CIO = Cl + OC10
HC1 + 0(1D) = Cl + OH
0 + HC1 = OH + Cl
NO + OC10 = N02 + CIO
H + OC10 = OH + CIO
Cl + H02 = HC1 + 02
*CFC13 + 0(1D) = 2C1
Cl + H_ = HC1 + H
C10N02 + 0 = CIO + NO-
NO + N03 = 2N02
= NO
M =
N2°5 + H2° = 2HN03
M
CHEMICAL EQUATIONS PHOTOLYSIS REACTIONS
o2 = o + o
HN03 = OH + N02
H02 = OH + 0
HC1 = H + Cl
CIO = Cl + DTD)
OC10 = CIO + 0
CFC13 = 3C1
°3 = ° + °2
N02 = NO + 0
NO = N + 0
H0 = 20H
C10N02 = CIO + N02
CIO = Cl + 0
C1N02 = Cl +
*CF2C12 = 2C1
*CC14 = 4C1
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- 13 -
CH3C1 = 2H02 + CO + Cl N205 = 2N02 + 0
N03 = NO + 02 N03 = N02 + 0
H00 = H + OH
£»
Products such as F, H,/ CO, COF- etc. are not followed when
produced from halocarBons.
Methyl radical production assumed to yield 2H02 + CO
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- 14 -
Although the understanding of the possible effects on climate
are not fully understood, an estimate can be made of the magnitude.
Using a one-dimensional model it has been predicted that
if the atmospheric concentration of C02 were to increase by
25% the global temperature would increase by 0.79°K. If the
concentration of N?0 were to double after a period of several
years the global temperature would increase by 0.68°K.
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- 15 -
II. Troposheric NO
.X
Atmospheric or air chemistry for NO involves two major
^C
regions, the troposphere and the stratosphere, where the
kinds of chemical reactions that occur are different. In the
troposphere, gases are well mixed and homogeneous. This region
is shielded by stratospheric gases, mainly ozone and water
vapor which absorb much of the ultraviolet light from the sun.
As gases move away from the Earth the altitude increases and the
temperature generally drops until a reversal occurs, primarily
due to the absorption of ultraviolet photons. The position of
the reversal is called the tropopause and it is the dividing
line between the troposphere and the stratosphere. In the
stratosphere the mixture of gases is nonuniform and in general
residence times are long if the possibility of chemical reaction
is excluded.
A major chemical characteristic of NO is their oxidative
X
nature as compared to SO which are reductive. This leads to
X
the primary deliterious effect of the NO cycle in the troposphere
X
which is often called smog or photochemical oxidations. Photo-
chemical smog involves the input of nitric oxide, sunlight and
hydrocarbons. The kinetic chemical reactions that occur in the
troposphere involve such intermediate as N02 and PAN (see figure
1). These reactions have been studied since the 1950's when
this problem manifest itself on a significant level in Los
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R = CH3, C2H5, ETC.
/VBCOO,,^+R + C02
V..X
R02 -.-^
+RO
+H0
DRY DEPOS.T.ON
NITROGEN COMPOUNDS
BACTERIA
N2 + °2
Figure 1 Simplified schematic description of NO chemical
kinetics in the troposphere. Although not inclusive of all
possible detail, the main pathways can be seen.
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- 17 -
Angeles. It has been demonstrated that the chemical compounds
involved vary in concentration throughout the day depending on
the amount of sunlight and input gases. The intermediates are
the compounds responsible for eye irritation, adverse respiratory
effects, plant damage and the browning of the atmosphere known
(4)
as smog.v ' Some of the color is due to adsorption 'of sunlight
by the intermediate products and some is due to polymerizing
of the oxidation products forming aerosols.
The two major nitrogen oxide constituents in the atmosphere
are NO (nitric oxide) and N02 (nitrogen dioxide). The major
oxide, NO is produced by the high temperature combustion process
in the presence of N~ in the air. Some N02 is produced by
lower temperature 300°K oxidation. The amount produced is
dependent more on the concentration of NO than 02 in the
reaction.
2NO + 0 2N02
As seen in figure 1 nitric oxide reacts commonly with the
oxidizing agents such as the transient hydroperoxyl (H02),
alkylperoxyl (R02) and acylperoxyl (RC002) free radicals. In
these reactions R represents methyl (CH3), ethyl (C2H5) and
higher alkyl groups.
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- 18 -
Nitrogen dioxide plays a very central role in tropospheric
chemistry being involved directly or indirectly in virtually
every kinetic pathway. It reacts with water and nitric oxide
to form nitric and nitrous acids in the reactions;
2 N0 + H0 HONO., + HONO
and
N02 + NO + H20 2 HONO
And as shown in figure 1, N0« finds a sink in the reactions
with HO, R02 and RC002. If R = CH3 then the latter reaction
involves PAN (CH.,C002N02) or peroxyacylnitrate. This compound
is the mysterious compound X of the 1950's involved in photo-
chemical smog.
A source of ozone in photochemical smog involving NO follows
from the photodissociation of N0_;
N02 + hv (<4300 A) !>0(3P) + NO
followed by;
0(3P) + 0 + M y 0 + M
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- 19 -
The ozone is either quickly ozidized by NO or photolyzed
to produce HO;
»-2HO
The oxidation of N02 by O., leads to the production of
N205 which forms on aerosol or reaches the ground by dry
deposition. As shown in figure 1 the acids fromed by N02 also
form aerosols and become part of the clouds eventually reaching
the ground by rain out.
It is generally concluded by researchers of kinetic chemical
reactions in the troposphere that the major reactions and pathways
are now "reasonably well" understood. However some reactions
have been recommended for future studies.
One majo^r area requiring further study is the reaction of
longer chain R02 with NO and N02 as a function of temperature
and pressure. The chemistry of PAN, particularly for higher
analogues such as PPN need to be investigated. Particularly
information is required concerning thermal decomposition.
are;
The reactions of NO., that require further investigation
(5)
NO
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- 20 -
N03 + hv > NO + 02
as well as the reaction
N2°5 + H2°
In summary, NO tropospheric chemistry involves some
it
mechanisms which require further study. Besides those mentioned
above research is needed in areas such as aerosol production
(heterogenous chemistry) and acid rainfall. However those
processes that are related to chemicals involved in ozone
depletion appear to be well enough known that the major
attention should be focused on what happens in the stratosphere
and how the chemicals that get there are produced.
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- 21 -
III. STRATOSPHERIC NO.
A.
Introduction
As shown in Table III the NO catalytic cycle is a major
X
cause of depletion of stratospheric ozone. One of the gases
involved in stratospheric NO chemistry that is of concern
A.
with regard to ozone depletion is nitrous oxide (N20).
TABLE III
Approximate mechanisms for the Depletion of Stratospheric
Ozone * ' (average for the region from 25 to 40 km in altitude) .
Mechanism
Percent of 03 destruction rate
Photolysis
Transport to the Troposphere
Hydroxyl and Hydroperoxyl
Radicals
NO Catalytic Cycle
CIO Catalytic Cycle
20
0.5
10
50-70
10-40
As with many of the atmospheric reactions which interrelate
different chemical cycles, the CIO and NO cycles are so
H A,
related. For example, chlorofluoromethane decomposes by photolysis
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- 22 -
in the stratosphere releasing chlorine which consumes ozone
by the reaction;
Cl + 03 > CIO + 02
But the abundance of CIO may be influenced by
CIO + NO ^ Cl + N02
and by
CIO + N02 + M >C1N03 + M
and
CIO + HO 5" H02 + Cl
Nitrous oxide is produced naturally by microbial action
on any nitrifiable substance and from anthropogenic sources
among which are fertilizers and burning of fossil fuels. It
has been used as an anesthetic and for that use has been
called laughing gas.
A recent Study suggests that nitrous oxide (laughing
gas) may have some direct toxicological effects on anesthesiol-
ogists and dentists. This conclusion is based on largely
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VALENCE
+5
+4
+3
+2
+1
0
-1
-2
-3
TYPICAL COMPOUND
NO:
NO,
NO;
NO
N2O
No
HONH.
REACTION
<
o
NH3,,NH4
O
u.
QC
<
X
Figure 2 Relationship between nitrification, denitrification
and fixation as they relate to valence and compound(reference 42).
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- 24 -
circumstantial evidence. However there is some evidence that
the gas increases the fetal death rate and may cause leukopenic
effects.
Animals and plants cannot use nitrogen directly in the
form of N« to produce nitrogen compounds such as amino acids
and proteins. The nitrogen must be first made availabe in
a fixed form. This is a process that dissociates molecular
nitrogen and uses the resulting atomic nitrogen to form
compounds such as ammonium (NHt via biological fixation
or nitrate (N0~) or nitrite (NO-) via a biological fixation.
On land the biological fixation is performed mainly by
organisms symbiotically associated with leguminous plants.
For continuous high levels of productivity over large land
areas fertilizer must be used, but high yields over limited
areas on a continuous basis or on extensive areas inter-
mittently can be obtained by recycling biologically fixed
N2. In aqueous media fixation is primarily accomplished
by blue-green algae.
Nitrous oxide can in principle be produced by either
nitrification or denitrification. Nitrification is the
microbial oxidation of ammonia to nitrate and nitrite.
Biological nitrification is the major source of N20 in the
biosphere, and is mediated primarily by obligate aerobic
bacteria.
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- 25 -
Denitrification is carried out by bacteria that can use
nitrate as a terminal electron acceptor when oxygen has been
depleted via the sequence;
NO
nitrate nitrite nitric nitrous molecular
oxide oxide nitrogen
(see also figure 2)
The above sequence is somewhat oversimplified. For example,
it appears that between NO and N~0 is an intermediate some-
(2)
times referred to as X. It is possible that X is hyponitrous
acid but the present evidence is not positive and no identi-
fication can be made at this time. It has been estimated that
about 25-30% of the time does fixed nitrogen go into denitri-
fication in soils^ and about 6% in oceans.
The ability of soil microorganisms to reduce N-0 to N2
(62)
is inhibited by the NO^ content of the soil. All nitrogen
fertilizers provide nitrate to the soil, either directly, because
they may be wholly or in part in the nitrate form, or because
they are in the ammonium form or are changed to the ammonium
form, which can then be converted to nitrate. All crop manage-
ment inputs and techniques which increase plant growth and return
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- 26 -
more plant biomass to the soil and increase the amount of nitrate
produced in the soil.
Without the denitrification process the oxygen and
nitrogen of the atmosphere would long ago have been depleted.
Thus one would expect that the rate of denitrification is
on the average equal to the rate of fixation.
As long as there is sufficient free oxygen the aerobic
microorganisms use it for respiration. However when the
oxygen level falls below 5% in soils the microbial popu-
lation may use both free oxygen and combined oxygen in the
form of nitrate thus forming N-O. Since N20 is a gas, some
of it in any case will diffuse away before the denitrification
becomes complete. Under the situation of extreme lack of
oxygen, N^O will be an intermediate with N~ the final product.
It is thus important to measure the N20/N2 ratio in the
denitrification process for different ecosystems.
Factors that affect denitrification are; ^ ' '
I. Free oxygen content must be very small (e.g. in soils,
water, sediments). The redox potential should be 330 mv. '
2. The temperature range must be between 5 C and 60 C, although
individual species have narrower acceptable temperature ranges. ' '
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- 27 -
The production rate usually follows the well known Arrhenius
equation with rate changing expedientially with temperature.
3. When the pH is lower than 6.5 generation of N20 compared to
(12)
N2 is favored although the total denitrification rate is
(9)
decreased.
4. There must be an available carbon source.
B. Atmospheric Concentration and Residence Time
In the troposphere up to 12 km the concentration of N~0 is
fairly constant. However there is some disagreement about the
absolute value of the concentration. Hahn and Junge put the
(14)
value at 260 ppb while Singh et. al. v ' find it to vary less
than 4% between northern and southern hemisphere at an average
concentration of 311± 2.6 ppb. A recent urban study puts the
value at 329.5i 3.35 ppb. The value has been shown to
increase 1.5% over the last decade by Weiss & Craig and 0.3%/yr
(14)
for the last two years by Singh et al. However the latter
experimenter observes that they could not detect a change if it
were less than 0.5%/yr. Some researchers using the above
information claim that there is no N~0 problem. From balloon
(58)
measurements the change of N~0 concentration is less than
1%/yr.
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- 28 -
An early study of N~0 concentration showed a concentration
increase of roughly 20 ppb/yr during the period 1967-1969. An
extrapolation of this increase would predict the 1978 atmospheric
concentration to be roughly 460 ppb. Since the concentration is
nowhere near this high and since the data has not been reproduce-
able it is highly suspect.
According to Tony Brodrick and Paul Crutzen there
is no convincing hard evidence that the concentration of N20 is
changing in the atmosphere. Crutzen feels that the study by
Weiss is "coming close" but he feels that additional data are
needed.
In the troposphere the concentration thus is approximately
310 ppb and decreases to about 50 ppb at 35 km. This leads to
15 *
a tropospheric burden of M = 1.6 X 10 gm. A similar calcu-
lation for the stratosphere leads to an additional burden of
0.16 X 10 gm. Using an average removal or residence time of
T = 80 yr. then the input flux to the troposphere is;
n - M _ 1.6 X 10 5 gm on v in12 * vr n /
Q - - 80 yrs. = 20 X 10 gm of N20/yr.
*
This value' can be arrived by using the information that
1 m of air = 1.2 kg at ground level and approximately 0.4 kg
at 12 km. Then let the average mass density for the troposphere
mixing ratio (by volume) of N20 X average density of the tropo-
Q -3
sphere X volume of the troposphere = (310 x 10 ) (0.8 kg/m )
4
(TIT) Jj6412)3 - (6400) 3J m3 = 1.6 X 1015 gm. of N0.
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- 29 -
Uncertainties still exist concerning the value of the
tropospheric residence time of N20. A method for estimating
(18)
residence time (Test) has been described by Junge, Let;
= real standard deviation (for concentrations)
= overall standard deviation
= variability due to precision
then
(14)
Quantitatively Singh et al finds these values to be
= (0.8)2 - (0.3)2 = (0.7)2
Then using the general Junge criterion
Test = L4 = 20 years
a
It seems surprising that any meaningful value emerges from this
analysis since
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- 30 -
(52)
It has been observed that Junge ' s model represents a
valid -lower limit for lifetime. Other modelers put the lifetime
at, 150 yrs^60^ 130 years ^~"'' and 100 years VJ~''. These are
lifetimes with respect to stratospheric photochemical destruction.
Thus they are upper limits and other sinks could shorten the
photochemical lifetimes. Note that if the variability is less
than 1% in the troposphere this is consistent with a lifetime
larger than 100 yrs.
Some authors use different names for this quantity. For
(14)
instance Singh et al. ' determines the turnover rate to be
(18)
67 years. The term removal rate is sometimes used.v If a
gas is consumed the description used is lifetime but if the
gas is merely passing through to the stratosphere perhaps a
better description is residence time.
Crutzen and Ehhalt^ ' note that in 1967-68 there was a sharp
increase in N~0 levels indicating that the turnover rate must
be "short". Using Junge's relationship described above they get
Test = 10 yrs with an uncertainty of a factor of two.
The estimates of the residence time of N~0 range from
5 to 160 years. It appears that a more reasonable range
of possiblities is 100-15-0 years.
-------
- 31 -
The equation used above, M = QT shows that far a given burden
M, Q and T are not independent. Using the known concentration
of N-O as shown above led to 1.6 x 10 gm. It would appear that
atmosphere concentration is know within an accuracy of 10%. Thus
M is the best known quantity in the equation. The above range
of residence time (100-150 yrs) has a larger uncertainty. This
range of lifetimes implies an injection rate of between 16 and
11 Mt N20/yr.
C. Sources of N~0
1. Land
a. Review
The denitrification losses in soils depends on a number of
factors among which are the availability of organic carbon sub-
strate, the moisture content of the soil (02 content or redox
potential), the presence or absence of a cover crop and the type of
fertilizer applied. The proportion of nitrogen liberated as
N20 varies from 0 to 100%. Another example of wildly varying
fluxes is shown in the fact that the N20/N2 ratio can vary between
1/0.06 and 1/6. (9)
There are three main methods used to measure fluxes of
nitrous oxide from the soil, viz;
-------
- 32 -
1. use soil profile concentration measurements and
classical molecular diffusion theory (e.g. reference
20,21,22)
2. the box method measuring how concentration changes
with respect to time (27,68,69)
3. the micrometeorological method which applies turbu-
lent diffusion theory to air concentration profiles (24).
Example of fluxes of N~0 that have been measured from
soil using the profile method generally are of the order of
>
g
magnitude of 10~12g cm"2 s"1 of N2O (3.4 x 10~12 cm"2 s"1 for
12 2 1
soil covered with grass and trees to less than 0.1x10 gems
for desert soil). Cropland soils show fluxes of approximately
-12 -2 -1
0.3 x 10 gm cm s . Compost piles show concentrations as
high as 20 ppm. '
As an example of the third method one worker uses the
aerodynamic profile method which calculates fluxes using the
equation; <19'24>
F = K2 V2 (C - C /
where
1 n[Z-d)
-------
- 33 -
2
F = flux density (gm/cm /sec)
K = von Karman's constant (0.41)
U*,V* = friction velocity (cm/sec)
C = N^O concentation (gin/cm ) at height, Z(cm)
u = windspeed (cm/sec) at height Z (cm)
d = crop height (cm)
The method does allow correction for thermal instability.
Using the above method one experimenter has determined
that N»0 can emanate or be absorbed by soil. His calculations
for two separate days showed a flux towards the soil of ^230 X
-12 -2 -1 -12
10 gm cm s on a warm dry day call a flux of ^160 X 10
-2 -1
gm cm s on a colder damper day.
These values are surprising in two ways. First they are
considerably larger than most other measurements (a recent
study shows that the concentrations change by as much as an
order of magnitude in one day ) and secondly they show that
in one case the soil is a sink. The method used is the closest
to a direct measurement but it does depend on a theoretical
model. One possible explanation of the large fluxes calculated
in this way is that in general the fluxes are both in and out
of the soil and that those experimentally measured by the first
-------
- 34 -
two methods is the difference between the emitted and absorbed
fluxes. A recent estimate indicates that the N20 supplied to
the atmosphere by soils probably does not exceed about 7 M tons
N/yr.<">
b. Analysis
To understand the magnitude of a flux from soil of magnitude
2 1
100 gin cm s consider the situation where 2/3 of the global
land mass (1/3 the earths surface area) is emitting at that
rate;
_,3 _ gm
100 X 10 2
cm sec
[3.15 X 107 secfl2\ ( l) ~\ fl010cm21= 3000 Mt
I JisJ Uj 411(6400 ) ^ kmT . . 2 N00 per
t 1 yr. ^.l ' J.j_ 1 km I 2 r
year
This can be compared to the total global soil nitrogen loses
(231
due to agricultural activites of 6000-13000 MT. v ' This is
larger than the total flux from all sources estimated as 210
Mt/yr earlier. This is clearly much too large a flux and suggests
that the problem is more complex than has previously been
expected. It also demonstrates the danger of extrapolating
from a single site to a global level.
Evidence for the complexity of the relationship between
N20 and soil is shown in a study of Iowa surface soils showed
that their capacity for uptake of N20 under conditions favor-
-------
- 35 -
able for denitrification of nitrate was much greater than
their capacity for the release of the gas. ^ ' The experiments
involved incubating the soils in anerobic conditions and showed
that the soils had the capacity to take up all the N20 produced
by denitrification. At least 90% of the gas released was in
the form of N2. This information casts doubt on techniques
used to measure the flux of N20 which rely on soil concentration
profiles and indicates that the use of chambers placed over soil
surface is a superior technique.
An upper limit on the flux of N20 possible from any system
is given by the rate of fixation. In some cases the rate of
fixation is known more precisely than the rate of N20 emission.
(23)
From some "very patchy" evidence ' it is estimated that nitrogen
-12 -2
fixation occurs in forests to the extent of 3-5 X 10 gm cm
-1 9
s . For an area of global forests of about 4.1 X 10 ha
8 2 17 2
(lha = 10 cm ) or 4.1 X 10 cm this gives a global average of
approximately 50 Mt/yr nitrogen fixed in forests. A similar
-12 -2 -1
analysis for unused land yields ^-0.6 X 10 gm cm s for
Q
4.9 x 10 ha and a global total for nitrogen fixed of 10 Mt/yr.
The emission and absorption of N20 by soils is the most
diverse and complex part of the situation and would thus deserve
the most attention and investigation in the future (note also
in the section and sinks that soils could be a very large sink for
N20) .
-------
- 36 -
In summary, although some experimental measurements have
been made to determine the N20 levels for soil systems they
are too few and specialized to make accurate generalizations
from. Studies need to be made involving several parameters
among which should be; type of ground cover, kind and amount
of cultivation, moisture and humidity, pH, meteorological
conditions, sunlight, fertilizers or nutrient and soil com-
position. The measurement of N-O from land areas will likely
be the most difficult complex and involved of any of the
natural flux determinations.
2. Freshwater and Sediments
a. Review
As recently as 1977 no known measurements had been made
(13)
for fresh waters. However recently it has been shown that
fresh surface water is supersaturated with N20 bearing about
(19)
20% more than the air immediately above it. ' Significantly
larger concentrations supersaturated by factors up to five
have been found by Cicerone et al which are due to anthropogenic
(27)
activities such as sewage treatment. v ' Also municipal tap
water samples from U.S. cities show saturation factors between
1.8 and 89.2 (a factor of one indicates equilibrium and a
factor of 89.2 means that the dissolved N20 was 89.2 times
more concentrated than in equilibrium with the atmosphere).
-------
- 37 -
It was suggested that chlorination of river water and wastes
might be a key step in the production of N20.
Within a few cm. of the surface of sediments exist the
ideal denitrification condition. A possible mechanism for
nitrate transport involves the upward seepage of groundwater.
Estimates for the amount of nitrogen denitrified by this process
range from 23 to 81% of that retained in the lake. '
In lakes it was found that only waters deeper than 100 ft.
(19)
produced significant amounts of N20. It is suggested that
this occurs since photoplankton are absent and the supply of
organic matter is scarce. In these circumstances the nitrifying
microorganisms can compete more successfully for the limited
amount of available ammonium. Also there are often more nutrients
below the discontinuity layer (50-60 M) than in surface water.
b. Analysis
12
The average flux from some lakes is approximately 2 x 10
gm cm" s~ ). It might be expected that the average flux
from fresh water sources would be of this order of magnitude but
16 2
slightly higher. Using 2 x 10 cm as an estimate of the area
-12 -2 -1
of freshwater and 5 x 10 gm cm s as a rough estimate of
the average flux of N20 from freshwater sources than the annual
flux would be:
-------
- 38 -
-12 gm
5 X 10
cm sec
\2 X lO^cmf UJLJLJLOII f-lMt_ 1
L JL 1 yr. J[10-" gmj
= 3.15 Mt of N20/yr.
Some heavily polluted rivers such as the Rhine show fluxes as
large as 70 X 10~12gm cm"2s~1.(28)
It has been suggested that N20 is produced in water and
soil systems by the oxidation (nitrification) of ammonia in
addition to that produced by denitrification. If this is so
monitoring N~0 could be used as a way to study nitrification.
Thus the contribution from freshwater sources appears to
be small however it would appear that a more careful and closer
examination should be made of the contribution of sewage treat-
ment plants.
3. Oceans
Nitrification appears to take place in the soil, water,
the upper layers of sediment and in the ocean where it is
(1 o \
most efficient below 200 M. Hahn and Jungev ' suggested that
this occurs since photoplankton are absent and the supply
of organic matter is scarce. In these circumstances the
nitrifying microorganisms can compete more successfully for
-------
- 39 -
the limited amount of available ammonium. Also there are
often more nutrients below the discontinuity layer (50-60M)
than in surface water.
The range of fluxes measured from oceans vary widely.
These fluxes are not measured directly but involve using
concentration data with a model. The most common model is
called a stagnant film diffusion model or a two layer
(29)
model. The model assumes that both phases (air and sea)
are well mixed and that there is no turbulence or wind. The
flux is then proportional to the difference between the con-
centration of N20 in the water and the water concentration
when in equilibrium with the atmospheric abundance. This is
derived from the one dimensional relationship for molecular
diffusion.(29)
- n
- - D
where F is the flux, c the concentration, Z the depth and D
the coefficient of molecular diffusion of gas in the layer (air-
sea layer). The rate of evaporation/condensation determines
the exchange parameters. The proportionately factor in the
above relationship can be derived for ideal conditions.
Measurements of 42 ocean samples from the North Atlantic
by Hahn and Junge , some taken at depths of 3000 m, show
-------
- 40 -
a concentration of N20 in the water of 0.4 ppm v. Some of the
samples were slightly below equlibrium concentration and some
were saturated by as much as 280%. The flux using the above
simplified model is 70 Mt N20/yr for all the oceans if the average
concentration is 0.4 ppm v. The individual flow ranged from (0.82
to 1.66) x 10~12g cm~2s~1 near the equator to (0.05 to 0.66)
12 2 1
X 10 g cm s at 60 degrees north latitude.
The global number was determined by substitution in the
relationship.
F = - K (C - Ceq)
where C = 0.4 ppm v was the concentration found on the water
and Ceq = 0.25 ppm v was the assumed equilibrium value (Ceq is
actually closer to 0.31 ppmv).
(14)
Singh et al have measured water concentrations in
the Pacific ocean with an average surface saturation of 133%
(a super saturation of 33%). They found that some of the
concentration below the mixed layer to vary between 1 and 7
times that of the surface concentration. These values lead
12 ? 1
via the model to an oceanic flux of 0.359 x 10 gm cm s
or 32.4 Mt N20/yr. If fresh water sources are included the
total goes up to 35 Mt N,,0/yr.
-------
- 41 -
A survey was made of several oceans of the world sampling
N-0 in the surface water and the air by Weiss et al. Each
day analysis was made of the atmosphere and the ocean for a total
of approximately 8000 analysis in all. They were analyzed by gas
chromotagraphy with a precision of 0.3 to 0.4%.
The trip covered the ocean from California through the
Panama Canal to Spain, then through the Mediterranean Sea to the
Arabian Sea, and Mauritius. The trip then went to Antartica and
Australia.
The overall average super saturation of ocean waters with
respect to the atmosphere was approximately 4% with a few
occasional spikes as much as two fold. These values are much
(13)
lower than those of Hahn and Junge.
The procedure used to determine the flux from the ocean
was to determine the partial pressure of N-O and then estimate
how much was released. A check is provided in the levels
of radon gas released from the radium in sea water. The
atmosphere has very little natural radium). This mechanism will
show the rate of release of a gas from the ocean.
In general the saturation levels were very near the
equilibrium level except in regions where there is upwelling
-------
- 42 -
bringing N20 up from the lower depths or frontal boundaries
formed by sharp temperature changes.
Contrary to the findings of Hahn and Junge and Singh
et al^14^, the levels found by Weiss in the North Atlantic
were remarkably uniform and averaged no more than a few percent
from saturation. The western basin of the Mediterranean showed
some supersaturation and the Red Sea was slightly undersaturated.
But as samples were taken further south in the Red Sea values
they showed significant spikes. These high values correlated well
with oceanic mixing boundaries. The highest value of the
expedition of two fold supersaturation was recorded at one point
in this area.
The Northern Arabian Sea showed isolated high values of
320-330 ppb due to upwelling. In the Antartic a polar front
structure reflected N-O distribution. This is a region where
warm water is moving one way on one side and cold water another
way on the other side.
Using the above concentrations, the model, and the
radon flux information a global flux estimate can be made. This
estimate assumes that the tract followed by Weiss was representative
of all the ocean. The estimate of approximately 4 Mt N20/yr and
is considerably lower than the 70 Mt N20/yr estimated by Hahn &
Junge. Due to the major differences in measurements in the
-------
- 43 -
ocean and their importance (see discussion in Global Balance
N~0 section) the magnitude oceanic fluxes needs to be measured
by an independent researcher. In terms of biological activity
the surface of the oceans over the world is very heterogerous.
Hahn, Weiss & Singh have simplified different parts of the large
and highly variable ocean.
4. Fertilizers
a. Review
The dramatic increases in crop yields since WW II have
been largely due to an increased use of nitrogen fertilizers
and plant nutrients. This has been mainly due to better
genetic stock. Higher yielding hybrids require more nutrient
and care; therefore, the agricultural chemicals become very
important in allowing the plants to reach their genetic potential.
The mechanism by which N20 is produced usually involves
an anerobic bacterial activity denitrification. It is estimated
that about 1/3 of fertilizer used (57 Mt was fixed industrially
in 1974-reference 23) in agriculture is returned to the atmosphere
by denitrification in a relatively short time (decades or so).
The yield of N20 from fertilizers is between 5 and 20%. Thus the
fertilizer used may end up as nitrous oxide although it may take
several years for this to happen. The time estimates of this
phenomena show a large range of uncertainty.
-------
- 44 -
Commercial fertilizers are made up of nitrogen in the
form of nitrate, ammonium, and/or organic nitrogen compounds.
They are used to fertilize arable soil and in some cases on
pasture land. They add to the substrate to enhance denitri-
fication and nitrogen uptake.
Different chemical forms of a fertilizer added to the soil
can lead to N20 production.^ ' If it is added in the form of
a nitrate the mechanism of denitrification may follow and if
it is added in the form of an ammonium it will follow a cycle
of bacteria nitrification leading from NH3 to NH2OH and the
intermediate X.(13'31)
If fertilizers are added to the soil in the form of
urea they are hydrolyzed by soil urease to form (NH4)2 C0_.
It was observed that this ammonium is nitrified under aerobic
/ on \
conditions to form nitrate. The amount of the resulting N20 '
released can be greatly reduced by the use nitropyrin. However,
this has not been demonstrated under field conditions.
High N20 fluxes are found immediately after a rain or
in anerobic conditions. Using NaNO produced fluxes of 50 x
10 g cm s for recently cropped areas after a rain, '
and applying KN03 has yielded fluxes of up to 480 x 10~12g
cm s . This latter flux was 10 days after application.
-------
- 45 -
The N20 flux increased until the 18th day when it reached
-12 -2 -1
1350-X 10 g cm s Thus N20 can be generated by the
anerobic denitrification of nitrate fertilizer. However the
values were determined by the soil gradient technique and thus
are questionable.
b. Analysis
An estimate can be made of N~0 flux from fertilizers by
assuming that each person on earth requires 1/2 acre for food
9 2
production (4 x 10 people and 1 acre = 0.004 km ). Using a
11 2 1
flux of 10 gm cm s produces 25 Mt N90/yr from fertilizers,
The contribution of fertilizer usage to atmospheric N«0
appears to be the largest anthropogenic source and should be
expected to increase significantly in the future.
5. Other Anthropogenic Sources
a. Review
The relationship between natural and anthropogenic N-O
is demonstrated by comparing the natural rate of fixation with
that due to industry. The natural rate of nitrogen fixation
is about 200 Mt/yr.
Industrial fixation for agricultural purposes was 3.5 MT/
yr. in 1950 and it is predicted to be between 100 and 200 Mt/
-------
- 46 -
yr. (the rate of application of nitrogen fertilizer in 1977 was
about 40 Mt.) (8'23) by the end of the century. It is
estimated as 79 Mt/yr.*16^ For all other industrial processes
the rate was 15 Mt/yr in 1950, 40 Mt/yr in 1974 and could grow
to 100 Mt/yr by 2000. These values should be compared to the
natural rate of about 160 Mt/yr. Thus it can be seen that
the rate of N?0 released from anthropogenic sources could be
growing at a rapid rate.
As pointed out earlier the concentrations of N^O can be
(27 28)
larger near wastewater treatment plants. ' This occurs
in part because the easiest and most economical way to treat
wastes involves nitrification followed by denitrification. It
(32)
is the latter step that produces nitrous oxide.
Samples of air had been taken, sealed and kept from as
far back as 1961 by Barman Craig of the Scripps Institute of
Oceanography. They have been analyzed for N2O and the con-
centrations show a slow statistically significant increase with
time. They go from 291 ppb in 1961 to 300 ppb in 1978. This
increase is very similar to that of CO- for the same time period.
For the last decade the N20 atmospheric concentration has increased
1.5% while that of C02 has increased 2%. It suggests from
these measurements that the N20 change in the atmosphere may be
anthropogenic.
-------
- 47 -
Samples from the exhaust stacks fossil fuel power plants
have been taken and analyzed for the N-O/CO- ratio. The CO-
concentration value is well known for combustion processes
being in the range 12-14%. The exhaust stack concentrations
of N20 for two large fossil fuel plants were 25.8 and 25.0 ppm
by volume. The equilibrium values are of the order of 0.1 ppm
showing concentrations of least two orders of magnitude above
equilibrium. It was suggested that the major source of N2
-------
- 48 -
the value at 2.2 Mt N20/yr. It is also predicted that this flux
is increasing at a rate of 3.5% per year.
It has been suggested that the use of catalytic converters
(2)
could lead to the production of N20 by the reactions;
6NO + 4NH3
and
2NO
b. Analysis
The N20 in automobile exhausts was found to be less than
ambient levels unless catalytic converters were used and then
the concentration was found to be in the range of 1-10 ppm by
volume. ' Using an average value of 6 ppm for the
concentration of N20 from autombiles their contribution to the
global flux can be estimated. Assume that there are 100
million automobiles with an average engine displacement of 2
liters running at 2000 rpm for 2 hours each day each with a
catalytic converter. Then the total contribution from
automobiles would be
2000 revolutions honr-Qi (2000 cm3) ... -6, (1.2 kg) ,nn8x
\ * «» j u _> Li\j LIJ. o) ... ^ ^ 2C JL U ) " ( J_ (J J
(revolution) (m )
= 0.13 Mt N20/yr
-------
- 49 -
Thus the contribution from automobiles would be minimal
even with the catalytic converters.
Thus there are several anthropogenic sources other than
fertilizer usage that need further study. Possible significant
contributions can be made by fossil fuel combustion and sewage
treatment processes.
D. Sinks of N00
1. Stratospheric Sinks
Once N20 survives long enough to reach the stratosphere
it has two possible sinks '
N20 + O^D) 9- 2NO K = 6.3 x 10"11
** N + 0 K = 5.1 x 10"11
Stratospheric N20 can be destroyed by photolysis
hv
The resulting nitric oxide is an ozone depleter
NO + 03 > N02 + 02 K = 2.1 x 10 12 exp (-1470/T)
-------
- 50 -
2 -1
The meaning of a rate constant (unit cm molecule ) is
found in the relationship
1
T = k~c
where T is the lifetime, k the rate constant and c the concen-
tration.
The amount of N20 consumed by the reaction
K = 6.3 x 10'11
molecule
can be determined using the following information
Altitude 0(D ) Atmospheric N20 Atmospheric
(km) from Mixing Ratio denisty-from
reference ppb(v) from reference (45)
(45) reference 13 (particles/cm )
particles/cm
20 1 180 1.7 x 1018
25 6 100 7.7 x 1017
-------
- 51 -
30
35
40
45
50
55
60
30
100
330
700
60
45
300
70
40
10
-
-
-
Then using the relationship
-d[N20] = K[N20]
dt
Altitude
(km)
20
25
30
35
40
[O1!
»] [N20]
(particles/cm )
1
6
30
100
330
3060 x 108
770 x 108
252 x 108
63 x 108
8.1 x 108
3.6 x 1017
1.7 x 1017
8.1 x 1016
4.3 x 1016
2.3 x 1016
1.3 x 1016
7.2 x 1015
[N2o]
molecules/cm /sec
19.3
29.1
47.6
42.8
16.8
The volume of a spherical shell at these altitudes with a
9 3
width of 5 km is = 2.6 x 10 km .
Thus we have at 20 km for example:
-------
- 52 -
19.3 molecules [2.6 x 109 km3] lO^cm3! 44 gm ^Ys.15 x 107sec1
cm3 sec 1 km I 6 x 10 molecules 1 yr
(19.3) [6.0 x 109] gm
yr
= 11 x 1010 22 - >
then add the contribution for all altitudes
(19.3 ^ 29.1 + 42.8 +16.8) (6.0xl09) gm =
yr
1,0 Mt N20yr/ for consumption to form NO
also 1.0 Mt/yr to the reaction producing N~ and 02
Thus most of stratospheric N-0 must be photolyzed in order
to have an overall sink of the order of 20-40 Mt N2O/yr. in
the stratosphere.
Most of the estimates of the strength of the sink for
N20 with 0(1D) fall around 1.0 x 108 molecules cm~2s~1. (25)
p
A more recent value using Crutzen's 2D model is 5.1 x 10
-2 -1
molecules cm s . These lead to annual values of 1.2
Mt N20/yr and 7 Mt N20/yr.
-------
- 53 -
(33) 9 2
Crutzen* estimates that 3.4 x 10 molecules/cm /sec of
N_0 cross the tropopause at 15 km and are either photolyzed or
1 8
react with 0( D). This latter reaction returns 2.5 x 10
2
molecules/cm /sec of NO, N02 and HN03 back to the troposphere.
9 2
The net result is that 3.1 x 10 molecules/cm /sec are consumed
in the stratosphere. This corresponds to consumption of approxi-
mately 33 Mt of N20/year in the stratosphere.
Recently reactions in the stratosphere involving H0_ have
been investigated in some detail and new rate ' ' constants
and temperature dependence has been determined. This new
information indicates that H0~ consumes NO faster than had been
-------
- 54 -
2. Other Possible Sinks
It has been demonstrated that soils may act as sinks
as well as sources for N20 through the uptake of atmospheric
N-0 by soil microorganisms. ' This suggested idea has
(18)
also been suggested as a mechanism involving oceans.
Several possible tropospheric reactions have been
investigated. The rate constant for N20 + OH is less than
4 x 10~ cm /sec with a corresponding removal time of more
than 10 years. The experimental turnover time determined
by direct measurement is 5-15 years. A recent report shows
the rate constant to be (3.8 1.2) x 10 cm molecule"
sec~ .
It is suggested that the reactions
H02 > N2 + 02 + OH
> 2NO + OH
will be slower than that with OH by factor of 100.
Using the data of Blackmer and Bremner ' an estimate
can be made of the possible magnitude of the soil as a sink.
They showed that approximately 1 yg of N20 can be absorbed
by a gm of soil in 10 hrs. This rate was measured for 30 gm
-------
1 yg N20
gm 10 hr
'0.6 gmj
.cm2 J
"24 hr"
. 1 day
jH
[1 Mt "\
1018 pgj
- 55 -
2
density of roughly 0.6 gm/cm . Then assuming that 1/6 of
the earth is soil and that the radius of the earth is 6400
km the annual absorption of N20 by solid would be:
rin5 2-
j-i 99 -i-U cm
-x-4 n (6400) km2T^ I
^ 112 Mt N20/yr.
Assuming that soil conditions will be appropriate about 1/3 of
the year yields on annual absorption of ^ 40 Mt N20/yr.
This estimate is derived from an enclosed bench top experiment
and there is only weak evidence that measurements in the field
demonstrate such an effect. (Cicerone showed that on 8 of 60
days his soil was a sink^ and similar results have been
found by Rolston^ & Piebie. It would seem that before
the microorganisms involved in denitrification can consume
the nitrates and start consuming the N-0 it is blown away.
However in the sense that everything comes from somewhere and
goes somewhere the N20 transported by the wind could be
absorbed at another site. Recent diurnal variations in N20
atmospheric concentrations found by Bruce, Eggleton and
(43)
Penkett suggest that there is an important ground surface
sink for N-O. This uptake appears to be due to the reduction
(44)
of N20 to N2. l '
-------
- 56 -
It has been shown that nitrous oxide will undergo both
a thermal and a photochemical decomposition at 296°K when
it is absorbed on various dry sands. The rate of
decomposition was dependent on pressure and moisture. The
destructive efficiency under optimum conditions was the order
of 2 x 10 molecules/incident photon. Assuming that 1/3 of
the global land mass is desert or near-desert and 1.7% of solar
intensity is in the range 280-320 nm then approximately 70 Mt/yr
of nitrous oxide could be consumed at the above rate. Thus
decomposition by desert soil is a potential sink for N0.
Thus although some possible sinks for N»0 in the atmosphere
other than photolysis and reaction 0( D) have been suggested,
none has been definitely shown to be a viable mechanism.
IV. Global Mass/Materials Balance of N.,0
In analyzing a complex system such as the way that
nitrogen and its compounds move through the environment the
mass or materials balance approach can be a useful organizational
tool. This model examines all possible sources and sinks allowing
the comparison of different fluxes of N20. It also provides
insight and understanding into seemingly unrelated components,
flows between reservoirs, the relationships between anthro-
pogenic and natural sources and allows for a better design
of monitoring on data gathering activities.
-------
- 57 -
However the data regarding fluxes of N20 are somewhat
limited and have a large uncertainty. For example the amount
of N20 evolved from land was estimated as being between 16 and
69 Mt N/yr by one research group based on only two experiments.
And recently it has been shown that the soil could be a sink
of the order of 60 Mt N20/yr. This wide range of values makes
it very difficult to predict the effect on the stratosphere
due to N.,0. To get a rough of the order of magnitude of global
flux units assume that the average global production of N20 is
-12 -2 -1
1 x 10 gm cm sec . Then for the earth:
1 x io12 -2f - 4 n [*6400)2 km2!3'15 x 107 secjf lOJ^ITj
cm sec u 1 yr. -" 1 kmj. 1
10 gm
= 160 Mt N20/yr is emitted into the atmosphere
The global net production of N20 is estimated by others to
be between 125 and 475 million metric tons (Mt) of N20 per year
with 210 Mt as the most likely value. A recent review
listed the estimates for sources and sinks of N,,0. These and
other values are shown in Table IV.
-------
- 58 -
TABLE IV
Amount (10 metric tons of N20/yr)
Sources
Likely value
Range
Marine
land (soil)
fixed nitrogen
including fertilzer
man made (direct)
fresh water (13)
Thunderstorms
70 (5)'
25
20
4
5
10b
5-250
10-100
6-38
2-6
3-40
8-40C
Total:
144 (79)
34-474
Sinks
Stratosphere; Reaction
with UV and 0 ^D) 20
10-45
Troposphere; Reaction
with 0( D), OH, negative
ions & HO-
Soil
23
0-5
20-40
15-650
a) recent measurement by Weiss,
c) reference
(30)
b) educated evaluation,
As can be seen from Table IV a large unknown sink could
exist in the atmosphere for N20. At this time there are some
guesses as to what that sink might be e.g., the soil). At least
-------
- 59 -
one researcher has suggested that the above marine estimates
are much too large and thus there is no missing sink.
Some controversy centers on how much comes from (or goes
to) the oceans. Hahn,(13) Yoshinari(37) and Singh(14) et al
have estimated fluxes from oceans that require a tropospheric
sink. Singh concludes that since the oceans are so large,
the contribution from anthropogenic sources must be small.
Weiss with his lower estimate of oceanic flux would say
all the sources & sinks are known and balance (his flux
value combined with Hahn & Junge's other estimates place the
lower bound of sources much closer to estimate value for
sinks). Some support for this latter view comes from a review
by McElroy et al.(28)
The Marine estimate shown in Table IV comes from an average
of the few measurements made over oceans. It would appear
that more measurements are needed over oceans. The most
important areas are those which are nutrient rich such as the
upwelling zone along the West African coast, the Indian Ocean
and the Arabian sea, and coastal areas near some rivers
connecting to highly populated areas.
One source puts the estimated input of N~0 from fertilizer
at 3 to 11 Mt N/yr as the result of a postulated fixation rate
-------
- 60 -
of 200 Mt N/yr. For comparison with Table IV this would be
5 to 16 Mt N20/yr.(8)
The land estimate range of values in Table IV could also
be narrowed by better numbers for the fluxes over crops and
( JC\
forests. Using the data given by Blackmer & Bremmer the
estimate of a soil being a sink could be as large as 40 Mt
N~0/yr. Thus the soil could turn out to be a significant sink of
N2°'
The fertilizer estimate is derived from the amount used and
estimate of how much is released. Two projects are underway
(19 38)
to start such measurements. ' However, it appears that
the precision of 0 ppb can only detect fluxes down to about
10~ gm cm" sec which is two orders of magnitude above
the flux estimates from soils and water. A recent study
-13 -2
claimed that a detection limit for flux of 2 x 10 gm cm
_ -I
s can be acheived using samples taken over even 30 minute
(39)
durations. The discrepancy concerning the possible limits
of detection appears to be caused by different sample handling
techniques, and the theoretical flux models used.
In summary the estimates of fluxes associated with sources
and sinks of N20 are based on very few actual measurement. They
are for the most part rough first order estimates and in some
respects are confusing. With some authors suggesting that
-------
- 61 -
the oceans and soil are potential sinks and that the flux may
be into and out of these areas indicates that there are many
open questions and much more to be learned about how the
N20 cycle works.
A recent estimate indicates that the N20 supplied to the
atomosphere by soils probably does not exceed about 7 M tons
N/yr. (63)
A not unreasonable approach to determining the best overall
guess from the current inadequate information would be as
follows:
1. Use the estimate of N20 flux that is determined
by residence; viz, 11-16 Mt N^O/yr.
2. Assume that all sinks are in the stratosphere and
we use the upper limit; viz, 16 Mt N20/yr.
3. Then assume that freshwater and ocean sources
(including sediments) yield 7 Mt N20/yr. and other
sources contribute another 1 Mt N20/yr.
4. Then on balance land/crop/fertilizer systems must
contribute 8 Mt N20/yr. This could be all due to
sources or a combination of sources and sinks.
-------
- 62 -
II. Effect of NO on Stratospheric Ozone
Predictions have been made of the increase in N20 burden
in the atmosphere due to fertilizer use. Assuming a 6%
annual increase in the use of fertilizers, the increase
by the year 2000 will be between 7 and 31% of the 1975
atmospheric burden of N?0. ' It is possible that at some
future date (approx. 100 yrs) the N20 production rate
(28)
will be 3 times what it is now due to anthropogenic sources.
Models have been used to predict the change in stratospheric
ozone due to N20. ' ' ' Using the first order approxi-
mation that the rate of nitrogen fixation is the same as that
for N20 production, the % decrease can be calculated as a
function of increase in nitrogen fixation.
It has been observed that the interaction of N~0
in the atmosphere and its resulting effect on 0_ is non-
linear. Figure 3 shows the relationship between the
multiple of present N-O release and change in total ozone.
However the analysis shown below does give an indication of
the variables involved. Since this analysis is only a first
order approximation its usefulness is restricted.
Stratospheric NO is produced primarily by nitrous oxide
and the relationship between NO and 0_ has been shown to be^
J$* .3
-------
LU
I
O
O
z
111
O
X
O
I I
2 3
MULTIPLE OF PRESENT N2°
Figure 3 Effect on stratospheric ozone of increases in the
present rate of release of N2O (reference 67).
-------
- 64 -
A[0-J A[NO 1
£_ = x
[03] [N0x]
However since the primary source of stratospheric NO is
X
nitrous oxide, then;
A[0 ] A[N~0]
= X
[03] [N20]
where [0_] and [N20] are the amounts of ozone and nitrous oxide.
Recent information ' would suggest that X is quite small
and positive (perhaps 1/100). Previous values were in the range
-1/5 to -1/10. Thus it is suggested that N20 actually leads to
an increase in ozone concentration. Such assessments should be
I
treated with some caution. There are still some unknowns that could
change the value of X. Some key measurements that remain involve
methane chemistry, water vapor effects, and tropospheric con-
centrations of HO.
The main reasons for this recent change in effect are:
1. CIO has more of an effect than previously thought.
2. More NO is consumed by H02 than previously thought.
3. The ozone is found in 3-D models to be less sensitive
to N20 than previously thought.
-------
- 65 -
The important variables used in the analysis of N-0 mass
balance are:
ANF = The increase in global nitrogen fixation due to
human activities (Mt/yr) (from combustion,
manufacture, and use of fertilizers and
cultivation of legumes - this assumes that the
major changes are due to human activities)
ot = Fraction of denitrified nitrogen that is N20
3 = Fraction of fixed nitrogen that is denitrified
i
within a few decades
T = Atmospheric residence time of N^O
Of the estimated 200 to 350 Mt N/yr of nitrogen fixation
it is estimated that 79 Mt N/yr results from the synthesis
of nitrogen fertilizer and the cultivation of legumes and another
21 Mt N/yr is fixed as NO by combustion, another source puts
ji
(28)
this at 26 Mt N/yr. These fixation processes have been
increased rapidly in the last three decades so that between
1/4 & 1/2 of the total global rate of nitrogen fixation is due
to anthropogenic activities. As part of the nitrogen cycle
this leads to release of gases as shown;
-------
- 66 -
1-oC
N0= bacteria and
organic matter
, NO
N2°
thus
[N20]
[N2] + [N20] + i [NO]
In soils o( varies widely as temperature, pH, moisture and the
availability of oxygen change. Typical ranges for these
parameters are
land 0.025 <
-------
- 67 -
from the large global reservoir. Their conclusion is based
on the fact that the global reservoir of fixed nitrogen is
estimated to be 2 x 10 Mt so that adding even as much as
200 Mt by the year 2000 will be only a very small contribution.
(28)
However McElroy et al point out that in general
agricultural life times are much shorter.
The rate at which nitrogen is retained in living plants is
directly affected by their life cycle. For cereal crops and
leaves of trees the lifetime is one year while the average
time nitrogen is retained in living plants is of the order of
(9)
15 years. It is generally felt that in the natural nitrogen
(8)
cycle the rate of denitrification equals the rate of retention.
However the increase of anthropogenic N^O may upset the equilibrium.
The production, ApSI00, of nitrous oxide is: (where P _
^* IN A U
is the global rate of production)
APN 0 =*( 3 A N F
Where 1SL, is the global rate of nitrogen fixation. The
E
change in the N20 inventory must equal the difference between
the production rate and the destruction rate.
d[N20] _ [N20]
dt " PN20 - ~^~
-------
- 68 -
and when a steady state is acheived
[N20] = T PN 0
It is thought that at present the situation is close
to steady state.
Consider now the situation where the global rate of
production of nitrous oxide is increased from P Q to P Q +
AP n and kept here for a time long compared to the atmos-
pheric residence time. A new steady state inventory will be
achieved. If the change is small the new inventory will be
The relationship between the production of N20 and fix-
ation of nitrogen is quite complex. However a first order
approximation may be made using the expression mentioned above,
A PN() =o( 0 A NF
Where N^ is the natural (background) global rate of fixation
r
of nitrogen. Substituting this expression and P n = [N_0] T
IN n \J £
in the above yields.
A[0 ] A[N20] A P n AN
_ 1 = x __ _ = x __i_- x _
[03] [N20] PQ [N20]
-------
- 69 -
The natural global inventory [N20] is known to be about
1500 Mt N
Assume that the rate of anthropogenic fixation of nitrogen
is 100 Mt N/yr (this is the increase N^ over the natural rate
r
of fixation) then
At°3'
1500
If we assume that all the fixed nitrogen is denitrified
in a few decades then 3=1. Using T = 150 yrs. ando^ = 0.1
and assuming that X = 1/100 then
A[°3] = 0.
01
[Q3]
or the ozone concentration would increase by 1%
Some support that the increased N20 production will deplete
stratospheric 0., is found in the phenomena that followed a large
(54)
solar proton event that occurred in the stratosphere in 1972.v '
The proton event produced NO in the stratosphere that led to a
20% decrease in stratospheric ozone at that time.
-------
- 70 -
VI. Conclusion
A broad and general conclusion regarding the ozone depleting
capability of anthropogenically produced nitrous oxide emerges
from the preceding analysis. There is a problem which may
be very important on the time scale of 20-50 years. The
problems could be a serious one but it is not an immediate crisis.
Recommendations
The two main areas that need further study are
1. a more accurate determination of estimated global
flux from soil/fertilizer/crop systems
2. a determination of how much (if any) the N-0 atmospheric
concentration is changing.
The role of N~0 in nitrogen cycle has been the subject of
much study in recent years. Although N~0 rates have been
deduced from these studies, much more specific information is
needed to determine with some reasonable accuracy the role
of each source in the overall cycle and thus its ultimate
effect or stratospheric ozone. Perhaps the most important
information that is needed is to know under what conditions
N«0 is released from soil and also how that is related to
-------
- 71 -
the amount of and type of fertilizer and how it is used. These
studies should be conducted in natural field conditions. Also
the variation of the N-O/N^ ratio for different crops, soils
and growing conditions needs to be determined. The magnitude
of N20 release must be measured in the field and under different
conditions of moisture, weather, soil pH, type of ground cover
and type of soil.
A long term (perhaps 10 yrs) study should be made of
atmospheric nitrous oxide to determine if it is changing.
In particular any possible change in the quantity of nitrous
oxide reaching the stratosphere needs to be determined.
With regard to possible future ocean studies it was
observed that some more coverage is needed. Particularly
the following areas should be sampled include: the upwelling
areas of western Africa and the eastern coasts of most continents
and the eastern tropic areas of the Pacific Ocean.
Also more studies should be made of fresh waters. It was
noted that the Panama Canal showed high values for N20 and this
suggests a study of tropical inland waters (a study of the
Amazon River is planned in the near future).
In view of the studies of exhaust stack concentrations of
N»0 from fossil fuel power plants it would seem advisable to
-------
- 72 -
monitor those in a more careful and complete way. Care should
be taken concerning where the sample is collected from and what
possible reactions could produce or consume nitrous oxide. A
similar study should be made of exhaust gases from automobiles
with catalytic converters.
Since the contributors to the knowledge and understanding
of global nitrous oxide fluxes are from diverse disciplines,
a symposium or workshop would be important and valuable, perhaps
every 2 years. The major workers appear to be from the areas of
atmospheric chemistry, atmospheric physics, oceanography, micro-
biology and soil sciences. It is essential that there be direct
communication between workers in these diverse areas.
Other potentially sizable sources of N~0 such as sewage
treatment plants and feedlots need to be monitored.
Should cultivation and fertilizer practices be changed?
It appears that anthropogenic sources of N20 can have a sizeable
effect on stratospheric ozone. The most important of these
contributions involve planting, cultivating and fertilizing
crops. The implications of the problem on world food production
is immense and beyond the present analysis. However it can
be seen that political and economic decisions in the not to
distant future will require a better knowledge of the impact
-------
- 73 -
of nitrous oxide of stratospheric ozone than is now known. Some
continued and new studies, investigations and monitoring are
clearly indicated.
-------
- 74 -
Appendix
IMPORTANT NUMERICAL DATA FOR N,,0
" ^~~
Tropospheric Burden 1.7 x 10 gm
Stratospheric Burden 0.17 x 10 gm
Residence Time 100 - 150 years
12
Guesstimate of Global Flux 210 x 10 gm N-O/yr
Soil Flux 0.1 - 3.4 x 10~12gm cm~2s~1
-12 -2 -1
Land Flux up to 40 x 10 gm cm s
-12 -2 -1
Water Flux 'v 2 x 10 gm cm s
12 1 1
Oceans .82 - 1.66 x 10 gm cm s
Fertilizer N20 5-20%
I /- O
Area of: freshwater 2 x 10 cm
17 2
forests 4.1 x 10 cm
17 2
unused land 4.9 x 10 cm
From Crutzen
atmospheric scale height _ ,-
- deposition velocity = . . . ., . . ~ "l3
c J atmospheric residence time 10 yr.
= 2.7 x 10"3 cm sec"1
- flux of N20 into stratosphere = 9 Mt N/yr.
9 2 1
= 1.2 x 10 molecules cm s
Atmospheric concentration of 0.3 ppm v is equivalent to a
concentration in the soil of 0.013 yg N-O-N/g^ '
12 6
10 gm = 1 Mt (million metric tons) = 10 tonnes
1 ha = 104 m2
-------
- 75 -
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- 76 -
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-------
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59. Private Communication, Art Hornsby, EPA, Korr Laboratory,
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MJ.S. GOVERNMENT PRINTING OFFICE: 1979 620-007/3750 1-3
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing]
1. REPORT NO.
EPA-560/6-79-001
3..RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
AjPreliminary Analysis of Nitrous Oxide (N20)
including a Materials Balance
5. REPORT DATE
January 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
C. Richard Cothern
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
1J. SPONSORING AGENCY NAME AND ADDRESS
Environmental Protection Agency
Office of Toxic Substances
401 M St., S.W.
Washington, D.C. 20460
13. TYPE OF REPORT AND PERIOD COVERED
In House
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRAC1
This preliminary analysis of the sources, sinks and effect of nitrous
oxide(N2O) in the atmosphere is a snapshot in time of a rapidly moving
object. Much has been learned about this gas in recent years but much
remains to be determined. The most reasonable residence time for N2O in
the atmosphere is in the range 100-150 years. Since the change in atmos-
pheric concentration of N20 is less than 1% per year, this implies that
the release rate is in the range 10-15 Mt N2O/year. The combination of
freshwater.- ocean and sediment sources appears to be about 7 Mt N2O/year
which means that the most that land/crop/fertilizer systems can contribut
is 8 Mt N2O/year. The unknown in this balance is the concentration of
land/crop/fertilizer systems. Measurement of such fluxes are needed. It
appears that the only sinks for N20 are in the stratosphere. The overall
effect of N20 on stratospheric ozone is predicted to be small and could
lead to an increase in stratospheric ozone. The contribution of man
made nitrous oxide appears to be small at the present time but is
expected to grow. The overall problem is an important one on the time
scale 20-50 years and is not an immediate crisis.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
8. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)'
Unclassified
21. NO. OF PAGES
79
20. SECURITY CLASS (Thispage)
22. PRICE
EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
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