Nitrous Oxide (N2O) and
Stratospheric Ozone Layer Depletion
A. R. Ravishankara
Earth System Research Laboratory, Chemical Sciences Division,
National Oceanic and Atmospheric Administration,
Boulder, CO, USA
1. Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century,
A. R. Ravishankara, John S. Daniel, and Robert W. Portmann, SCIENCE, Vol. 326, Pages:
123-125, 2009 (OCT 2)
2. Options to accelerate ozone recovery: ozone and climate benefits, J. S. Daniel, E. L. Fleming,
G. J. M. Velders, C. H. Jackman, and A. R. Ravishankara, Atoms. Chem. Phys., 10, 76977707, 2010.
3. Ozone depletion due to N2O: influences of other gases, R. W. Portmann, J. S. Daniel, A. R.
Ravishankara, Phil. Trans. .Royal Soc., B (Biology), The proceedings of the discussion
meeting entitled ‘Nitrous oxide, the forgotten greenhouse gas’, held between 23-24 May, 2011
at The Royal Society Kavli Centre. In press, 2011/2012.
Disclaimer:
I am not talking as a co-Chair of WMO/ENEP SAP panel
Opinions expressed are mine and not necessarily those of NOAA
What we know: Stratospheric Ozone Depleted via
catalytic cycles that include nitrogen oxised
Ozone layer depletion has been focused on halogens….
But, there are other catalytic cycles
HOx catalysis
ClOx catalysis
NOx catalysis
OH + O 3 ® HO 2 + O2
NO + O3
HO 2 + O3 ® OH + O 2 + O2
net: 2 O3 ® 3 O 2
O + NO2
net: O + O 3
NO2 + O 2
NO + O 2
2 O2
Homogeneous gas phase catalytic ozone loss controls the O3 layer
Gas phase homogeneous catalytic reactions that can destroy millions
of times more ozone- small amounts can cause a big change
Nitrogen oxides can catalytically destroy ozone
First time the ozone layer depletion was identified
What we know: N2O is the main source of
stratospheric NOx
From mesosphere
NO
NO2
NOx
N2O (from trop)
N2O is very stable in the troposphere
Atmospheric lifetime of ~100 years
 N2O (~10%) is converted to NOx in the stratosphere
 N2O is the largest source of NOx in the stratosphere
Many studies have examined what happens to the stratospheric ozone layer if nitrous
oxide emissions are altered. Conclusion- Increasing N2O leads to decreased in O3.
3
What we know: A significant part of N2O
emission is of human origin
 Preindustrial level ~270 parts per billion (ppb)
 Current level ~325 ppb
 Concentration continues to increase at roughly 0.25% (of total) per year;
i.e., ~1% of anthropogenic component per year.
 All Increases in N2O is due to anthropogenic activity– looks like other
anthropogenic emissions, e.g., CO2, CH4
 Anthropogenic sources: agricultural fertilization, combustion, industrial
production, etc.
4
What we know: Others have studied the effect of
N2O increases on ozone layer
Many studies have examined what happens to the stratospheric
ozone layer if nitrous oxide emissions are altered
Examples:
oKinnison et al. (1988)
oRandeniya et al. (2002)
oChipperfield and Feng (2003)
oKonopka et al. (2007)
oetc.
All indicate that ozone would decrease if N2O increases, as
expected from emission trends
5
What we did
View anthropogenic N2O as a potential ozone
depleting substance
- the same way as the ozone depleting
substances (ODSs) controlled by the Montreal
Protocol
6
Ozone Depletion Potential of N2O
Calculated ODP using Garcia-Solomon 2D model for 2000 conditions
ODP of N2O = 0.017
N2O is an ozone-depleting gas!
This positive number for ODP is comparable to those of some of
the HCFCs: HCFC-123 = 0.02; HCFC-124 = 0.022; HCFC-225ca =
0.025; HCFC-225cb = 0.033
To our knowledge, this is the first time N2O is
suggested to be an ozone-depleting substance in the
same way that other Montreal Protocol gases
7
N 2O
=
ODS
Implications of our findings
Anthropogeni
c
6.7 TgN/yr
11 TgN/yr
1
2
Current Emissions
Natural



55 ppbv
270 ppbv
1
2
Current Atmospheric Concentrations
Large Natural Emissions
Even larger natural concentrations
Anthropogenic concentrations growing rapidly
Two Key Points:
Our discussion is restricted to anthropogenic emissions
- the ones that are under human control
N2O’s ODP is small
- but its emissions are large
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ODP alone does not tell the story
Compare the ODP-weighted anthropogenic N2O emissions- not ODP- with
those of CFCs and other ozone-depleting substances
 Anthropogenic ODP-weighted-emissions of N2O were the fourth largest ODS
emissions even in 1987, at the height of the CFC emissions prior to the MP.
 Anthropogenic N2O is now the largest ozone depletion gas emission; it will
continue to be so and get even larger in the 21st century if the anthropogenic N2O
emissions are unabated.
N2O: the dominant ozone depleting
substance emitted in the 21st century
Obs Scenarios
 N2O is already the dominant ozone depleting substance being emitted today!
 Continued growth in N2O, combined with decreasing chlorine loading, makes
it even more important in the future.
 There are uncertainties in projections of N2O growth- but even the most
optimistic projections shows an increasing N2O trend.
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 Large uncertainties lead to large uncertainties in any potential actions!
Climate benefits of reduced N2O
Ozone
depletion
Climate
Forcing
Both climate AND O3 layer benefit by reductions in N2O emissions
— a “win-win” for both ozone and climate.
But, what sources to target? What to reduce?
12
A few other points of note
 N2O does not contribute to the Antarctic ozone hole.
It influences global ozone
 Changes in anthropogenic N2O emissions will affect
the estimated date for the recovery of the global
ozone layer
 Calls in to question the “baseline” for ozone recovery
 Anthropogenic N2O could be an unintended
byproduct of climate mitigation strategies, e.g.,
biofuel, iron fertilization
 Ozone depletion by anthropogenic N2O is roughly the
same as that from the original projections for 500 US
supersonic transport SSTs.
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Where does N2O come from?
The crux of the issue
 Total anthropogenic source is well
known
 Many sources of N2O
 Diffuse sources
 Large uncertainties in source strengths
now and in the future
Science issues for N2O:
 “Global” monitoring will not provide individual source
strengths
 Additional in-situ monitoring using intensives and other
methods would be a path forward
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Influence of N2O in a changing climate
Portman, Daniel, and Ravishankara,
Ozone depletion due to N2O:
influences of other gases, 2011
“Isolated” individual source
contributions.
Perturbation from the A1B/A1 scenario
minus constant 1900 source gas levels




Non-linear interactions are important- (solid vs dashed lines)
Non-linearities limit unambiguous isolation of a gas’ effect.
Clearly, N2O continues to “deplete” ozone
Need to think beyond 2100 also!
N2O decreases vs. Other options
4
DU
3
N2O Emission (Anthr)
CFC Bank
HCFC Bank
HCFC Production
Halon Bank
CH3Br Emission (Anthr)
CCl4 Emission
2
1
0
2000
2020
2040
2060
Year
2080
2100
 The elimination of anthropogenic N2O emission has the largest
potential for reducing ozone depletion in the future.
 More beneficial than:
• CFC banks,
• HCFC production and banks,
• Halon banks,
• Anthropogenic methyl bromide,
• Carbon tetrachloride
Key Points of Our Studies
1. Fact: NOx from N2O leads to ozone depletion; N2O is not
regulated under the Montreal Protocol.
2. Treated N2O like any other ozone-depleting substance (CFCs,
Halons, methyl bromide,…). Calculated ODP. Compared ODPweighted emissions of anthropogenic N2O to the ODP-weighted
anthropogenic emissions of other ozone-depleting substances.
3. Looked at the influence of “climate change” on N2O’s ozone
depletion. Compared Magnitudes of anthropogenic N2O-induced
depletion with other standard measures- e.g., destruction of banks
4. Findings: Anthropogenic N2O is now the largest manmade ozonedepleting gas emission (a recent development owing to the successful
abatement of CFCs and other ODSs under the Montreal Protocol!),
and it will remain so for the next century if anthropogenic
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emissions of N2O are unabated.
Thank you
for
your attention
Backup slides
What we know: Current state and
outlook based on assessments
Ozone layer depletion has been focused on halogens….
Findings from 2002 and 2006 SAP of UNEP/WMO:
 The Montreal Protocol is a success!
 The chlorine (and bromine) containing ODSs are decreasing in
the atmosphere
 The ozone layer is expected to recover
 It is showing signs of recovery
 Climate change and atmospheric composition will influence the
recovery
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A few points about calculated ODP
Calculated ODP is robust
ODPs calculated in this study for
CFC-12 is 1.03 and HCFC-22 is 0.06agree with literature values.
O3 depletion dominated by NOx in midstratosphere, where 2D models do well
ODP is influenced by
amount of sulfate aerosol
and Chlorine in the
stratosphere
Choice of ODP = 0.017 is a “conservative” estimate.
It may change with better information.
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Why is the ODP of N2O ~0.02?
NOx catalysis is roughly similar in efficiency to that by ClOX
Main reason for smaller value:
Only 10% of N2O is converted to NOx and while 3 Cl atoms are
produced from CFC-11 (CFCl3)
There are some differences in efficiency, fraction in active form,
etc. that account for the other difference
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Calculated “steady state” ODP
There are many different “ODPs!” – e.g.,
Semi-empirical
Time dependent
Steady state …. Chosen by Montreal Protocol for regulations
ODP =
Integrated Ozone depleted by emission of 1 kg of N 2 O
Integrated Ozone depleted by emission of 1 kg of CFC -11
 The Ozone Depletion Potential (ODP) is defined by the time-integrated change of
global ozone due to a unit mass emission of the ODS relative to that of CFC-11.
 Our model incorporates a mixing ratio lower boundary condition rather than an
emission boundary condition.
Garcia-Solomon 2D model
 Full photochemistry
 Full heterogeneous chemistry
 Takes care of “self-healing”
How good is transport? Not as big an issue because lifetimes are long!
Other issues
Changes in anthropogenic N2O emissions will affect:
(a)the estimated date for the recovery of the ozone layer;
(b)imply that the use of a single parameter such as Equivalent Effective
Stratospheric Chlorine, EESC, to estimate the recovery of the ozone layer
should be reevaluated;
(c)implications for the recovery of the polar ozone hole that might differ from
that of global ozone;
(d)Calls in to question the “baseline” for ozone recovery.
N2O could be an unintended byproduct of enhanced crop growth for biofuel
production or iron fertilization to mitigate CO2 emissions. Such an enhancement
would lead to the unintended ‘indirect’ consequence of ozone layer depletion
and increased climate forcing from N2O produced for alternative fuel used to
curb global warming
For history: Ozone depletion by anthropogenic N2O is roughly the same as that
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from the original projections for 500 US supersonic transport SSTs.
Postscript: Right way to think about N2O
O3 BG, changing T; ODS=0
O3 with changing T; ODS = 0 from 2010
O3 with changing T; Halogens = 0 from 2010
Daniel et al., submitted to ACPD, 2010
Figure 1. (a) Globally averaged total column ozone, (b) ozone depletion relative to a case in which no ODSs were or will be emitted
(“background” case), and (c) EESC time series. Cases shown are the baseline scenario, in which future ODS emissions follow a path
consistent with current growth and Montreal Protocol regulations and IPCC scenario A1B for N 2O, CH4, and CO2, a case in which no
anthropogenic chlorine- or bromine-containing ODSs are emitted after 2010, and a case in which no ODSs are emitted (including anthropogenic
N2O) after 2010. The ozone time series for the background case is also shown. Solid lines are calculations from the GSFC model; dashed are
for the NOCAR model. The ozone depletion from the NOCAR model (panel a) is increased by 3% so the 1980 levels of ozone depletion are
equal. The dotted lines represent the 1980 benchmark levels that are used in previous ozone assessments and are also often considered in
Montreal Protocol discussions.
Postscript: N2O influences many other facets
N2O influence ozone “recovery”
Nitrous oxide delays ozone recovery, Martyn
Chipperfield, Nature Geoscience 2, 742 - 743
(2009); doi:10.1038/ngeo678
News and Views on our paper
N2O emissions will continue to increase! We may want to think beyond 2100
(It is one of the “longer-lived” GHGs)
Influence of N2O in other parts of the stratosphere can be important