Nitrous Oxide N 2 O The Dominant OzoneDepleting Substance Emitted in the 21 st Century A R Ravishankara John S Daniel and Robert W Portmann SCIENCE Vol 326 Pages 123125 2009 OCT 2 ID: 1010682
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1. Nitrous Oxide (N2O) and Stratospheric Ozone Layer DepletionNitrous 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)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, 7697-7707, 2010.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.A. R. RavishankaraEarth System Research Laboratory, Chemical Sciences Division, National Oceanic and Atmospheric Administration,Boulder, CO, USADisclaimer: I am not talking as a co-Chair of WMO/ENEP SAP panelOpinions expressed are mine and not necessarily those of NOAA
2. What we know: Stratospheric Ozone Depleted via catalytic cycles that include nitrogen oxisedHomogeneous gas phase catalytic ozone loss controls the O3 layerNOx catalysisNO+O3NO2+O2O+NO2NO+O2O+O32O2net:Gas phase homogeneous catalytic reactions that can destroy millions of times more ozone- small amounts can cause a big changeClOx catalysisNitrogen oxides can catalytically destroy ozoneFirst time the ozone layer depletion was identifiedOzone layer depletion has been focused on halogens….But, there are other catalytic cycles
3. What we know: N2O is the main source of stratospheric NOxN2O (~10%) is converted to NOx in the stratosphereN2O is the largest source of NOx in the stratosphere N2O (from trop)NONO2NOxFrom mesosphereN2O is very stable in the troposphereAtmospheric lifetime of ~100 yearsMany 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
4. What we know: A significant part of N2O emission is of human originPreindustrial level ~270 parts per billion (ppb)Current level ~325 ppbConcentration 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, CH4Anthropogenic sources: agricultural fertilization, combustion, industrial production, etc.4
5. What we know: Others have studied the effect of N2O increases on ozone layerMany studies have examined what happens to the stratospheric ozone layer if nitrous oxide emissions are alteredExamples: Kinnison et al. (1988)Randeniya et al. (2002)Chipperfield and Feng (2003)Konopka et al. (2007)etc.All indicate that ozone would decrease if N2O increases, as expected from emission trends5
6. What we did6View anthropogenic N2O as a potential ozone depleting substance- the same way as the ozone depleting substances (ODSs) controlled by the Montreal Protocol
7. Ozone Depletion Potential of N2OThis 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 = 0.033Calculated ODP using Garcia-Solomon 2D model for 2000 conditionsODP of N2O = 0.017N2O is an ozone-depleting gas!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 gases7
8. N2O=ODS
9. Implications of our findingsTwo Key Points:Our discussion is restricted to anthropogenic emissions - the ones that are under human controlN2O’s ODP is small - but its emissions are large9Large Natural EmissionsEven larger natural concentrationsAnthropogenic concentrations growing rapidlyCurrent EmissionsCurrent Atmospheric ConcentrationsAnthropogenicNatural6.7 TgN/yr11 TgN/yr55 ppbv270 ppbv
10. ODP alone does not tell the storyCompare the ODP-weighted anthropogenic N2O emissions- not ODP- with those of CFCs and other ozone-depleting substancesAnthropogenic 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.
11. N2O: the dominant ozone depleting substance emitted in the 21st centuryN2O 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.Large uncertainties lead to large uncertainties in any potential actions!11ObsScenarios
12. Climate benefits of reduced N2OBoth 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?Ozone depletionClimate Forcing12
13. A few other points of noteN2O does not contribute to the Antarctic ozone hole. It influences global ozoneChanges in anthropogenic N2O emissions will affect the estimated date for the recovery of the global ozone layerCalls in to question the “baseline” for ozone recoveryAnthropogenic N2O could be an unintended byproduct of climate mitigation strategies, e.g., biofuel, iron fertilizationOzone depletion by anthropogenic N2O is roughly the same as that from the original projections for 500 US supersonic transport SSTs. 13
14. Where does N2O come from? The crux of the issueTotal anthropogenic source is well knownMany sources of N2ODiffuse sourcesLarge uncertainties in source strengths now and in the future14Science issues for N2O:“Global” monitoring will not provide individual source strengthsAdditional in-situ monitoring using intensives and other methods would be a path forward
15. “Isolated” individual source contributions.Perturbation from the A1B/A1 scenario minus constant 1900 source gas levels Influence of N2O in a changing climatePortman, Daniel, and Ravishankara, Ozone depletion due to N2O: influences of other gases, 2011Non-linear interactions are important- (solid vs dashed lines) Non-linearities limit unambiguous isolation of a gas’ effect.Clearly, N2O continues to “deplete” ozoneNeed to think beyond 2100 also!
16. 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 N2O decreases vs. Other options
17. Key Points of Our StudiesTreated N2O like any other ozone-depleting substance (CFCs, Halons, methyl bromide,…). Calculated ODP. Compared ODP-weighted emissions of anthropogenic N2O to the ODP-weighted anthropogenic emissions of other ozone-depleting substances.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 banks171. Fact: NOx from N2O leads to ozone depletion; N2O is not regulated under the Montreal Protocol. Findings: Anthropogenic N2O is now the largest manmade ozone-depleting 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 emissions of N2O are unabated.
18. Thank you for your attention
19. Backup slides
20. What we know: Current state and outlook based on assessmentsFindings from 2002 and 2006 SAP of UNEP/WMO:The Montreal Protocol is a success!The chlorine (and bromine) containing ODSs are decreasing in the atmosphereThe ozone layer is expected to recoverIt is showing signs of recoveryClimate change and atmospheric composition will influence the recovery20Ozone layer depletion has been focused on halogens….
21. A few points about calculated ODPCalculated ODP is robustODPs calculated in this study for CFC-12 is 1.03 and HCFC-22 is 0.06- agree with literature values. O3 depletion dominated by NOx in mid-stratosphere, where 2D models do well21ODP 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.
22. Why is the ODP of N2O ~0.02?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 differenceNOx catalysis is roughly similar in efficiency to that by ClOX22
23. Calculated “steady state” ODP 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 modelFull photochemistryFull heterogeneous chemistryTakes care of “self-healing”How good is transport? Not as big an issue because lifetimes are long!There are many different “ODPs!” – e.g., Semi-empiricalTime dependentSteady state …. Chosen by Montreal Protocol for regulations
24. Other issuesChanges in anthropogenic N2O emissions will affect: the estimated date for the recovery of the ozone layer; 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; implications for the recovery of the polar ozone hole that might differ from that of global ozone;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 warmingFor history: Ozone depletion by anthropogenic N2O is roughly the same as that from the original projections for 500 US supersonic transport SSTs. 24
25. Daniel et al., submitted to ACPD, 2010Postscript: Right way to think about N2OFigure 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 N2O, 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.O3 BG, changing T; ODS=0O3 with changing T; ODS = 0 from 2010O3 with changing T; Halogens = 0 from 2010
26. Postscript: N2O influences many other facetsNitrous oxide delays ozone recovery, Martyn Chipperfield, Nature Geoscience 2, 742 - 743 (2009); doi:10.1038/ngeo678News and Views on our paperN2O influence ozone “recovery”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