A. R. Ravishankara, John S. Daniel, Robert W. Portmann (2009) Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century A. R. Ravishankara, John S. Daniel, Robert W. Portmann (2009)

ODS: ozone-depleting substances ODP: ozone depletion potential Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century Abbreviations: ODS: ozone-depleting substances ODP: ozone depletion potential GWP: 100-year global warming potential Similarities between CFCs and N2O: both are very stable in the troposphere, where they are emitted both are transported to the stratosphere, where they release active chemicals destroying ozone both have substantial anthropogenic sources, but N2O also has natural sources in addition ↔ despite this similarities, N2O remains unregulated by the Montreal Protocol N2O’s reactions in the stratosphere: 90% 4.4% 5.6% N2O + hv  N2 + O N2O + O(1D) N2 + O2 2NO

ODPX = mCFC−11 ∙ ∆μCFC−11 ∙ τX ∙ ∆O3 X mX ∙ ∆μX ∙ τCFC−11∙ ∆O3 CFC−11 Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century NO catalytically destroys Ozone as shown by Crutzen and Johnston: NO + O3  NO2 + O2 O + NO2  NO + O2 O + O3  2 O2 The paper’s authors quantify the ozone destruction of N2O by determing the ODP for N2O: ODPX = mCFC−11 ∙ ∆μCFC−11 ∙ τX ∙ ∆O3 X mX ∙ ∆μX ∙ τCFC−11∙ ∆O3 CFC−11 m: molecular weight of the ODS Δμ: change in the mixing ratio boundary condition τ: atmospheric lifetime ΔO3: change in globally averaged column ozone computed by the model

Comparison of different ODPs: Factors that influence N2O‘s ODP: Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century Comparison of different ODPs: Factors that influence N2O‘s ODP: N2O contribute most to depletion where ozone concentrations are the largest Conversion rate of N2O to NOX levels of odd hydrogen, bromine and methan stratospheric volcanic sulfate aerosols decrease the ODP by sequestering NOX to HNO3 Cly – concentrations due to the formation of ClONO2 and conversion of ClO to Cl by NO ODPCFCl3 = 1 ODPHCF-225cb = 0.033 regulated by the Montreal Protocol ODPHCF-225ca = 0.025 ODPHCF-124 = 0.022 ODPHCF-123 = 0.020 ODPN2O = 0.017

Comparison of ODP-weighted N2O emissions with other ODS Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century Comparison of ODP-weighted N2O emissions with other ODS Anthropogenic emissions of 10 million tons N2O per year make up for its smaller ODP The emissions of all CFCs together in 1987 were around one ton per year. The uncertainties come from uncertainties in the emission estimates, not from the ODP anthropogenic N2O emissions are the single most important of the anthropogenic ODS emissions today

Looking into the future Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century Looking into the future In any case N2O emissions are unlikely to be lower than they are today For all scenarios, N2O is expected to remain the largest ODS emission throughout the rest of this century. The authors regard the regulation of N2O as largest option for protection of the ozone layer N2O is important for both the future ozone layer and climate → „Limiting future emissions would enhance the recovery of the ozone layer from its depleted state and would also reduce the anthropogenic forcing of the climate system, representing a win-win for both ozone and climate.“

ODPX = mCFC−11 ∙ ∆μCFC−11 ∙ τX ∙ ∆O3 X mX ∙ ∆μX∙ τCFC−11∙ ∆O3 CFC−11 Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century Follow-up papers and how ODPN2O might change ODPX = mCFC−11 ∙ ∆μCFC−11 ∙ τX ∙ ∆O3 X mX ∙ ∆μX∙ τCFC−11∙ ∆O3 CFC−11 Ravell et al. (2015): The changing ozone depletion potential of N2O in a future climate GHG Tstr cat. NOx - cylcle Brewer-Dobson circ. time N2O -> NOx N2Ostr UVstr N2O CH4 HOx NO2 Cly ClO OH + NO2 + M  HNO3 + M ClO + NO2 + M  ClONO2 + M

Follow-up papers and how ODPN2O might change Ravishankara et al. (2009): Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century Follow-up papers and how ODPN2O might change Ravishankara et al. (2009): now: 0.017 2100: 0.026 +50% calculating with 1959 Cly concentrations; Flemming et al. (2011): now: 0.019 2100: 0.018 -5% one medium WMO scenario; 2D-model Ravell et al. (2015): now: 0.015 2100: 0.015-0.030 +0% - +100% 9 simulations of a medium-high scenario; 3D-model Ravell et al. (2015) Figure 1