The impact of the rise in atmospheric nitrous oxide on stratospheric ozone
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The impact of the rise in atmospheric nitrous oxide on stratospheric ozone This article belongs to Ambio’s 50th Anniversary Collection. Theme: Ozone Layer Rolf Mu¨ller
Published online: 21 November 2020
More than forty years ago Ambio published a paper that raised concerns about the increased use of fixed nitrogen as fertiliser in that the resulting increase in nitrous oxide (N2O) emissions to the atmosphere could result in a significant reduction of the Earth’s ozone shield (Crutzen and Ehhalt 1977). It presented another case of human activities at the Earth’s surface impacting on the stratospheric ozone layer. Today, it is known that atmospheric N2O, which is present in the atmosphere in 2020 at a mixing ratio of 332.8 ppb1, is not only important for stratospheric ozone but also constitutes the third most important long-lived greenhouse gas (after CO2 and CH4). N2O is essentially inert in the troposphere and has no significant sinks at the surface of the Earth. However, when transported to the stratosphere it will be broken down mainly via photolysis at short wavelengths (below 200 nm) N2 O þ hm ! N2 þ O 1 D :ðR1Þ To a lesser extent, N2O is also broken down by reaction with O(1D) N2 O þ O 1 D ! N2 þ O2 ðR2Þ and
N2 O þ O 1 D ! 2 NOðR3Þ where the primary source of O(1D) in the atmosphere is the photolysis of ozone (at wavelengths below 320 nm). Reaction R3 constitutes the major source of nitrogen oxides in the stratosphere, but less than 10% of the atmospheric N2O is converted to nitrogen oxides (e.g. Ravishankara et al. 2009; Portmann et al. 2012; Fleming et al. 2015). The breakdown in the stratosphere determines the atmospheric lifetime of N2O; the SPARC (Stratosphere-
troposphere Processes And their Role in Climate) lifetime assessment estimated the N2O lifetime to be 123 years (with a 2-r range of 104–152 years; SPARC 2013), while more recently Prather et al. (2015) recommend a slightly shorter lifetime of 116 ± 9 years, which is however consistent with the SPARC value within uncertainties. In the stratosphere, nitrogen oxides (NO and NO2) destroy ozone through the following catalytic cycle (Crutzen 1970; Johnston 1971) NO þ O3 ! NO2 þ O2 ðR4Þ NO2 þ O ! NO þ O2 ðR5Þ ... net O3 þ O ! 2 O2 ðC1Þ Cycle C1 constitutes the dominant catalytic ozone loss cycle in the extrapolar regions (Fig. 1). This cycle is essential for quantifying the stratospheric ozone budget and, consequently, the total column of ozone (e.g. Crutzen et al. 1995; Grooß et al. 1999). At altitudes below 20 km and above 45 km, ozone loss driven by the HOx catalytic cycle dominates ozone loss rates (Fig. 1; Crutzen et al. 1995; Grooß et al. 1999; Mu¨ller 2009; Portmann et al. 2012). The paper by Crutzen and Ehhalt (1977) was based on the discussions in the Nobel symposium No. 38, ‘‘Nitrogen - An essential life factor and a growing environmental hazard’’ in 1976, the proceedings of which were published as well in Ambio (Bolin and Arrhenius 1977). At that time, many aspects of atmospheric N2O were not known and the knowledge o
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