Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.2.3.1 Atmosphere ====

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The tropospheric abundance of N <sub>2</sub> O was 332.1 ± 0.4 ppb in 2019 (Figure 5.15), which is 23% higher than pre-industrial levels of 270.1 ± 6.0 ppb ( ''robust evidence, high agreement'' ). Current estimates are based on atmospheric measurements with high accuracy and density ( [[#Francey--2003|Francey et al., 2003]] ; [[#Elkins--2018|Elkins et al., 2018]] ; [[#Prinn--2018|Prinn et al., 2018]] ), and pre-industrial estimates are based on multiple ice-core records [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.2.3|Section 2.2.3.2.3]] ). The average annual tropospheric growth rate was 0.85 ± 0.03 ppb yr <sup>–1</sup> <sub></sub> during the period 1995 to 2019 (Figure 5.15a). The atmospheric growth rate increased by about 20% between the decade 2000–2009 and the most recent decade of 2010–2019 (0.95 ± 0.04 ppb yr <sup>–1</sup> ) ( ''robust evidence, high agreement'' ). The growth rate in 2010–2019 was also higher than during 1970–2000 (0.6–0.8 ppb yr <sup>–1</sup> ; [[#Ishijima--2007|Ishijima et al., 2007]] ) and the 30-year period prior to 2011 (0.73 ± 0.03 ppb yr <sup>–1</sup> ), as reported by AR5. New evidence since AR5 (WGI, Section 6.4.3) confirms that, in the tropics and subtropics, large interannual variations in the atmospheric growth rate are negatively correlated with the multivariate ENSO index (MEI) and associated anomalies in land and ocean fluxes ( [[#Ji--2019|Ji et al., 2019]] ; [[#Thompson--2019|Thompson et al., 2019]] ; S. [[#Yang--2020|]] [[#Yang--2020|Yang et al., 2020]] ) (Figure 5.15a).

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'''Figure 5.15 |''' '''Changes in atmospheric nitrous oxide (N''' <sub>2</sub> '''O) and its isotopic composit''' '''ion since 1940''' . '''(a)''' Atmospheric N <sub>2</sub> O abundance (parts per billion, ppb) and growth rat (ppb yr <sup>–1</sup> ); '''(b)''' δ <sup>15</sup> N of atmospheric N <sub>2</sub> O; and '''(c)''' alpha-site <sup>15</sup> N–N <sub>2</sub> O. Estimates are based on direct atmospheric measurements in the Advanced Global Atmospheric Gases Experiment (AGAGE), Commonwealth Scientific and Industrial Research Organisation (CSIRO), and National Oceanic and Atmospheric Administration (NOAA) networks ( [[#Prinn--2000|Prinn et al., 2000]] , 2018; [[#Francey--2003|Francey et al., 2003]] ; [[#Hall--2007|Hall et al., 2007]] ; [[#Elkins--2018|Elkins et al., 2018]] ), archived air samples from Cape Grim, Australia ( [[#Park--2012|Park et al., 2012]] ), and firn air from the North Greenland Ice Core Project (NGRIP) Greenland and H72 Antarctica ( [[#Ishijima--2007|Ishijima et al., 2007]] ), Law Dome Antarctica ( [[#Park--2012|Park et al., 2012]] ), as well as a collection of firn ice samples from Greenland ( [[#Prokopiou--2017|Prokopiou et al., 2017]] , 2018). Shading in (a) is based on the multivariate El Niño–Southern Oscillation (ENSO) index, with red indicating El Niño conditions ( [[#Wolter--1998|Wolter and Timlin, 1998]] ). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

As assessed by SRCCL ( [[#IPCC--2019a|IPCC, 2019a]] ), combined firn, ice, air and atmospheric measurements show that the <sup>15</sup> N/ <sup>14</sup> N isotope ratio ( ''robust evidence'' , ''high agreement'' ) and the predominant position of the <sup>15</sup> N atom in atmospheric N <sub>2</sub> O ( ''limited evidence'' , ''low agreement'' ) in N <sub>2</sub> O has changed since 1940 (Figure 5.15b, c) whereas they were relatively constant in the pre-industrial period ( [[#Ishijima--2007|Ishijima et al., 2007]] ; [[#Park--2012|Park et al., 2012]] ; [[#Prokopiou--2017|Prokopiou et al., 2017]] , 2018). The SRCCL concluded that this change indicates a shift in the nitrogen-substrate available for denitrification, and the relative contribution of nitrification to the global N <sub>2</sub> O source ( ''robust evidence'' , ''high agreement'' ), which are associated with increased fertilizer use in agriculture ( [[#Park--2012|Park et al., 2012]] ; [[#Snider--2015|Snider et al., 2015]] ; [[#Prokopiou--2018|Prokopiou et al., 2018]] ).

Since AR5 (WGI, Section 6.4.3), the mean atmospheric lifetime of N <sub>2</sub> O has been revised to 116 ± 9 years ( [[#Prather--2015|Prather et al., 2015]] ). The small negative feedback of the N <sub>2</sub> O lifetime to increasing atmospheric N <sub>2</sub> O results in a slightly lower residence time (109 ± 10 years) of N <sub>2</sub> O perturbations compared with that assessed by AR5 (118–131 years) ( [[#Prather--2015|Prather et al., 2015]] ). The dominant N <sub>2</sub> O loss occurs through photolysis and oxidation by O(1D) radicals in the stratosphere and amounts to approximately 13.1 (12.4–13.6) TgN yr <sup>–1</sup> ( [[#Minschwaner--1993|Minschwaner et al., 1993]] ; [[#Prather--2015|Prather et al., 2015]] ; [[#Tian--2020|Tian et al., 2020]] ).

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