Editing IPCC:AR6/WGI/Chapter-6 (section)

==== 6.3.3.1 Nitrogen Oxides (NO <sub>x</sub> ) ====

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The distribution of tropospheric NO <sub>x</sub> is highly variable in space and time owing to its short lifetime coupled with highly heterogeneous emission and sink patterns. NO <sub>x</sub> undergoes chemical processing, including the formation of nitric acid (HNO <sub>3</sub> ), nitrate (NO <sup>–</sup> <sub>3</sub> ), and organic nitrates (e.g., alkyl nitrate and peroxyacyl nitrate), atmospheric transport, and deposition. Despite challenges in retrieving quantitative information from satellite observations ( [[#Duncan--2014|Duncan et al., 2014]] ; [[#Lin--2015|Lin et al., 2015]] ; [[#Lorente--2017|Lorente et al., 2017]] ; [[#Silvern--2018|Silvern et al., 2018]] ), improved accuracy and resolution of satellite-derived tropospheric NO <sub>2</sub> columns over the past two decades have advanced understanding of the global distribution, long-term trends and source attribution of NO <sub>x</sub> . Long-term average tropospheric NO <sub>2</sub> column based on multiple satellite-borne instruments (Figure 6.6a) reveals the highest NO <sub>2</sub> levels over the most populated, urbanized and industrialized regions of the world corresponding to high NO <sub>x</sub> emissions source regions ( [[#Krotkov--2016|Krotkov et al., 2016]] ; [[#Georgoulias--2019|Georgoulias et al., 2019]] ). Enhanced but highly variable NO <sub>2</sub> columns are also associated with biomass-burning regions as well as areas influenced by lightning activity ( [[#Miyazaki--2014|Miyazaki et al., 2014]] ; [[#Tanimoto--2015|Tanimoto et al., 2015]] ).

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[[File:54fa0416445da6c9329b02a4769f09b2 IPCC_AR6_WGI_Figure_6_6.png]]

'''Figure 6.6 |''' '''Long-term climatological mean (a) and time evolution (b) of tropospheric nitrogen dioxide (NO''' <sub>2</sub> ''') vertical column density.''' Data are from the merged GOME/SCIAMACHY/GOME-2 (TM4NO2A version 2.3) dataset for the period 1996–2016 ( [[#Georgoulias--2019|Georgoulias et al., 2019]] ). Time evolution of NO <sub>2</sub> column shown in panel (b) is normalized to the fitted 1996 levels for the 10 regions shown as boxes in panel (a). Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

Observational constraints derived from the isotopic composition of atmospheric nitrate inferred from ice cores provide evidence of increasing anthropogenic NO <sub>x</sub> sources since pre-industrial times ( [[#Hastings--2009|Hastings et al., 2009]] ; [[#Geng--2014|Geng et al., 2014]] ). Global NO <sub>x</sub> emissions trends in bottom-up inventories (Section 6.2.1) as well as model simulations of nitrogen deposition ( [[#Lamarque--2013a|Lamarque et al., 2013a]] ) are in qualitative agreement with these observational constraints. CMIP6 ESMs exhibit stable NO <sub>x4</sub> burden over the first half of the 20th century and then a sharp increase driven by a factor of three increase in emissions, however, the magnitude of this increase remains uncertain due to poor observational constraints on pre-industrial concentrations of NO <sub>x</sub> <sub></sub> ( [[#Griffiths--2021|Griffiths et al., 2021]] ).

The AR5 reported NO <sub>2</sub> decreases by 30–50% in Europe and North America, and increases by more than a factor of two in Asia, over the 1996–2011 period based on satellite observations ( [[#Hartmann--2013|Hartmann et al., 2013]] ). Extension of this analysis covering the time period up to 2015 reveals that NO <sub>2</sub> has continued to decline over the USA, Western Europe and Japan ( [[#Schneider--2015|Schneider et al., 2015]] ; [[#Duncan--2016|Duncan et al., 2016]] ; [[#Krotkov--2016|Krotkov et al., 2016]] ) because of effective fossil fuel NO <sub>x</sub> emissions controls (Section 6.2), although this rate of decline has slowed down post-2011 ( [[#Jiang--2018|Jiang et al., 2018]] ). Satellite observations also reveal a 32% decline in NO <sub>2</sub> column over China after peaking in 2011 (Figure 6.6b), consistent with declining NO <sub>x</sub> emissions (Section 6.2) due to the implementation of emissions-control strategies (de Foy et al. , 2016; Irie et al. , 2016; F. Liu et al. , 2016) . Over Southern Asia, tropospheric NO <sub>2</sub> levels have grown rapidly with increases of 50% during 2005–2015, largely driven by hotspot areas in India experiencing rapid expansion of the power sector ( [[#Duncan--2016|Duncan et al., 2016]] ; [[#Krotkov--2016|Krotkov et al., 2016]] ). Further analysis indicates that many parts of India have also undergone a reversal in NO <sub>2</sub> trends since 2011 that has been attributed to a combination of factors, including a slowdown in economic growth, implementation of cleaner technologies, non-linear NO <sub>x</sub> chemistry, and meteorological variability ( [[#Georgoulias--2019|Georgoulias et al., 2019]] ). Satellite data reveals spatially heterogeneous NO <sub>2</sub> trends over the Middle East with an overall increase over 2005–2010 and a decrease over large parts of the region after 2011–2012. The reasons for trend reversal within individual areas are diverse, including warfare, imposed sanctions, and air-quality controls ( [[#Lelieveld--2015a|Lelieveld et al., 2015a]] ; [[#Georgoulias--2019|Georgoulias et al., 2019]] ). Satellite-derived tropospheric NO <sub>2</sub> levels over Africa and Latin America do not show a clear trend; both increasing and decreasing trends are observed over large agglomerations in these regions since the early 2000s ( [[#Schneider--2015|Schneider et al., 2015]] ; [[#Duncan--2016|Duncan et al., 2016]] ).

In summary, global tropospheric NO <sub>x</sub> abundance has increased from 1850–2015 ( ''high confidence'' ). Satellite observations of tropospheric NO <sub>x</sub> indicate strong regional variations in trends over 2005–2015. There is ''high confidence'' that NO <sub>2</sub> has declined over the USA and Western Europe since the mid-1990s and increased over China until 2011. NO <sub>2</sub> trends have reversed (declining) over China beginning in 2012 and NO <sub>2</sub> has increased over Southern Asia by 50% since 2005 ( ''medium confidence'' ).

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'''Table 6.4 |''' '''Summary of the global CO trends based on model estimates and observations.'''

{| class="wikitable"
|-
! '''Analysis Period'''

! '''Trends: Regions'''

! '''Reference/Methodology'''

|-
| colspan="3"| '''Global/Hemispheric'''

|-
| 2003–2015

| –0.86% yr <sup>–1</sup>

| [[#Flemming--2017|Flemming et al. (2017)]] Model assimilating MOPITT

|-
| 2002–2013

| –1.4% yr <sup>–1</sup>

| [[#Gaubert--2017|Gaubert et al. (2017)]]

Model assimilating MOPITT

|-
| 2002–2018

| –0.50 ± 0.3% yr <sup>–1</sup> : 60°N–60°S (MOPITT)

–0.56 ± 0.3% yr <sup>–1</sup> ; <sup>–</sup> 0.61 ± 0.2% yr <sup>–1</sup> : 0°–60°N

–0.35 ± –0.3% yr <sup>–1</sup> ; -0.33±0.3% yr <sup>–1</sup> : 0°–60°S

| [[#Buchholz--2021|Buchholz et al. (2021)]]

Satellite Observations

MOPITT; AIRS

|-
| 2000–2017

| –0.32 ± 0.05% yr <sup>–1</sup>

| [[#Zheng--2019|Zheng et al. (2019)]]

Satellite Observations

MOPITT

|-
| 2003–2014

| around –2.5 to 0.5 ppb yr <sup>–1</sup> : Northern Hemisphere around –0.5 to 0 ppb yr <sup>–1</sup> : Southern Hemisphere

| [[#Flemming--2017|Flemming et al. (2017)]] NOAA Carbon Cycle Cooperative Global Air Sampling Network

|-
| 2001–2013

| –2.19 to –0.80 ppb yr <sup>–1</sup> : Northern Hemisphere (Upper Troposphere/Tropopause Layer)

| [[#Cohen--2018|Cohen et al. (2018)]] IAGOS Airborne

|-
| colspan="3"| '''Pacific/Tropics'''

|-
| 2004–2013

(Spring Mean)

| –2.9 ± 2.6 ppb yr <sup>–1</sup> : Mauna Loa (19.54°N, 155.58°W)

| [[#Gratz--2015|Gratz et al. (2015)]] Ground-based

|-
| 2004–2013

(Spring Mean)

| –2.6 ± 1.8 ppb yr <sup>–1</sup> : Sand Island Midway (28.21°N, 177.38°W)

| [[#Gratz--2015|Gratz et al. (2015)]] Ground-based

|-
| colspan="3"| '''Europe'''

|-
| 1996–2006

| –0.45 ± 0.16% yr <sup>–1</sup> : Jungfraujoch (46.6°N, 8.0°E)

–1.00 ± 0.24% yr <sup>–1</sup> : Zugspitze (47.4°N, 11.0°E)

–0.62 ± 0.19% yr <sup>–1</sup> : Harestua (60.2°N, 10.8°E)

0.61 ± 0.16% yr <sup>–1</sup> : Kiruna (67.8°N, 20.4°E)

| [[#Angelbratt--2011|Angelbratt et al. (2011)]] Ground-based

|-
| 2001–2011

May to Sep

| –3.1 ± 0.30 ppb yr <sup>–1</sup> : Pico Mt. Obs (38.47°N, 28.40°W) –1.4 ± 0.20 ppb yr <sup>–1</sup> : Mace Head, Ireland

| [[#Kumar--2013|Kumar et al. (2013)]] Ground-based

|-
| 2002–2018

| –-0.89 ± 0.1% yr <sup>–1</sup> : Europe (45°N–55°N, 0°E–15°E)

| [[#Buchholz--2021|Buchholz et al. (2021)]]

Satellite Observations

MOPITT

|-
| colspan="3"| '''North America'''

|-
| 2001–2010

| –2.5 ppb yr <sup>–1</sup> : Thompson Farm (43.11°N, 70.95°W)

–2.3 ppb yr <sup>–1</sup> : Mt. Washington (44.27°N, 71.30°W)

+2.8 ppb yr <sup>–1</sup> : Castle Springs (43.75°N, 71.35°W)

–3.5 ppb yr <sup>–1</sup> : Pack Monadnock (42.86°N, 71.88°W)

–2.8 ppb yr <sup>–1</sup> : Whiteface Mountain (44.40°N, 73.90°W)

–4.3 ppb yr <sup>–1</sup> : Pinnacle State Park (42.09°N, 77.21°W)

| [[#Zhou--2017|Zhou et al. (2017)]] Ground-based

|-
| 2004–2013

(Spring Mean)

| –3.2 ± 2.9 ppb yr <sup>–1</sup> : Mt. Bachelor Observatory

| [[#Gratz--2015|Gratz et al. (2015)]] Ground-based

|-
| 2004–2012

(Spring Mean)

| –2.8 ± 1.8 ppb yr <sup>–1</sup> : Shemya Island (55.21°N, 162.72°W)

| [[#Gratz--2015|Gratz et al. (2015)]] Ground-based

|-
| 2002–2018

| –0.85 ± 0.1%yr <sup>–1</sup> : Eastern USA (35°N–40°N, –95°E–75°E)

| [[#Buchholz--2021|Buchholz et al. (2021)]]

Satellite Observations

MOPITT

|-
| colspan="3"| '''Asia'''

|-
| 2005–2018

| –0.46 ± 0.14% yr <sup>–1</sup> : Eastern Asia

| [[#Zheng--2018a|Zheng et al. (2018a)]]

WDCGG Ground-based

|-
| 2005–2018

| –0.41 ± 0.09% yr <sup>–1</sup> : Eastern Asia

| [[#Zheng--2018a|Zheng et al. (2018a)]]

MOPITT

|-
| 2002–2018

| –1.18 ± 0.3% yr <sup>–1</sup> : (Northeast China 30°E–40°E, 110°E–123°E) –0.28 ± 0.2% yr <sup>–1</sup> : (North India 20°N–30°N, 70°E–95°E)

| [[#Buchholz--2021|Buchholz et al. (2021)]]

Satellite Observations

MOPITT

|}

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