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

==== 6.3.2.1 Tropospheric Ozone ====

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About 10% of the total atmospheric ozone column resides in the troposphere. The ozone forcing on climate strongly depends on its vertical and latitudinal distribution in the troposphere. The lifetime of ozone in the troposphere ranges from a few hours in polluted urban regions to up to few months in the upper troposphere. Observed tropospheric ozone concentrations range from less than 10 ppb over the tropical Pacific Ocean to as much as 100 ppb in the upper troposphere and more than 100 ppb downwind of major ozone precursor emissions regions. An ensemble of five CMIP6 models including whole atmospheric chemistry and interactive ozone has been shown to simulate consistently the present-day ozone distribution (north to south and latitudinal gradients) and its seasonal variability when compared with observations from sondes, background surface stations and satellite products ( [[#Griffiths--2021|Griffiths et al., 2021]] ). The biases, whose magnitude is similar to AR5, are lower than 15% against climatological seasonal cycles from ozonesondes with an overestimate in the Northern Hemisphere and an underestimate in the Southern Hemisphere ( [[#Griffiths--2021|Griffiths et al., 2021]] ). The CMIP6 multi-model ensemble estimate of the global mean lifetime of ozone for present-day conditions is 25.5 ± 2.2 days ( [[#Griffiths--2021|Griffiths et al., 2021]] ), which is within the range of previous multi-model estimates ( [[#Stevenson--2006|Stevenson et al., 2006]] ; [[#Young--2013|Young et al., 2013]] ), indicating a ''high level of confidence'' .

The AR5 assessed the tropospheric ozone burden to be 337 ± 23 Tg for the year 2000 based on the ACCMIP ensemble of model simulations ( [[#Myhre--2013|Myhre et al., 2013]] ). Multiple satellite products, ozonesondes and CCMs are used to estimate tropospheric ozone burden (Table 6.3). Satellite products provide lower-bound values as they exclude regions under polar night conditions ( [[#Gaudel--2018|Gaudel et al., 2018]] ). The tropospheric ozone burden values from multi-model exercises are within the range of the observational estimates despite different definitions of the tropopause for multi-model estimates which can lead to differences of about 10% on the ozone-burden model estimates ( [[#Griffiths--2021|Griffiths et al., 2021]] ). Weighted by their number of members, CMIP6 and CCMI multi-model estimates and observational estimates of tropospheric ozone burden in about the year 2010, lead to an assessment of the tropospheric ozone burden of 347 ± 28 Tg for 2010.

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'''Table 6.3 |''' '''Global tropospheric ozone budget terms and burden based on multi-model estimates and observations for present conditions.''' All uncertainties quoted as '''±''' 1 standard deviation. Values of tropospheric ozone burden with asterisk indicate average over the latitudinal zone 60 <sup>°</sup> N–60 <sup>°</sup> S. STE = stratospheric–tropospheric exchange.

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

| '''Burden'''

'''(Tg)'''

| '''Production'''

'''Tg y''' '''r''' <sup>–1</sup>

| '''Loss'''

'''Tg y''' '''r''' <sup>–1</sup>

| '''Deposition'''

'''Tg y''' '''r''' <sup>–1</sup>

| '''STE'''

'''Tg y''' '''r''' <sup>–1</sup>

| '''Number of Models/Reference'''

|-
| colspan="7"| Models

|-
| ~2000

time slice (1995–2004)

| 347 ± 30

| 4510 ± 566

| 3948 ± 379

| 846 ± 44

| 284 ± 193

| rowspan="2"| CMIP6 <sup>a</sup> (5 Earth system models for burden and 4 models for budget terms)

( [[#Griffiths--2021|Griffiths et al., 2021]] )

|-
| ~2010

time slice (2005–2014)

| 356 ± 31

| 4708 ± 589

| 4122 ± 399

| 863 ± 40

| 277 ± 201

|-
| ~2000

| 341 ± 31

(309 ± 31)*

|
| rowspan="2"| CCMI <sup>b</sup> (9 models) ( [[#Archibald--2020|Archibald et al., 2020]] )

|-
| 2010

| 345 ± 30

(314 ± 29)*

|
|-
| '''~''' 2000

| 340 ± 34

| 4937 ± 656

| 4442 ± 570

| 996 ± 203

| 535 ± 161

| TOAR <sup>c</sup> (based on 32–49 models participating in inter-model comparisons and single-model studies)

( [[#Young--2018|Young et al., 2018]] )

|-
| colspan="7"| Observations

|-
| 2010–2014

| 338 ± 6

|
| TOST <sup>d</sup> , IASI <sup>e</sup> -FORLI, and IASI-SOFRID

( [[#Gaudel--2018|Gaudel et al., 2018]] )

|-
| 2010–2014

| 302 ± 12*

|
| TOST, IASI-FORLI, IASI-SOFRID, OMI <sup>f</sup> /MLS, OMI-SAO and OMI-RAL

( [[#Gaudel--2018|Gaudel et al., 2018]] )

|}

<sup>a</sup> CMIP6: Coupled Model Intercomparison Project Phase 6; <sup>b</sup> CCMI: Chemistry–Climate Model Initiative; <sup>c</sup> toAR Tropospheric Ozone Assessment Report; <sup>d</sup> toST Trajectory-mapped Ozonesonde dataset for the Stratosphere and Troposphere; <sup>e</sup> IASI Infrared Atmospheric Sounding Interferometer; <sup>f</sup> OMI Ozone Monitoring Instrument.

The tropospheric ozone budget is controlled by chemical production and loss, by stratospheric–tropospheric exchange (STE), and by deposition at the Earth’s surface, whose magnitude are calculated by CCMs (Table 6.3). Despite The high agreement of the model ensemble mean with observational estimates in the present-day tropospheric ozone burden, the values of individual budget terms can vary widely across models in CMIP6, consistent with previous model intercomparison experiments ( [[#Young--2018|Young et al., 2018]] ). Furthermore, single-model studies have shown that the halogen chemistry, which is typically neglected from model chemistry schemes in CCMs, may have a notable impact on the ozone budget, as halogens, particularly of marine origin, take part in efficient ozone-loss catalytic cycles in the troposphere (Saiz-Lopez et al. , 2012; Sarwar et al. , 2015; Sherwen et al. , 2016) .

Because of the heterogeneous distribution of ozone, limited observations or proxies do not provide accurate information about the global pre-industrial abundance, posing a challenge to the estimation of the historical evolution of tropospheric ozone. Therefore, global CCMs complemented by observations are relied upon for estimating the long-term changes in tropospheric ozone. The AR5 concluded that anthropogenic changes in ozone precursor emissions are unequivocally responsible for the increase in tropospheric ozone between 1850 and the present ( [[#Myhre--2013|Myhre et al., 2013]] ). Based on limited isotopic evidence, [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] assesses that the global tropospheric ozone increased by less than 40% between 1850 and 2005 ( ''low confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.5.3|Section 2.2.5.3]] ). The CMIP6 models are in line with this increase of tropospheric ozone with an ensemble-mean value of 109 ± 25 Tg (model range) from 1850–1859 to 2005–2014 (Figure 6.4). This increase is higher than the AR5 value of 100 ± 25 Tg from 1850–2010 due to higher ozone precursor emissions in CMIP6. However, the AR5 and CMIP6 values are close when considering the reported uncertainties. The uncertainties are equivalent in CMIP6 and AR5 despite enhanced inclusion of coupled processes in the CMIP6 ESMs (e.g., biogenic NMVOC emissions or interactive stratospheric ozone chemistry).

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[[File:5358ba242352e968d18a4ead22513723 IPCC_AR6_WGI_Figure_6_4.png]]

'''Figure 6.4 |''' '''Time evolution of global annual mean tropospheric ozone burden (in Tg) from 1850 to 2100.''' Multi-model means for CMIP6 historical experiment (1850–2014) from UKESM1-LL-0, CESM2-WACCM, MRI-ESM2-0, GISS-E2.1-G and GFDL-ESM4 and for ScenarioMIP SSP3-7.0 experiment (2015–2100) are represented with their inter-model standard deviation (±1 standard deviation, shaded areas). Observation-based global tropospheric ozone burden estimate (from Table 6.3) is for 2010–2014. Tropospheric Ozone Assessment Report (TOAR) multi-model mean value (from Table 6.3) is for 2000 with a ±1 standard deviation error-bar. Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) multi-model means are for 1850, 1930, 1980 and 2000 time slices with ±1 standard deviation error-bars. The troposphere is masked by the tropopause pressure calculated in each model using the WMO thermal tropopause definition. Further details on data sources and processing are available in the chapter data table (Table 6.SM.3).

Since the mid-20th century, the CMIP6 model ensemble shows a higher global trend (Figure 6.4). Since the mid-1990s, the trends are better documented by observations ( [[IPCC:Wg1:Chapter:Chapter-2#2.2.5.3|Section 2.2.5.3]] ) and indicate spatial heterogeneity. In particular, in situ observations at remote surface sites and in the lower free troposphere indicate positive trends that are far more common than negative trends, especially in the northern tropics and across Southern and Eastern Asia (Figure 6.5). The CMIP6 ensemble and observations largely agree on the magnitude of the global positive trend since 1997 (0.82 ± 0.13 Tg yr <sup>–1</sup> in the model ensemble; 0.70 ± 0.15 Tg yr <sup>–1</sup> in the ozonesonde dataset; 0.83 ± 0.85 Tg yr <sup>–1</sup> in the satellite ensemble) and qualitatively reproduce positive trends in the Southern Hemisphere ( [[#Griffiths--2021|Griffiths et al., 2021]] ). More analyses are needed for evaluation in other parts of the world to assess the skill of the recent ensemble based on CMIP6 emissions.

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[[File:ce65b712100397d7960e48942eac57b6 IPCC_AR6_WGI_Figure_6_5.png]]

'''Figure 6.5 |''' '''Decadal tropospheric ozone trends''' '''since 1994.''' Trends are shown at 28 remote and regionally representative surface sites ( [[#Cooper--2020|Cooper et al., 2020]] ) and in 11 regions of the lower free troposphere (650 hPa, about 3.5 km) as measured by In-Service Aircraft for a Global Observing System (IAGOS) above Europe, north-eastern USA, south-eastern USA, western North America, north-east China, South East Asia, southern India, the Persian Gulf, Malaysia/Indonesia, the Gulf of Guinea and northern South America ( [[#Gaudel--2020|Gaudel et al., 2020]] ). High-elevation surface sites are &gt;1500 m above sea level. All trends end with the most recently available year but begin in 1995 or 1994. The sites and datasets are the same as those used in Figure 2.8, further details on data sources and processing are available in the [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] data table (Table 2.SM.1).

In summary, there is ''high confidence'' in the estimated present-day (about 2010) global tropospheric ozone burden based on an ensemble of models and observational estimates (347 ± 28 Tg), but there is ''medium confidence'' among the individual models for their estimates of the tropospheric ozone-related budget terms.

Evidence from successive multi-model intercomparisons and the limited isotopic evidence agree on the magnitude of the increase of the tropospheric ozone burden from 1850 to the present day in response to anthropogenic changes in ozone precursor emissions corroborating AR5 findings. This increase is assessed to be 109 ± 25 Tg ( ''medium confidence'' ). The CMIP6 model ensemble shows a constant global increase since the mid-20th century whose rate is consistent with that derived from observations since the mid-1990s.

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