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

==== 6.3.3.2 Carbon Monoxide (CO) ====

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About half of the atmospheric CO burden is due to its direct emissions and the remainder is due to the atmospheric oxidation of methane and NMVOCs. Reaction with OH is the primary sink of CO with a smaller contribution from dry deposition.

Since AR5, advances in satellite retrievals (e.g., Worden et al. , 2013; Warner et al. , 2014; Buchholz et al. , 2021) , ground-based column observations (e.g., Zeng et al. , 2012; Té et al. , 2016) , airborne platforms (e.g., [[#Cohen--2018|Cohen et al., 2018]] ; [[#Petetin--2018|Petetin et al., 2018]] ), surface measurement networks (e.g., Andrews et al. , 2014; Schultz et al. , 2015; Prinn et al. , 2018; Pétron et al. , 2019) and assimilation products (e.g., [[#Deeter--2017|Deeter et al., 2017]] ; [[#Flemming--2017|Flemming et al., 2017]] ; [[#Zheng--2019|Zheng et al., 2019]] ) have resulted in better characterization of the present-day atmospheric CO distribution. Typical annual mean surface CO concentrations range from around 120 ppb in the Northern Hemisphere to around 40 ppb in the Southern Hemisphere ( [[#Pétron--2019|Pétron et al., 2019]] ). The sub-regional patterns in CO reflect the distribution of emissions sources. Seasonal hotspots are linked to areas of biomass burning in tropical South America, equatorial Africa, South East Asia and Australia. A study using data assimilation techniques estimates a global mean CO burden of 356 ± 27 Tg over the 2002–2013 period ( [[#Gaubert--2017|Gaubert et al., 2017]] ).

Global models generally capture the global spatial distribution of the observed CO concentrations but have regional biases of up to 50% (e.g., [[#Emmons--2020|Emmons et al., 2020]] ; [[#Horowitz--2020|Horowitz et al., 2020]] ). Despite updated emissions datasets, the global multi-model and single-model simulations persistently underestimate observed CO concentrations at northern high and mid-latitudes as well as in the Southern Hemisphere, but with smaller biases compared with that in the Northern Hemisphere (Naik et al. , 2013; Stein et al. , 2014; Monks et al. , 2015; Strode et al. , 2015). Models are biased high in the tropics, particularly over highly polluted areas in India and Eastern Asia ( [[#Strode--2016|Strode et al., 2016]] ; [[#Yarragunta--2017|Yarragunta et al., 2017]] ).

Estimates of global CO burden simulated by global models generally fall within the range of that derived from data assimilation techniques, though the spread across the models is large (Naik et al. , 2013; Stein et al. , 2014; Zeng et al. , 2015; Myriokefalitakis et al. , 2016) . There is a large diversity in model-simulated CO budget driven by uncertainties in CO sources and sinks, particularly those related to in situ production from NMVOCs and loss due to reaction with OH ( [[#Stein--2014|Stein et al., 2014]] ; [[#Zeng--2015|Zeng et al., 2015]] ; [[#Myriokefalitakis--2016|Myriokefalitakis et al., 2016]] ). Global CO budget analysis from a multi-model ensemble for more recent years, including results from the CMIP6 model runs, are not yet available.

Reconstructions of CO concentrations based on limited ice-core samples in the Northern Hemisphere high latitudes suggest CO mole fractions of about 145 ppb in the 1950s, which rose by 10–15 ppb in the mid- 1970s, and then declined by about 30–130 ppb by 2008 ( [[#Petrenko--2013|Petrenko et al., 2013]] ). The negative trends since the 1990s are often attributed to emissions regulations from road transportation in North America and Europe. Due to limited observations prior to the satellite era, long-term global CO trends are based on estimates from models. An increase of global CO burden of about 50% for the year 2000 relative to 1850 is found in CMIP6 ( [[#Griffiths--2021|Griffiths et al., 2021]] ).

The AR5 reported a global CO decline of about 1% yr <sup>–1</sup> based on satellite data from 2002–2010, but biases in instruments rendered ''low confidence'' in this trend. The AR5 also indicated a small CO decrease from in situ networks but did not provide quantitative estimates. New analysis of CO trends performed since AR5 and based on different observational platforms and assimilation products show a decline globally and over most regions during the last one to two decades with varying amplitudes partly depending on the period of analysis (Table 6.4). Inversion-based analysis attributes the global CO decline during the past two decades to decreases in anthropogenic and biomass-burning CO emissions despite probable increase in atmospheric CO chemical production (Gaubert et al. , 2017; Jiang et al. , 2017; Zheng et al. , 2019). Furthermore, [[#Buchholz--2021|Buchholz et al. (2021)]] report a slowdown in global CO decline in 2010–2018 compared to 2002–2010, although the magnitude and sign of this change in the trend varies regionally. Global models prescribed with emissions inventories developed prior to the CMIP6 inventory capture the declining observed CO trends over North America and Europe but not over Eastern Asia ( [[#Strode--2016|Strode et al., 2016]] ). CMIP6 models driven by CMIP6 emissions simulate a negative trend in global CO burden over the 1990–2020 period ( [[#Griffiths--2021|Griffiths et al., 2021]] ), however the simulated trends have not yet been evaluated against observations.

In summary, our understanding of present-day global CO distribution has increased since AR5 with newer and improved observations and reanalysis. There is ''high confidence'' that global CO burden is declining since 2000. Evidence from observational CO reanalysis suggests this decline is driven by reductions in anthropogenic CO emissions, however this is yet to be corroborated by global ESM studies with the most recent emissions inventories.

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