CO – Carbon monoxide

CO molecule

What is CO?

CO is an important gas used as tracer for air pollution and with large connections to climate. It has a relatively long lifetime (a few weeks to a few months depending on latitude and time of year).

How is CO produced?

CO is primarily released in the atmosphere by incomplete combustion of biomass and fossil fuels (e.g. transport, heating, industrial activities and biomass burning). In the atmosphere, oxidation of CH4 – Methane and non-methane hydrocarbons (NMHC), like isoprene, accounts for nearly half of the global CO production (Brenninkmeijer and Novelli, 2003, Duncan et al., 2007). Linked with biomass burning, CO concentrations can exhibit large spatial (with concentrations varying by more than one order of magnitude between background and source regions) and inter-annual variations, while some industrialised regions (e.g. East-China) are permanent emission hot-spots. Sources of secondary importance include emission by vegetation and oceans.

At surface level, the volume mixing ratios range from a background concentration of 50 ppbv to excess of 700 ppbv where high emissions occur. Large uncertainties remain in the estimated strengths of both natural and anthropogenic sources. In the lower atmosphere, the lifetime of CO varies from a few weeks to a few months, depending on time of the year and the latitude. Its observation allows the characterization of both emission sources and atmospheric transport of pollution plumes (Logan, 1981). In the upper troposphere, CO can also be transported across the tropical tropopause.

Standard atmospheric CO profile, with the different sources of production (blue) and destruction / sinks (red) as a function of altitude. The CO profile was constructed from averaged ACE-FTS data over China and completed with TES observations over the same area below 6 km. The vertical sensitivity of each CO sounding type of instrument is also reported on the right-hand side of this plot (Clerbaux et al., 2008).

The removal of CO is largely (for as much as 90 %) determined by the reaction with the hydroxyl (OH) radical. The remaining 10 % is removed by soils (Brenninkmeijer and Novelli, 2003)

In the stratosphere, CO is produced by the oxidation of methane and is converted to CO2 – Carbon dioxide by reaction with OH. Above 50 km, in the mesosphere and thermosphere, photolysis of CO2 is the main source of CO, which reaches a concentration of 5-20 ppmv at 80 km. At these altitudes, CO is also a useful dynamical tracer which can be used to study atmospheric transport processes, and, in particular, upward transport in high altitude summer regions and downward transport in the high latitude winter regions (Solomon et al., 1985).

Why shall we observe atmospheric CO?

CO – Carbon monoxide is an important precursor of tropospheric O3 – ozone in the Earth atmosphere and poses serious health risk. Apart from its relevance to air quality as a primary pollutant, CO is the single largest sink of the hydroxyl radical (OH) and therefore affects the oxidizing capacity of our atmosphere. It is responsible for about 75% of the OH sink (Clerbaux et al., 2008). It hence influences concentrations of other species such as CH4 – Methane and tropospheric O3 – ozone and causes an indirect radiative effect. Therefore, CO prevents the atmosphere from removing other pollutants.

A typical CO satellite map?

IASI total CO-column (from MetOp-A & B) processed by LATMOS and ULB using the Fast Optimal Retrievals on Layers for IASI (FORLI) algorithm. More maps are made available by the EUMETSAT satellite application facility on atmospheric composition monitoring named AC SAF (Source:

IASI instrument, a nadir-looking high resolution Fourier Transform Spectrometer (FTS), allows to observe CO from the Thermal InfraRed (TIR) spectral band. It flies on the polar-orbiting MetOp (the 1st being launched on 19 October 2006), as a component of the space segment of the Eumetsat Polar System (EPS).

The map above shows high concentrations over central Africa caused by biomass burning while over China, it is related to industrial processes.

Some reference CO satellite missions / products?

In addition to the IASI instrument, several space-borne sensors allow to observe CO. Here is a non-exhaustive list:

  • ACE-FTS current mission (Canada), Thermal-InfraRed (TIR)
  • TES, on-board NASA EOS-AURA, TIR
  • MOPITT, on-boar the NASA Terra platform, early morning, TIR
  • ESA past mission MIPAS, on-board ESA ENVISAT, early morning (10:00)
  • MLS, on-board NASA EOS-AURA, mid-afternoon,
  • AIRS, early morning, Thermal InfraRed band
  • ESA past mission SCIAMACHY, on-board ESA ENVISAT, early morning (10:00),  Short Wave InfraRed (SWIR) band (also available on TEMIS website)
  • TROPOMI mission, on-board Sentinel-5 Precursor, mid-afternoon, SWIR


More information?

  • The ESA SIROCCO project, devoted to CO and CH4 spectral synergy retrieval here
  • The near-real time IASI CO product delivered by the EUMETSAT Atmospheric Composition SAF here


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