### Other atmospheric characteristics

#### Indicators:

- Lifetimes of CH4
- OH· concentration
- CO concentration

| #### Lifetimes of CH4
| unit: year dimension: chemical and atmospheric lifetime

The chemical lifetime of CH4 is determined by the amount of OH· radicals in the troposphere, giving the reaction:

The reaction rate of this reaction determines the loss-rate of CH4 and thus, the chemical lifetime of CH4.

The atmospheric lifetime of CH4 is an inverse sum of the chemical lifetime and the lifetimes due to stratospheric loss (assumed constant at 120 years) and soil-loss (assumed constant at 160 years):

According IPCC (2001), the chemical lifetime is considered 9.6 years in 1998, leading to an atmospheric lifetime of 8.4 years.

| #### OH· concentration
| unit: index dimension: -

The OH· concentration presented is the global mean concentration relative to 1990. The OH· radical is the main oxidant of the troposphere. Because of its high activity, the atmospheric lifetime of OH· is very short. The OH· radical is a fragment of the water molecule, to which it can revert by abstracting a hydrogen atom from a nearby molecular target. This explains the large abundance and high activity of OH· in the troposphere.

In IMAGE 2.4, OH· is considered a product of the reactions with the tropospheric compounds CH4, CO, NOx and NMVOC. These reactions are part of a complex reaction cycle with many regional differences. For example, the oxidation of CO and CH4 produces OH· at low NOx levels, while in areas with high NOx levels the OH· level decreases due to reactions with CH4 and CO. These interactions are taken into account with global mean sensitivity coefficients. The resulting OH· concentration determines the chemical lifetime of CH4 and the depletion factor of many tropospheric compounds like CO, HCFCs and HFCs.

| #### CO concentration
| unit: ppmv dimension: -

The CO concentration is the global mean atmospheric concentration. CO is not a greenhouse gas, but it is an important compound which influences the OH· concentration and is involved in ozone chemistry.

CO is a short-lived gas which responds rapidly to changes in its sources and sinks and contributes to formation of tropospheric ozone. Besides the CO oxidation by OH·, CO is removed from the troposphere by soil uptake and transport to the stratosphere. The loss factor of CO is set to 0.901 per year, according to a loss of 100 Tg per year to the stratosphere and 200 Tg per year due to dry deposition.

The sources of CO include direct emissions (e.g., biomass burning) and atmospheric oxidation of methane and non-methane volatile hydrocarbons (NMVOC). The CO generation from oxidation of NMVOC is calculated with an average yield factor of 0.4 (mass/mass).