### Atmospheric composition

#### Indicators:

CO2 concentration CH4 concentration | N2O concentration Tropospheric O3 concentration

This group of indicators presents the atmospheric concentrations of the most important greenhouse gases. The concentrations are calculated in the Atmospheric Chemistry Model (ACM) of IMAGE 2.4 on the basis of the emissions generated by the TIMER emissions model (TEM) and Land Use Emissions Model (LUEM). The global mean concentrations are used as input of the Upwelling-Diffusion Climate Model (UDCM) of IMAGE 2.4.

| #### CO2 concentration
| unit: ppmv (parts per million by volume) dimension: none

CO2 is inert in the troposphere. Hence, its concentration is determined by the balance between sources and sinks of CO2. Sources of atmospheric CO2 include combustion of fossil fuels and deforestation. Sinks for CO2 are uptake by vegetation and oceans. The strength of these sources and sinks is calculated by the ocean carbon model (OCM) and terrestrial carbon model (TCM).

| #### CH4 concentration
| unit: ppmv (parts per million by volume) dimension: none

CH4 has a shorter lifetime than CO2, because it is oxidized by OH radicals in the troposphere. The concentration of OH and thus the chemical lifetime of CH4 are temporally variable. Other sinks for CH4 are oxidation in soils and loss to the stratosphere. The lifetimes due soil oxidation (160 years) and loss to the stratosphere (160 years) are assumed to be constant.

The atmospheric lifetime of CH4 calculated from the three lifetimes determines the annual loss rate.

The various energy, industry and land-use related sources of CH4 are calculated by the TIMER emission model (TEM) and land use emissions model (LUEM). The natural emissions are considered constant at a level of 232 Tg per year.

| #### N2O concentration
| unit: ppbv (parts per billion by volume) dimension: none

N2O is inert in the troposphere. Its main sink is photodissociation in the stratosphere, which is proportional to the tropospheric concentration divided by the lifetime of N2O, which is 120 years. Possible uptake of N2O by soils, which is considered a minor sink, is not included in this lifetime. The various energy, industry and land-use related sources of N2O are calculated by the TIMER emission model (TEM) and land use emissions model (LUEM). A nmuber of natural sources (including oceans) of N2O are assumed constant in time.

| #### Tropospheric O3 concentration
| unit: DU (dobson unit) dimension: none

Ozone (O3) is not homogeneously distributed over the troposphere. The main sources of tropospheric O3 are transport from the stratosphere and photo-oxidation of CO, CH4 and other hydrocarbons in the presence of NOx. Sinks for tropospheric O3 are destruction at the Earth's surface and reaction with OH radicals. In the atmospheric chemistry model (ACM) of IMAGE 2.4, these reactions are not explicitly included. The concentration of O3 in the troposphere is calculated with tropospheric sensitivity coefficients relating the change in CH4 concentration and the CO, NOx and NMVOC emissions.