The Atmospheric Chemistry Model of IMAGE 2.2 (ACM) calculates the concentrations of the most important greenhouse gases and other reactive gases. ACM uses the sum of the emissions from natural and land-use related sources generated by the Land Use Emissions Model (LUEM), and emissions from energy systems and industrial activities from the TIMER Emissions Model (TEM). The output of ACM - gas concentrations - is the input for the Upwelling-Diffusion Climate Model of IMAGE 2.2 (UDCM).
The radiative forcing by aerosols are calculated directly from the sulphate emissions in UDCM. The stratospheric ozone forcing is calculated from the concentrations of all chlorine- and bromine-containing compounds, calculated in ACM.
The input and output of ACM are given below:
| Model input | Emissions of CH4, N2O, NOx, CO, NMVOC, CFCs, CCs, HCFCs, bromocarbons, PFCs, SF6 and HFCs |
| Model output | Concentrations of CH4, N2O, CO, tropospheric ozone, CFCs, CCs, HCFCs, bromocarbons, PFCs, SF6, HFCs and the OH-radical
Chemical and atmospheric lifetime of CH4 |
For an explanation of compounds and groups of compounds, see definitions - chemical compounds.
The hydroxyl radical (OH) is the primary oxidant of atmospheric CH4, CO and more complex hydrocarbon compounds (like NMVOCs) whereby the NOx concentration determines the reaction pathway. Therefore the OH concentration will depend on the emissions of CH4, CO, NOx and NMVOC, and determines the lifetimes of these compounds. In ACM, this dependency is represented by the linear interpolation mentioned in Table 4.11 in TAR (IPCC, 2001). The most important factor in this interpolation is the OH feedback, 
ln(OH)/ln(CH4), having a value of -0.32 (this is called the chemical feedback parameter). This means that tropospheric OH abundancy declines by 0.32% for every 1% increase in CH4. A similar interpolation with dependencies of CH4, NOx, CO and NMVOC is used for calculating of the increase in tropospheric ozone concentration (IPCC, 2001).
The OH concentration has a direct effect on the chemical lifetime of CH4 through the chemical reaction:
 CH3· + H2O |
The chemical lifetime of CH4 can be calculated from the rate of this reaction. Assuming a stratospheric lifetime of 120 years and a soil-loss lifetime of 160 years (taken from IPCC, 2001), the total atmospheric lifetime of CH4 is calculated as follows:
 |
with:
CH4 the atmospheric lifetime of CH4 (yr)
chemical = chemical lifetime of CH4 (yr)
stratospheric = stratospheric lifetime of CH4(yr)
soil loss = soil loss lifetime of CH4 (yr)
The rate of change of the CH4 concentration is calculated as:
 |
with:
CCH4 = the atmospheric concentration of methane (ppm)
ECH4 = the annual methane emissions (Tg/yr)
cv = a mass-to-concentration conversion factor (ppm/Tg)
The same approach as for methane is used for nitrous oxide (N2O), the CFCs, CCs, HCFCs, bromocarbons, PFCs, SF6 and HFCs. Thus the change in concentrations depends on the change in both emissions and the atmospheric removal, determined by its atmospheric lifetime. However, for N2O, CCl3, the CFCs, bromocarbons and PFCs, the chemical lifetime is assumed constant, as adopted in most simple climate models currently used (Harvey et al., 1997).
The calculation of the CO concentration is slightly modified from the approach used in IMAGE 2.0 (Krol and Van der Woerd, 1994). On the basis of recent literature, the CO yield factor for NMVOC emissions is 0.4; the lifetime of CO due to soil uptake and stratospheric loss is 1.10 year (Müller and Brasseur, 1995).
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