Atmospheric Chemistry Model

Model description

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, NO_x and NMVOC, and will determine 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, δ ln(OH)/ δ 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 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:

CH₄+ OH- → 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:
t _CH4 = atmospheric lifetime of CH₄ (yr)
t _chemical = chemical lifetime of CH₄ (yr)
t _{stratospheric} = stratospheric lifetime of CH₄(yr)
t _{soil loss} = soil-loss lifetime of CH₄ (yr)

The rate of change of the CH₄ concentration is calculated as:

with:
C_CH4 = the atmospheric concentration of methane (ppm)
E_CH4 = 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 on the atmospheric removal, as determined by its atmospheric lifetime. However, for N₂O, CCl₃, the CFCs, bromocarbons and PFCs, the chemical lifetime is assumed to be constant, as adopted in most of the 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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