(78b) Total Global Warming Potential: An Expanded Definition of GWP

Authors: 
Canter, C., The University of Arizona
Galvan, D. D., Univ. of Arizona


Total Global Warming Potential: An Expanded Definition of GWP

Paul Blowers,
Christina Canter, Daniel David Galvan

Global warming potential was created as a referenced value
for comparing chemicals that may contribute to global climate change on a
convenient basis.  The formal generally
accepted definition is:

GWP1.JPG

where TH is an arbitrary time horizon,
normally chosen to be 20, 100, or 500 years, ai is the radiative
forcing of a compound, i, defined as the energy absorbed per square meter of
atmosphere per ppb of concentration of compound i, and xi(t) is the
time dependent concentration time profile for chemical i.  This definition inherently assumes that the
parent compound i is the only species that will contribute to capturing energy
and it is removed from the atmosphere by hydroxyl radical attack, the dominant
removal mechanisms.  However, larger
compounds may fracture apart into daughter species that may form stable
compounds which go on to have their own atmospheric lifetimes and radiative
forcing values that may additionally contribute to global climate change.  This would change the above equation to:

GWP2.JPG

where i is the parent species and j represents
the various daughter species which may temporally form and dissipate.  The expanded definition opens up the need for
other data that may be experimentally inaccessible due to the difficulty of
examining total degradation mechanisms and in identifying other removal
mechanisms like dissolution into raindrops and then rain-out of species.  This work applies quantum chemical
calculations to evaluate several candidate species to highlight when it is
important to use an expanded GWP definition to capture potential environmental
liabilities of compounds.  Where
experimental data are unavailable, quantum chemical calculations are used to
predict intermediate values for radiative forcing and kinetic degradation rate
constants.