I believe that, for CO_2, we used the specific emissions factor for the particular fuel listed in the table. (Eg, for solids, we used the emissions factor for industrial coal). These emissions factors are reported in DUKES (the Digest of UK Energy Statistics, published by DECC).
It turns out that emissions factors expressed as emissions per unit of energy (rather than per unit of mass or volume) are reasonably constant for all hydrocarbon fuels of a particular physical state – certainly within the limits of uncertainty expected of a model like this.
For the other gases, CH_4 and N_2O, we used the ratio of these gases emitted to CO_2 emitted from combustion (of the appropriate fuel) for the total UK emissions, as reported in the Greenhouse Gas Inventory, and assumed this ratio applied to all sources. In summary: for each physical state of hydrocarbons, we first estimated CO_2 emission as above, then applied a constant multiplier to obtain the emissions of the other gasses.
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