Bit unusual to need to know CO2 output figures. Since CO2 + water (steam) are the resultant gasses produced during complete combustion, the ideal situation regards other arguably (not really even an argument) more harmful emissions is to have CO2 and water vapour exhaust gasses total 100% of emissions, because that will mean there's 0% CO emission, 0% HC emission, 0% NOx emission, etc..
I'm not a chemist but I'd bet that given the above situation, since both petrol and LPG are HC fuels, since H forms with O to produce water and C bonds with O to produce CO/2, since the engine must need similar amounts of H and/or C to produce similar amounts of heat (that will see the engine produce similar amounts of power - the conditions needed for a fair test) - that where CO2 emission is lower running on LPG, water vapour emission must increase... but water vapour emission isn't depicted in any emissions stats. That might be the next thing - vehicle drivers causing increasing sea levels.
But you're not talking % of emissions, you're talking volume of emissions - Volume of all emissions is likely to increase with engine size, engine load and with decreasing engine efficiency (many aspects to engine efficiency). If CO, HC, NOx etc are good then there's probably not much you can do to lower CO2 (or at least combined CO2 and H2O). If CO or HC increase when running on LPG that would point to a problem with the LPG setup, or the ignition system, or some other aspect of the engine (perhaps compression), any of which could cause the engine to need to use greater quantities of air/fuel to create the same power, including when the engine is just idling.
If nothing is found to be wrong with other emissions, a simple way of decreasing CO2 emissions would be to fit a smaller or more efficient engine but that won't suit your purpose. Another way is to make sure your engine is running under as little load as possible, test done with engine fully warmed up, use thinner oil, turn off all electrical loads and aircon etc. If the test is comparative between petrol and LPG with back to back tests on your car, could cheat by increasing CO2 emissions when running on petrol by turning on aircon and electrical items such as the heater fan and heated rear window.. turning everything off while testing on LPG!
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