In round numbers, burning Avgas or petrol produces 3 times its mass in carbon dioxide.
So if you're burning 40 litres an hour, that's about 29kg that will be putting out about 87kg of CO2 per hour of flight time.
LHR to LAX is about 4800 nm. So the equivalent in an Archer would be about 40 hours or about 3.5 tonnes of CO2.
If you have 4 on board that's less than a tonne each!