Sources: https://www.eia.gov/energyexplained/index.cfm?page=electrici...
You can reduce the emissions by adding technology to process the combustion products to clean them up before emitting them, but you are limited by the fact that your cleanup technology has to be small enough and light weight enough to include with the car.
If you burn that same fuel remotely to make electricity, and use that electricity to power the car, now the pollution is being emitted at a fixed location.
We can pick that location to minimize the harm from the pollution that is emitted, and we can use far more effective cleanup technology at a fixed plant than we can on a vehicle.
I'm not entirely sure how accurate my knowledge is though, it's based on a few sort of tangential comments that connected some other data points I know, and not from any one source I can cite.
Also, the impression I got was this ignores things like leaking oil, which isn't measured by my state's emissions test. So I'm not trying to make claims about overall pollution effects between gas and electric.