Its said to be because the power grid is not as congested at night due to closed businesses. Therefore the grid has less pollution.
As to cars running better @cd318 , I hadn't thought about that since I was a kid with my 1st car. Indeed that car DID run better at night. I suspect it did so because the air is cooler and provides more oxygen. It ran better in the rain also. But those old vacuum windshield wipers made you keep your foot out of the throttle too much. Pressing the accelerator hard caused the wipers to stop because vacuum was needed by the engine. Thanks for those memories. Hadn't remembered that in a long time