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Presuming the A320 is at least 80% full (of passengers) and comparing it to a typical road car with 2 people inside, which one is more environmentally friendly, per passenger mile?

I have attempted to do some rudimentary calculations, but then was reading that because aircraft release their emissions at altitude, this offsets (?) some of the environmental impact.

I'm going to simplify and assume that jets and cars burn the same fuel, and output the same exhaust, CO2, NOx and all. I'm going to compare only short-haul flights against cars.

According to Wikipedia, an A-320-NEO does 1.95L/100km per seat. Assuming flying at 80% capacity, that gives us 2.4L/100km per seat. According to
The Car Guide, a 2019 Honda Civic does 6.5L/100km. Assuming an average car carries 2 people, that gets 3.2L/100km. It would seem like flying is slightly better, using this simplistic measure.

Keep in mind that the longer you fly, the better the mileage, since the takeoff and climb are the fuel guzzling segments.

It depends on what you mean by "environmentally friendly." Just for an example, let's consider a 1000 mile trip.

An A320 burns about 5 gallons of fuel per set per hour, and with 150 seats this comes to 750 gallons per hour. A 1000 mile flight will take about 2.5 hours, so this comes to 12.5 gallons per seat, or 1875 gallons total. This means that 2 seats worth of fuel would be about 25 gallons total.

For driving, if you assume a fairly efficient car will get 50 miles per gallon, and the drive is not as direct and is 1300 miles, this comes out to 26 gallons total.

So, on the face of it, they are approximately equivalent in fuel usage.

However, if the flight is only 80% full, then if you choose not to take the flight, it's still going to happen. This means the difference in fuel usage will be from the reduction in weight of two people and their luggage. The fuel usage will decrease by about the same amount as the weight reduction compared to the total aircraft weight. If you approximate 2 passengers plus luggage at 400 lb, and aircraft weight at 140,000 lb, based on the 1875 gallon fuel consumption from before, the additional fuel burned by the two people choosing to fly is about 3 gallons total. This makes it much better than driving.

But not all emissions are equal. The aircraft emits most of the exhaust higher in the atmosphere, where it will create contrails that help trap solar energy.

In regards to lbs of fuel consumed per passenger nautical mile the A320 unquestionably is.