Urban areas tend to have higher smog concentrations, but most gases aren't known for their tendency to avoid travel and diffusion. Emissions have non-local effects. Some might even say global.

Additionally, gas taxes are about the most straightforward road use fee right now.

Still, non-metro areas tend to get a break. Never understood exactly why but my theory is either additional county/muni taxes, or real estate tends to be cheaper and with it station overhead. This goes out the window if it's an isolated refill chance, though, in which case it's whatever the market will bear.