system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is diesel no longer "green"?

When I was growing up, diesel was always considered the greener option than petrol, and this was reflected in the prices, diesel always cost less because it was taxed at a lower rate. 

Now they say it's worse than petrol and prices are now higher for diesel. 

So what changed, or what did we suddenly learn about diesel that we didn't know before? 

Bonus question, considering they're both made from crude oil, what's the difference in how they're refined?
gpt
Our definition of "green" changed. 

It used to be purely about fuel consumption...how many gallons do you burn. And Diesel engines are inherently more efficient that gasoline (technically Otto cycle) engines. So they burned less gas, and that was greener.

But because of diesel engines' different combustion behavior, you also get differences in the exhaust. Diesel tends to have more particulates, more NOx (contributes to smog), and more unburned hydrocarbons. You can manage that to some extent with better engine designs and emission controls but those are also pollutants and we take a lot more notice of them now than we used to.