Minor nitpick, but tailpipes aren't the primary source of emissions. The study is about PM2.5[0]. which will chiefly be tires and brake pads. Modern gasoline engines are relatively clean, outside of CO2, though diesel engines spit out a bunch of bad stuff.
Exactly. The noxious tailpipe emissions in a city are usually from diesel trucks, small vehicles like motorcycles (small or absent catalytic converters), modified vehicles (catalytic converter removed or diesel reprogrammed to smoke), but not modern gasoline ICE vehicles.
The love for diesel engines in many European countries was always confusing to me.
PM2.5 is also a broad category of particulates that come from many sources. The PM2.5 levels in the air depend on many sources, with wind being a major factor in changing PM2.5 levels. It’s hard to draw conclusions when a number depends on the weather and a lot of other inputs.
The idea is that diesel is the "work" fuel, for shipping, construction, etc... While gasoline is the "consumer" fuel, for personal use, motorsports, etc... Make the former expensive and it will affect the entire economy, everything will become more expensive and less competitive. Making gasoline more expensive will not have the same impact.
So, put high taxes on gasoline. The result was an increase in popularity of diesel cars, that cost less to run because of taxes.
Now, the situation is changing. Diesel, at least the one that is legal to use on the road is taxed at a level closer to gasoline. Diesel cars are also becoming less and less welcome with regards to low emission zones and green taxes, so many people are going back to gasoline.