Diesel and gas have different emission profiles and the regulations don't get evenly applied. For example, harsher regulations on oxides of nitrogen will impact diesels more than gas. As someone else eluded to, "soot" may not be the best proxy for "emissions".

True, there are plenty of invisible emissions causing chronic air quality issues, but um an over-rich diesel exhaust in your vicinity is rather acute.

There are tons of exhaust pollutants that are not visible.

Those are all exarcebated by the incomplete pollution of trucks rolling coal far more.

To say they are "all exacerbated" isn't quite accurate. Air/fuel mixture affects the various emissions differently. For example, going from lean to stochiometric will increase NOx but decrease CO.

Lean/Stoichiometric is one thing, you still can (in theory) have perfect combustion on that part of the axis. Running very rich will increase both NOx and CO, for example.

It's a bit more complicated than just air-fuel ratio. The primary mechanism in NOx formation is combustion chamber temperature, while the primary mechanism in CO formation is incomplete combustion of HC. If you look at most emission maps, the NOx is decreasing rapidly while CO is relatively flat or increasing. My main point is that combustion is complicated and not as simple as "just do this thing to reduce all emissions". It's often a balancing act, which is why European regulations may be more favorable to diesels than American regulations and the opposite is true for gasoline.

I definitely agree that within reasonable combustion regimens it's a balancing act.

However, when you run your fueling extremely rich like someone that is rolling coal, NOx is going to be higher than in any reasonable tradeoff, because if the irreversible damage to the catalytic converter. It will be far worse than any conceivable lean air mixture.

Ah, ok, I'm tracking your perspective now, thanks for continuing the conversation. Yes, I wasn't really speaking to the damage to the catalytic converter. But I think you may have it reversed?

Under lean conditions, the Rh2O3 layer in the cat reacts with the Al2O3 to create an inactive oxide. This degrades the ability for NOx reduction.

Under very rich conditions, unburnt fuel will combust in the catalytic converter, drastically increasing the temperature and greatly speeding up oxidation, which degrades its effectiveness with regard to all pollutants, including NOx, and oftentimes pretty dramatically.

High temps in the catalytic converter can sinter the precious metals, reducing the surface area of catalytically active surfaces, but I believe the effect is much more pronounced in CO and HC activation. The conversion of NOx is much more constant.

Ah, I was under the impression this wasn't the case, so you're probably right. Do you have a source I could refer to?

Internal Combustion Engine Handbook by van Basshuysen and Schafer. My edition is rather old (2004) so there may a newer one with different info as diesel and emission tech has changed a bit in the last 20 years.

Thank you! I'll take a look and try to find a more up to date source if possible.

Also generally speaking the volume of the motors, diesels are usually larger displacement engines