I know the supply chain got messed up, but given that trucks are so crucial to transporting goods (at least here in the US) I would think that trucks remained on the road during the pandemic while individuals driving cars all stayed home during quarantine.
I'm really curious how this conclusion ('trucks are the primary cause of pollution') was reached
> In the most recent year for which data are available, combination trucks made up only 1% of the vehicle population but were responsible for 6% of all vehicle miles traveled (VMT) and 17% of vehicle fuel use.
Given that diesel is often considered "more polluting" you can probably make it work out given a bunch of assumptions that may or may not apply. Trucks are the primary cause of road wear, for example.
Modern diesel engines are less polluting than unleaded cars, thanks to diesel particle filters:
"gasoline cars emitted on average 10 times more carbonaceous PM at 22°C and 62 times more at -7°C compared to diesel cars"