Wikipedia: "Of total global air emissions, shipping accounts for 18 to 30 percent of the nitrogen oxide and 9 percent of the sulphur oxides"
Everyone reading this, if they think for a few minutes, will be able to figure out what the technology is (because it's marine propulsion, and how do we make that zero-emission?) and who the political groups are who oppose it, so I'm not going to actually specify them because I think the point will have more impact if people work it out for themselves. The sad thing is that half of the people who do figure it out will have an immediate reaction of, "Well of course we can't do that!", thereby dismissing a practical, safe solution to 1/3 of global air pollution based on the nothing but politics, because what does the well-being of the planet matter when implementing a workable solution means changing your political stance on anything? That's something only those looney pro-science people do, right?
Yes, they are very dangerous in the middle of a city, and that's why urban vehicles almost don't emit them, making ships more representative.
The best I could find gives nitrogen-dioxyde a half life of about 30 hours at the atmosphere, and sulfur-dioxyde about 5 hours.
I'd expect it to vary widely with local humidity (something the oceans have a lot), and I'd also expect the corresponding acids to fall down at the first rain (another thing the oceans have a lot), although the acids are already reasonably non-toxic.
Also, I'd expect those gases to not disperse in upper atmosphere, because of their molecular weight. But it thrown with any speed (like in a volcano eruption), they'd have the very low humidity, very low pressure that increases their life to a maximum.
Anyway, all that expectation is from theory. References are hard to find.