New research suggests coastal cities benefit from their proximity to the ocean which absorbs up to 15% of their smog producing nitrogen oxides. The burning of fossil fuels produces nitrogen oxide which reacts with sunlight to form smog and ozone.
What happens is that some of the smog makes it way out over the ocean. The smog reacts with the the surface of the sea removing it permanently from the air.
It’s too soon to say whether seawater has a net benefit on the air quality of coastal cities.