Why is the Air Quality Index usually lower in the ocean?

[Aurorus]

For example: a good chunk of the northern Pacific is perpetually at Moderate, even when the adjacent mainland North America is at Good. Is this a real signal or just some artefacts caused by modelling and limited offshore data collection?

[Weather2004]

The reason the oceans are worse in air quality are because of Sea Salt aerosols, which given our oceans being salty, and the winds could pick up some concentrations of sea salt, which is a pollutant and contributes to a lower air quality. It rarely goes over 25 micrograms per cubic meter or above an AQI of 100.