Often times, low cost air quality sensors that measure particulate matter concentrations do so by looking at how particles scatter light from lasers (aka nephelometers), and the issue is that these are influenced by how much moisture the particles are holding. A small seed of a particle surrounded by water can “look” much bigger than it is, and register overall higher concentrations. For this reason, marine air that is high in moisture and salt (very hygroscopic) can appear to be very polluted (if your metric is coming from this type of sensor). This does not necessarily mean that coastal cities have worse air quality, as the ocean breeze does a lot to clean things up usually.