My sensor is about 1-1/4 miles from the beach in Hawaii. I have noticed during periods of stronger (20 MPH+) onshore wind conditions that my sensor and others in similar near-shore locations here record higher 2.5u readings.
According to https://en.wikipedia.org/wiki/Sea_salt_aerosol
“the size of sea salt aerosols ranges widely from ~0.05 to 10 μm in diameter”
I’m not a scientist, but I suspect what your (and my) device is measuring is sea salt aerosols carried by onshore winds.