Aerosols

Also called: AOD, aerosol optical depth, dust, smoke, Saharan dust.

Tiny particles in the air — dust, smoke, sea salt, pollution — that dim stars and brighten the sky.

Aerosol optical depth (AOD) measures how much they block: under 0.1 is very clean air, 0.3 is hazy, and Saharan dust or wildfire smoke can push it to 1 or more, when even bright stars dim and the sky turns milky. It's a major part of transparency.