Aerosol Optical Depth

The Ångström exponent (α) relates AOD to wavelength: AOD(λ) = β × λ^(-α). For small aerosols (like pollution), α ≈ 1.0–2.0 (steeper wavelength dependence). For large aerosols (like dust), α ≈ 0.2–0.5 (weak wavelength dependence). α can thus distinguish aerosol types: high α suggests pollution/biomass burning; low α suggests dust.

Satellite AOD is retrieved only for cloud-free pixels. Cloud detection algorithms identify and mask cloudy areas, but errors occur: thin clouds are sometimes missed (cloud contamination), and clear pixels adjacent to clouds may be incorrectly flagged as cloudy (loss of valid data). Ground validation with AERONET is essential.

AOD is never truly zero due to molecular (Rayleigh) scattering and trace aerosols. On very clear days (e.g., over oceans after rain), AOD may be ~0.05–0.10. Over polluted cities or near dust sources, AOD > 0.5 is common. AOD ≈ 0.1–0.3 is typical for relatively clean continental air.

No. AOD measures light extinction but not particle size or composition. Fine particulate matter (PM2.5, <2.5 µm diameter) poses greater health risks than coarse particles. Two locations with identical AOD may have very different health impacts if one is dominated by fine pollution particles and the other by coarse dust. PM2.5 is typically inferred from AOD using empirical relationships, but direct PM2.5 monitoring (ground stations, lidar) is more reliable.