The Apparent Optical Properties (OC_AOP) data suite provides satellite-derived measures of the interaction between solar radiation and the upper ocean. AOPs are "apparent" because they depend not only on the inherent optical properties (IOPs) of seawater and its constituents but also on the ambient light field. These variables are critical for studying light penetration, primary productivity, and biogeochemical cycling.

AVW (Apparent Visible Wavelength) quantifies the spectral "center" of ocean color by summarizing the shape of the water-leaving reflectance spectrum (Rrs) across the visible range. Reported in nanometers (nm), AVW is lower for bluer, clearer (oligotrophic) waters and higher for greener/browner, more optically complex or productive waters influenced by phytoplankton, colored dissolved organic matter (CDOM), or suspended sediments. Users apply AVW for water-type classification, algorithm selection/blending (e.g., guiding chlorophyll retrievals), regional change detection, and as context alongside chlor_a, Kd_490, IOPs, and PAR.

Fluorescence Line Height (FLH) quantifies the red chlorophyll-a fluorescence (~678-683 nm) emitted by phytoplankton, computed as the height of the measured water-leaving radiance above a spectrally interpolated baseline. FLH is widely used for HAB detection and tracking, identifying productive waters, and assessing phytoplankton physiology; values tend to be elevated in active blooms and can be reduced in complex waters.

Aerosol Optical Thickness (AOT) represents the column-integrated aerosol loading in the atmosphere, describing the attenuation of light due to scattering and absorption by aerosols. AOT is a key parameter in atmospheric correction and is also used to study aerosol transport, air quality, and aerosol-ocean interactions.

Remote Sensing Reflectance (Rrs) provides the remote sensing reflectance at wavelength λ (nm) - the ratio of water-leaving radiance to downwelling irradiance just above the surface (units sr^-1). Variables are named by their band center (e.g., Rrs_412, Rrs_443). Rrs_λ is the foundational observable used to retrieve products such as chlor_a, Kd_490, IOPs, and to support algorithm development, validation, water-quality, and ecosystem applications. Users should review per-file quality flags (e.g., land, cloud, sun glint, high aerosol) and mission documentation - especially in optically complex coastal/inland waters.

As with all ocean-color retrievals, uncertainties increase in optically complex coastal/inland waters or under high aerosol/glint; users should refer to per-mission algorithm notes and quality flags before analysis.

Usage notes:

• Rrs and Rrs_unc are stored as scaled integers; use the scale_factor and add_offset attributes to obtain physical values.
• Quality control should use l2_flags per the flag_meanings attribute to mask clouds, land, high glint, and other conditions.

Geophysical variables in this suite include:

• Rrs - Remote sensing reflectance (sr^-1)
• Rrs_unc - Uncertainty in remote sensing reflectance (sr^-1)
• aot_865 - Aerosol optical thickness at 865 nm (unitless)
• angstrom - Aerosol Angstrom exponent, 443-865 nm (unitless)
• avw - Apparent Visible Wavelength (nm)
• nflh - Normalized Fluorescence Line Height (W m^-2 um^-1 sr^-1)