National Aeronautics and Space Administration (NASA), Goddard Space Flight Center

Aerosol Optical Depth (AOD) – No units

A sunphotometer is used to measure the extinction of direct beam spectral solar radiation. Robots approximately 18" high with a 1-foot diameter base are used in conjunction with a data logger, data transmission device, batteries, solar panels, and antenna for data transmission. The system runs continuously and automatically at MLO.

AERONET can provide continuous measurements of aerosol optical depth (AOD), an important climate forcing agent. AOD is needed in satellite validation and atmospheric correction, and MLO is used to calibrate the instruments since the low and stable AOD are ideal for this purpose. AERONET can provide long term comparisons between the in situ CMDL measurements of size distribution at Bondville, IL and the retrievals measured from the CIMEL automatic sunphotometers. The MLO instrument will periodically be swapped with a field instrument to provide a well calibration instrument for field intercomparisons. Long term particle size distribution comparisons will provide validation of the radiometric inversion scheme.

The trends are seasonal at MLO, with a maximum in spring (March-May) resulting from aerosols transported from Asia. No inter-annual long term trends have been detected at MLO.

AERONET is a global network of sun/sky radiometers that is monitoring AOD and aerosol optical properties for AOD trend analysis, optical properties characterization, and for validation of satellite retrievals. Aerosol radiative forcings are one of the largest uncertainties in climate change studies.

Calibration of AERONET sun/sky radiometers at MLO is critical in maintaining a high level of accuracy and consistency to the network database.