AOD

Marine ARM GPCI Investigations of Clouds (MAGIC): Atmospheric Optical Depth (AOD) by Microtops

1 September 2012 - 30 September 2013

Lead Scientist: R. Reynolds

Observatory: OSC, MAG

The Microtops, manufactured by Solar Light, has been proven capable of making good measurements of atmospheric optical depth (AOD) from moving platforms such as ships at sea. Dr. A. Smirnov, NASA, maintains a network of calibrated units and has provided one to the MAGIC program. Microtops data providde valuable support to the AMF2 aerosol instrumentation and the rotating shadowband radiometer. The instrument is equipped with five (5) accurately aligned optical collimators, with a full field view of 2.5°. Internal baffles are also integrated into the device to eliminate internal reflections. Each channel is fitted with a narrow-band interference filter and a photodiode suitable for the particular wavelength range. The collimators are encapsulated in a cast aluminum optical block for stability. A sun target and pointing assembly is permanently attached to the optical block and laser-aligned to ensure accurate alignment with the optical channels. When the image of the sun is centered in the bull’s-eye of the sun target, all optical channels are oriented directly at the solar disk. A small amount of circumsolar radiation is also captured, but it makes little contribution to the signal. Radiation captured by the collimator and bandpass filters radiate onto the photodiodes, producing an electrical current that is proportional to the radiant power intercepted by the photodiodes. These signals are first amplified and then converted to a digital signal by a high resolution A/D converter. The signals from the photodiodes are processed in series. However, with 20 conversions per second, the results can be treated as if the photodiodes were read simultaneously. AOT and water vapor column are determined assuming the validity of the Bouguer-Lambert-Beer law. The optical depth due to Rayleigh scattering is subtracted from the total optical depth to obtain AOT. Optical depth from other processes such as O3 and NO2 absorption are ignored. The water vapor column is determined based on measurements at 936nm (water absorption peak) and either 870nm or 1020nm (no absorption by water vapor). Ref: Smirnov et al. (2009) Marine Aerosol Network as a component of the Aerosol Robotic Network, JGR, 114, D06204.