Aerosol optical depth (AOD) is a measure of the extinction of sunlight reaching the Earth’s surface by aerosol particles. It is related to the aerosol burden in the atmospheric vertical column over the measurement location. This VAP retrieves the AOD from total and diffuse solar irradiances measured by the multifilter rotating shadowband radiometer (MFRSR) at the Earth’s surface.
The AOD VAP algorithm includes (1) routine computation of Langley retrievals that yield first-order calibration data, (2) generation of a robust calibration time series from these first-order values, (3) subsequent application of this calibration time series to the MFRSR and measurements, (4) retrieval of optical depth at several wavelengths, (5) calibration of irradiances, and (6) final application of an autonomous cloud screen to the AOD.
Data are available at 20-second temporal resolution.
Measurements of AOD are used to evaluate aerosol radiative forcing in earth system models. Because AOD is a measure of the aerosol burden in the atmosphere, this VAP also allows a user to analyze variability of aerosol loading. The spectral dependence of AOD, typically described by the Angstrom exponent, is also an indicator of particle size, with large particles having Angstrom exponent values near zero and smaller particles exhibiting larger Angstrom exponent values.
The cloud‐screening process relies on time variations in AOD over a specified time interval. If the AOD variability is small, the AODs are assumed to be good. If this variability is large, the AODs are considered cloud-contaminated. There are periods in which AOD has moderate temporal variability and separation of good and cloud-contaminated AODs can be problematic. Therefore, AODs are less reliable for these periods. The calibration uncertainty of MFRSR-derived AOD is estimated as ±0.005+0.01/m, where m is the optical air mass relative to the path in the zenith direction.