AVHRR satellites, which provide data for now two decades, have become probably the most popular tool to derive aerosol optical depth. Although limited to regions over (deep water) oceans, AVHRR retrievals in this comparison are the only satellite data, whose monthly statistics is based on data from more then two years. In our comparison we present results from three different AVHRR statistics. Two monthly statistics, AVHRR-noaa and AVHRR-giss are based on the identical data of one satellite, NOAA-9, but employ different cloud-screening and retrieval assumptions. Two other monthly statistics AVHRR-noaa and AVHRR, 8yrs are based on the same retrieval but differ by considering once 4 years of data from one satellite (NOAA-9) and another time a total of 8 years of data from three satellites (NOAA-7,-9and-11).

NOAA: The AVHRR-noaa statistics, despite its simple aerosol assumption, is usually in good agreement with the AERONET statistics. Underestimates or no data, however, occur at dust-dominated sites, primarily due to the inability to distinguish between thick dust and clouds and the subsequent removal of dust scenes during the cloud-screening process. It appears that when aerosol properties are close to assumptions of the retrieval (i.e. non-absorbing and a relatively large aerosol spheres) agreement is better (e.g. Lanai). A lack of absorption biases towards smaller optical depths, whereas overestimates in size tend to bias towards larger optical depths at viewing geometries with higher sun-elevations (Mishchenko et al. 1999). Stronger aerosol absorption is often associated with aerosol sizes that are smaller than the assumed size radius for the retrieval Thus, the agreement to AERONET is often improved by partially compensating biases.

The AVHRR, 8yrs statistics includes in addition to the NOAA-9 data (2/85-10/88), data from NOAA-7 (7/81-3/82, 4/84-12/84) and data from NOAA-11 (11/88-6/91). Despite the additional data and using the same retrieval algorithm as AVHRR-noaa, more (not less) deviations to the AERONET statistics are detected. A comparison of monthly averages on a year-to-year basis showed that the variability increases as the additional years are added. It is unclear, to what degree these variations are real or at what degree calibration issues, the drift to later overpass times and discontinuities in data-set transitions contributed.

GISS: The AVHRR-giss statistics, although based on the same data-set as AVHRR-noaa, on average suggests slightly larger aerosol optical depths. The inclusion of (moderate) aerosol absorption certainly contributes, but differences with respect to cloud-screening may have contributed as well. (Recall that AERONET cloud-screening is conservative, thus, small overestimates by satellite retrievals could be expected). Large underestimates near a dust-site (Dakar) are again attributed to dust-removal during cloud-screening. As second parameter this retrieval determines aerosol size, which can be evaluated by comparisons of the derived Angstrom parameter to those from AERONET statistics. Sizing is well represented for sites dominated by urban-industrial outflow (re about However, aerosol size is sharply underestimated at dust-dominated sites, as derived Angstrom parameters rarely fall below values of 0.7. Since the retreival's Angstrom parameter is used to convert the AVHRR optical depth to a smaller mid-visible wavelengths AVHRR-giss underestimates of aerosol optical depths at these dust regions are even larger.

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