Satellite description

In the comparison to AERONET monthly statistics only satellite derived aerosol properties are included for which monthly averages were calculated on a routine basis in the past. Recent and future satellites and their capabilities (e.g. King et al. 1999) will not be discussed. Here, only satellites, whose data contributed with monthly averages to the intercomparison are introduced.

3.2.1 AVHRR

AVHRR is a 5 band (vis: 1, n-IR: 2, IR: 2) cross track scanning radiometer flown on many NOAA polar-orbiting satellites since 1978. Swath width and spatial resolution are about 2800km and 1km. Aerosol optical depths are derived from visible (0.63(am - as the nominal value for the .58-,68(j,m band) and near-infrared (0.83|im - as the nominal value for the 0.721.1 |am band) reflection anomalies for cloud-free 1 *4km GAC (Global Area Coverage) pixels over sun-glint free ocean scenes. Monthly averages are based on NOAA-9 (Feb. 1985 - Oct. 1988) data, which had fewer calibration drifts than AVHRR sensors on other NOAA satellites. Also during that time-period there was no major contamination by stratospheric aerosol from volcanic eruptions. Monthly averages include data from all four years.

NOAA: A 1-channel retrieval (NOAA) derives the aerosol optical depth at 0.63|nm. The retrieval assumes a log-normal size distribution with a [concentration-] mode radius ofO.ljam and a standard deviation of 2.03. This size assumption translates into an effective radius re of 0.35|am and an AERONET comparable Angstrom parameter of 0.5 (see

Figure 3 for comparisons). Other assumptions are no aerosol absorption (wo=l) and a spherical aerosol shape (Stowe et al., 1997). Cloud screening is done with the CLAVR-1 algorithm (Stowe et al., 1999) and the data were obtained from the Pathfinder ATMOSphere (PATMOS) electronic archive at NOAA. Clear-sky radiance statistics on a 110km* 110km quasi equal area grid (1deg latitude by 1deg longitude) are used to derive aerosol optical thickness. Surface reflection is assumed to be Lambertian outside of a 40deg cone. Reflectivities over water within that cone are affected by sun-glint (specular reflection) and are not considered in monthly statistics. In addition to the NOAA-9 data monthly averages are provided based on a 8-year dataset from the Jul.1981-Jun.1991 period (NOAA-7, NOAA-9 and NOAA-11) minus the two years following the El Chichon volcanic eruption in Apr. 1982.

GISS: A 2-channel retrieval (GISS) derives the aerosol optical depth at from an extrapolation with the Angstrom parameter. The Angstrom statistics is based on a power-law size-distribution, whose power exponent is inferred from the optical depth ratios of the two channels. Other retrieval assumptions are moderate absorption (.97<wo<.99) and a spherical aerosol shape (Mishchenko et al., 1999). Cloud screening is based on the ISCCP data-set (Rossow et al., 1993), with the additional constraints, as to include only the warmest pixels (only retaining pixels with IR temperatures warmer than the composite value).

3.2.2 OCTS

OCTS is a 12 band (vis: 6, n-IR: 3, IR: 3) cross track scanning radiometer flown on the ADEOS polar orbiting satellite. Swath width and spatial resolution are about 1400km and 0.7km. Although primarily designed to detect ocean color, two channels, a visible (0.67|_im) and a near-infrared channel (0.87|u.m) are used to derive over sun-glint free ocean scenes from reflectances the aerosol optical depths at and the Angstrom parame ter. The value for the Angstrom parameter is based on a bimodal log-normal size-distribution (effective radii of and where weights of the two modes are chosen to match the ratio of retrieved aerosol optical depths in the two satellite channels. Other retrieval assumptions are moderate aerosol absorption (.97<wo<.99) and a spherical aerosol shape (Nakajima et al., 1998, Hiragushi et al., 1999). From eight months of available data (Nov. 1996-Jun. 1997) monthly averaged were only processed for April, May and June 1997.

3.2.3 POLDER

POLDER is 7 band (vis: 4, n-IR: 3) bi-dimensional CCD with filters and polarizers flown on the ADEOS polar orbiting satellite. Swath width and spatial resolution are about 2200km and 6km. Due to the limited lifetime of ADEOS, data were only available from Nov. 1996 to Jun.1997. The optical thickness was derived using measuring changes to reflectance or using changes to polarization.

Reflection 'ocean': A dual channel retrieval (0.67|J,m and 0.87|im) determines the near-IR optical depth at 0.87fim from reflection changes over sun-glint free 18* 18km ocean scenes (Deuze et al., 1998). (The combination with the retrieval at 0.67(im provides values for the Angstrom parameter and estimates on aerosol size). A-priori assumptions on aerosol composition, size and shape are based on the 6S model (Vermote et al., 1996), which assumes regional dependent aerosol properties.

Polarization: A new (still experimental) 2-channel retrieval uses the same two channels, however, utilizing polarized radiances. Changes in clear-sky polarization at 0.67(j.m and 0.87|am are used to derive the aerosol optical depth at 0.87jim. The retrieval is based on the concept that the polarization signal detected by the satellite comes mainly from the atmosphere (Herman et al., 1997). Less important surface contributions to polarization are included based on a map with a minimum polarization. No assumption with respect to surfaces properties are required, thus retrievals of aerosol optical depth are not limited to ocean regions. The aerosol model assumes an Angstrom parameter of 1, creating a positive bias for urban-industrial aerosol and biomass burning aerosol. For (large) dust aerosol a negative bias is expected. This bias is further enhanced, because larger aerosol particles loose sensitivity to polarization. Aerosol a-priori assumptions are adopted from the 6S model.

3.2.4 SeaWifs

SeaWifs is a 8 band (vis: 6, n-IR: 2) cross track scanning radiometer flown on the SEASTAR polar orbiting satellite since 1997. Swath width and spatial resolution are (AVHRR-similar) about 2800km and 1km. Data are based on the Sep. 1997-Nov. 1998 time-period. SeaWiFS was designed to detect ocean color, thus it required the removal of atmospheric effects -including aerosol. Data for '1 deg latitude / 1 deg longitude' regions represent averages of sub-pixels values corresponding to minimum optical depths at - thus, a negative bias for retrieved aerosol optical depths can be expected. Different techniques were used to determine aerosol properties over water and land.

Over water, 1-channel retrievals derive aerosol optical depth from changes in reflectance at and From both independently determined optical depths an Angstrom parameter was determined. The aerosol optical depth retrieval is based on the 6S model (Vermote et.al., 1996), which makes assumptions about aerosol composition, size and shape. Retrieved aerosol optical depths for a 1 degree latitude / 1 degree longitude resolution are comprised of monthly sub-pixel 'minimum blue' data. Thus, these optical depths have a tendency to be too small.

Over land, the visible aerosol optical depth at 0.67(im was determined only over dark dense vegetation. These low reflecting regions were identified by 3.7|j,m AVHRR data from the same day. Based on the 'black pixel' approach, it is assumed that the aerosol optical depth can be neglected in the near-IR (except dust) and that spectral correlations for the surface reflectance apply (Kaufman et al., 1997). The optical depth retrieval, as for water, is based on the 6S model, which assumes regional dependent aerosol properties for size, composition and shape.

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