Ziauddin Ahmad

Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation : Theoretical basis

We discuss the theoretical basis of a recently developed technique to characterize aerosols from space. We show that the interaction between aerosols and the strong molecular scattering in the near ultraviolet produces spectral variations of the backscattered radiances that can be used to separate aerosol absorption from scattering effects. This capability allows identification of several aerosol types, ranging from nonabsorbing sulfates to highly UV-absorbing mineral dust, over both land and water surfaces. Two ways of using the information contained in the near-UV radiances are discussed. In the first method, a residual quantity, which measures the departure of the observed spectral contrast from that of a molecular atmosphere, is computed. Since clouds yield nearly zero residues, this method is a useful way of separately mapping the spatial distribution of UV-absorbing and nonabsorbing particles. To convert the residue to optical depth, the aerosol type must be known. The second method is an inversion procedure that uses forward calculations of backscattered radiances for an ensemble of aerosol models. Using a look-up table approach, a set of measurements given by the ratio of backscattered radiance at 340-380 nm and the 380 nm radiance are associated, within the domain of the candidate aerosol models, to values of optical depth and single-scattering albedo. No previous knowledge of aerosol type is required. We present a sensitivity analysis of various error sources contributing to the estimation of aerosol properties by the two methods.

Satellite estimation of spectral surface UV irradiance. 2. Effects of homogeneous clouds and snow

This paper extends the theoretical analysis of the estimation of the surface UV irradiance from satellite ozone and reflectivity data from a clear-sky case to a cloudy atmosphere and snow-covered surface. Two methods are compared for the estimation of cloud-transmission factor CT, the ratio of cloudy to clear-sky surface irradiance: (1) the Lambert equivalent reflectivity (LER) method and (2) a method based on radiative transfer calculations for a homogeneous (plane parallel) cloud embedded into a molecular atmosphere with ozone absorption. The satellite-derived CT from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground-based CT estimations from the Canadian network of Brewer spectrometers for the period 1989 -1998. For snow-free conditions the TOMS derived CT at 324 nm approximately agrees with Brewer data with a correlation coefficient of ;0.9 and a standard deviation of ;0.1. The key source of uncertainty is the different size of the TOMS FOV (;100 km field of view) and the much smaller ground instrument FOV. As expected, the standard deviations of weekly and monthly C T averages were smaller than for daily values. The plane-parallel cloud method produces a systematic CT bias relative to the Brewer data (17% at low solar zenith angles to 210% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering clouds and snow/ice if the regional snow albedo RS is known from outside data. Since RS varies on a daily basis, using a climatology will result in additional error in the satellite-estimated CT. The CT error has the same sign as the R S error and increases over highly reflecting surfaces. Finally, clouds polluted with absorbing aerosols transmit less radiation to the ground than conservative clouds for the same satellite reflectance and flatten spectral dependence of CT. Both effects reduce C T compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.

Properties of Mount Pinatubo aerosols as derived from Nimbus 7 total ozone mapping spectrometer measurements

The perturbations to the radiances measured by the Nimbus 7 total ozone mapping spectrometer (TOMS) during the 18 months after the eruption of Mount Pinatubo are used to derive weekly zonal mean values for the stratospheric aerosol optical thickness at 312.5 nm and zonal-mean values for the area-weighted or effective radius. The method uses the TOMS observations, on both sides of the orbital track, of the detailed structure in the backscattering region of the aerosol-scattering phase function. Spatial and temporal evolution of the aerosol optical thickness and effective radius is obtained for the tropical region (25°N to 25°S) during most of the period mid-July 1991 to December 1992. The largest derived value of optical thickness was 0.22 (+36%/−10%), obtained for the latitude zone from 5° to 15°S at the end of July 1991. By the end of 1992, tropical optical depths varied from 0.02 to 0.06 over the 25°N to 25°S geographical area. The main source of uncertainty in the derived optical depth is the altitude of the aerosol layer. The inferred time evolution of the effective radius clearly shows an increase in particle size. At the end of July 1991, effective radius values of about 0.5 μm were derived, while in the fall of 1992, these values were between 0.7 and 1.4 μm. Corrections and error estimates are obtained for the measured ozone amounts. The zonal average retrieved ozone amounts corrected for the presence of aerosols are within 1% of the unconnected zonal averages. Individual scan angles can have ozone amount corrections of ±3%, with a nadir view correction of 2%.

A fast, accurate algorithm to account for non‐Lambertian surface effects on TOA radiance

Surface bidirectional reflectance distribution function (BRDF) influences both the radiance just above the surface and that emerging from the top of the atmosphere (TOA). In this study we propose a new, fast, and accurate algorithm CASBIR (correction for anisotropic surface bidirectional reflection) to account for such influences on TOA radiance. This new algorithm is based on four-stream theory that separates the radiation field into direct and diffuse components in both upwelling and downwelling directions. Such a separation is important because the direct component accounts for a substantial portion of incident radiation under a clear sky, and the BRDF effect is strongest in the reflection of the incident direct radiation. The model is validated by comparison with a full-scale, vector radiative transfer model for the atmosphere-surface system [Ahmad and Fraser, 1982] for wavelengths from UV to near-IR over three typical but very different surface types. The result demonstrates that CASBIR is accurate for all solar and viewing zenith and azimuth angles considered, with overall relative difference of less than 0.7%. Application of this algorithm includes both accounting for non-Lambertian surface scattering on the emergent radiation above TOA and developing a more effective approach for surface BRDF retrieval from satellite-measured radiance. Comparison with the result from the Lambertian model indicates that surface BRDF influence on TOA radiance is both angle and wavelength dependent. It increases as solar zenith angle decreases or wavelength increases and becomes strongest in the view directions where the surface reflection is most anisotropic (such as in the hot spot or Sun glint regions).

Optical effects of polar stratospheric clouds on the retrieval of TOMS total ozone

Small areas of sharply reduced ozone density appear frequently in the maps produced from polar region total ozone mapping spectrometer (TOMS) data. These mini-holes are of the order of 1000 km in extent with a lifetime of a few days. On the basis of measurements from ground-based instruments, balloon-borne ozonesondes, and simultaneous measurements of aerosol and ozone concentrations during aircraft flights in the Arctic and Antarctic regions, the appearance of polar stratospheric clouds (PSCs) are frequently associated with false reductions in ozone derived from the TOMS albedo data. By combining radiative transfer calculations with the observed PSC and ozone data, it is shown that PSCs located near or above the ozone density maximum (with optical thickness greater than 0.1) can explain most of the differences between TOMS ozone data and ground or in situ ozone measurements. Several examples of real and false TOMS mini-hole phenomenon are investigated using data from the 1989 Airborne Arctic Stratospheric Expedition (AASE) and from balloon flights over Norway and Sweden.

Assessment of the ultraviolet radiation field in ocean waters from space-based measurements and full radiative-transfer calculations

Quantitative assessment of the UV effects on aquatic ecosystems requires an estimate of the in-water radiation field. Actual ocean UV reflectances are needed for improving the total ozone retrievals from the total ozone mapping spectrometer (TOMS) and the ozone monitoring instrument (OMI) flown on NASAs Aura satellite. The estimate of underwater UV radiation can be done on the basis of measurements from the TOMS/OMI and full models of radiative transfer (RT) in the atmosphere-ocean system. The Hydrolight code, modified for extension to the UV, is used for the generation of look-up tables for in-water irradiances. A look-up table for surface radiances generated with a full RT code is input for the Hydrolight simulations. A model of seawater inherent optical properties (IOPs) is an extension of the Case 1 water model to the UV. A new element of the IOP model is parameterization of particulate matter absorption based on recent in situ data. A chlorophyll product from ocean color sensors is input for the IOP model. Verification of the in-water computational scheme shows that the calculated diffuse attenuation coefficient Kd is in good agreement with the measured Kd.

EFFECT OF MOLECULAR ANISOTROPY ON BACKSCATTERED ULTRAVIOLET RADIANCE

The effect of molecular anisotropy on backscattered UV (BUV) radiances is computed by accounting for it in both Rayleigh optical thickness and the scattering-phase matrix. If the effect of molecular anisotropy is included only in the optical thickness and not in the phase matrix, then for high sun (θ(0) ∼ 0°), the nadir radiance (I(0)) leaving the top of the atmosphere is approximately 1.8% higher than the radiance (I(op)) computed with the effect included in the phase matrix. For very low sun (θ(0) > 80°), I(0) is approximately 2.3% lower than I(op). For off-nadir radiances the relative increase (decrease) depends on both the local zenith angle as well as the azimuth angle. Also, an increase in the surface reflectivity decreases the effect of molecular anisotropy on the upwelling radiances. Exclusion of the anisotropy factor in the Rayleigh-phase matrix has very little effect (<1%) on ozone retrieval from the BUV-type instruments. This is because of the ratio technique used in the retrieval algorithm, which practically cancels out the anisotropy effect.

Applications of ultraviolet absolute radiometry in satellite and surface-based remote sensing of atmospheric ozone

Side by side comparisons of Langley type Dobson AD double wavelength pair direct sun observations between SBUV/2, SSBUV flight models and the NOAA world primary standard Dobson spectrophotometer 83 and 61 show that the SBUV/2 type instruments yield column ozone amounts that are 2% higher than the NOAA Dobson spectrometers. Similar results have been obtained with a radiometrically stable multi-filter spectroradiometer (MFS) under less than ideal conditions. A new approach based on a modeled table look-up method for using zenith sky radiances to derive total column ozone from zenith clear sky conditions has been tested using SBUV/2, SSBUV flight models and the MFS equipped with narrow band interference filters at the Dobson AD wavelength pairs. These clear zenith sky observations yield total column ozone that is in good agreement with ozone from the NOAA Dobson spectrophotometer direct sun observations. New model calculations on the relationship between satellite nadir radiances and surface based zenith clear sky radiances suggest that the combination of a very radiometrically stable MFS combined with a compact high performance double monochromator could be used to derive a common radiometric scale among satellite borne ozone monitoring instruments using the solar backscatter ultraviolet technique, and to be able to determine the drift in the radiometric calibration of ozone remote sensing instruments in space.

A model for the assessment of UV penetration into ocean waters from space-based measurements and full radiative transfer calculations

Increased levels of biologically harmful Uv radiatonhave beenshown to affec aquatic ecosystems, marine photocynmetiry, and their imapct on carbon cycling. A quantiative assessment of UV effectw requires an estimate of the in-water raiationfield. An esitmate of underwater UV radiatonis porosed based on satellit meausrments fromthe TOMS and SeaWiFS and modesl fo radiatve transfer (RT). The Hydrolight code, modified toe xtnd it to the 290 - 400 nm wavleength range, is used for REt calucaitons in theocean. Solar direc tandidffuse radiances at the ocean surfce are calculated using a fulll RT code for clear-sky coditions, whicha re then modified for clouds and aerosols.Teh TOMS total column ozone and reflectivity productsa reinputs for RT calcuaitons in the atmosphere. An essential component of the in-water RT model is a model of seawater inherent optical properties (IOP). The IOP model is an extension of the Case-1 water model to the UV spectral region. Pure water absorption is interpolated between experimental datasets available in the literature. A new element of the IOP model is parameterization of particulate matter absorption in the UV based on recent in situ data. The SeaWiFS chlorophyll product is input for the IOP model. The in-water computational scheme is verified by comparing the calculated diffuse attenuation coefficient Kd, with one measured for a variety of seawater IOP. The calculated Kd is in a good agreement with the measured Kd. The relative RMS error for all of the cruise stations is about 20%. The error may be partially attributed to variability of solar illumination conditions not accounted for in calculations. The conclusion is that we are now able to model ocean UV irradiances and IOP properties with accuracies approaching those visible region, and in agreement with experimental in situ data.

Multipurpose Spectroradiometer for Satellite Instrument Calibration and Zenith Sky Remote Sensing Measurements

In the early 1990s a series of surface-based direct sun and zenith sky measurements of total column ozone were made with SBUV/2 flight models and the SSBUV Space Shuttle instrument in Boulder, Colorado which were compared with NOAA Dobson Instrument direct sun observations and TOMS instrument overpass observations of column ozone. These early measurements led to the investigation of the accuracy of derived total column ozone amounts and aerosol optical depths from zenith sky observations. Following the development and availability of radiometrically stable IAD narrow band interference filter and nitrided silicon photodiodes a simple compact multifilter spectroradiometer was developed which can be used as a calibration transfer standard spectroradiometer (CTSS) or as a surface based instrument remote sensing instruments for measurements of total column ozone and aerosol optical depths. The total column ozone derived from zenith sky observations agrees with Dobson direct sun AD double wavelength pair measurements and with TOMS overpass ozone amounts within uncertainties of about 1%. When used as a calibration transfer standard spectroradiometer the multifilter spectroradiometer appears to be capable of establishing instrument radiometric calibration uncertainties of the order of 1% or less relative to national standards laboratory radiometric standards.