Alexander P. Vasilkov

Global mapping of underwater UV irradiances and DNA‐weighted exposures using Total Ozone Mapping Spectrometer and Sea‐viewing Wide Field‐of‐view Sensor data products

The global stratospheric ozone layer depletion results in an increase in biologically harmful ultraviolet (UV) radiation reaching the surface and penetrating to ecologically significant depths in natural waters. Such an increase can be estimated on a global scale by combining satellite estimates of UV irradiance at the ocean surface from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument with the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color measurements in the visible spectral region. In this paper we propose a model of seawater optical properties in the UV spectral region based on the case 1 water model in the visible range. The inputs to the model are standard monthly SeaWiFS products: chlorophyll concentration and the diffuse attenuation coefficient at 490 nm. Penetration of solar UV radiation to different depths in open ocean waters is calculated using the radiative transfer quasi-single scattering approximation (QSSA). The accuracy of the QSSA approximation in the water is tested using more accurate codes. Sensitivity studies of underwater UV irradiance to atmospheric and oceanic optical properties have shown that the main environmental parameters controlling absolute levels of UVB (280–320 nm) and DNA-weighted irradiance underwater are solar zenith angle, cloud transmittance, water optical properties, and total ozone. Monthly maps of underwater UV irradiance and DNA-weighted exposure are calculated using monthly mean SeaWiFS chlorophyll and diffuse attenuation coefficient, daily SeaWiFS cloud fraction data, and the TOMS-derived surface UV irradiance daily maps. The results include global maps of monthly average UVB irradiance and DNA-weighted daily exposures at 3 and 10 m and depths where the UVB irradiance and DNA-weighted dose rate at local noon are equal to 10% of their surface values.

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.

Problems in assessment of the ultraviolet penetration into natural waters from space-based measurements

Satellite instruments currently provide global maps of surface UV irradiance by combining backscattered radiance data with radiative transfer models. The models are often limited by uncertainties in physical input parameters of the atmosphere and surface. Global mapping of the underwater UV irradiance creates further challenges for the models. The uncertainties in physical input parameters become more serious because of the presence of absorbing and scattering quantities caused by biological processes within the oceans. We summarize the problems encountered in the assessment of the underwater UV irradiance from space-based measurements, and propose approaches to resolve the problems. We have developed a radiative transfer scheme for computation of the UV irradiance in the atmosphere-ocean system. The scheme makes use of input parameters derived from satellite instruments such as the total ozone mapping spectrometer (TOMS) and sea-viewing wide field-of-view sensor (SeaWiFS). The major problem in assessment of the surface UV irradiance is to accurately quantify the effects of clouds. Unlike the standard TOMS UV algorithm, we use the cloud fraction products available from SeaWiFS and MODIS to calculate instantaneous surface flux at the ocean surface. Daily UV doses can be calculated by assuming a model of constant cloudiness throughout the day. Both SeaWiFS and a moderate resolution imaging spectroradiometer (MODIS) provide some estimates of seawater optical properties in the visible. To calculate the underwater UV flux, the seawater optical properties must be extrapolated down to shorter wavelengths. Currently, the problem of accurate extrapolation of visible data down to the UV spectral range is not solved completely, and there are few available measurements. The major difficulty is insufficient correlation between photosynthetic and photoprotective pigments of phytoplankton absorbing in the visible and UV, respectively. We propose to empirically parameterize seawater absorption in the UV on a basis of available datasets of bio-optical measurements from a variety of ocean waters. Another problem is the lack of reliable data on pure seawater absorption in the UV. Laboratory measurements of the UV absorption of both pure water and pure seawater are required.

A new technique for retrieval of tropospheric and stratospheric ozone profiles using sky radiance measurements at multiple view angles: Application to a Brewer spectrometer

[1]xa0This paper describes and applies a new technique for retrieving diurnal variability in tropospheric ozone vertical distribution using ground-based measurements of ultraviolet sky radiances. The measured radiances are obtained by a polarization-insensitive modified Brewer double spectrometer located at Goddard Space Flight Center, in Greenbelt, Maryland, USA. Results demonstrate that the Brewer angular (0–72° viewing zenith angle) and spectral (303–320 nm) measurements of sky radiance in the solar principal plane provide sufficient information to derive tropospheric ozone diurnal variability. In addition, the Brewer measurements provide stratospheric ozone vertical distributions at least twice per day near sunrise and sunset. Frequent measurements of total column ozone amounts from direct-sun observations are used as constraints in the retrieval. The vertical ozone profile resolution is shown in terms of averaging kernels to yield at least four points in the troposphere–low stratosphere, including good information in Umkehr layer 0 (0–5 km). The focus of this paper is on the derivation of stratospheric and tropospheric ozone profiles using both simulated and measured radiances. We briefly discuss the necessary modifications of the Brewer spectrometer that were used to eliminate instrumental polarization sensitivity so that accurate sky radiances can be obtained in the presence of strong Rayleigh scattering and aerosols. The results demonstrate that including a site-specific and time-dependent aerosol correction, based on Brewer direct-sun observations of aerosol optical thickness, is critical to minimize the sky radiance residuals as a function of observing angle in the optimal estimation inversion algorithm and improve the accuracy of the retrieved ozone profile.

Ultraviolet radiation in the atmosphere-ocean system: a model study

A radiative transfer model is developed for the calculation of ultraviolet (UV) fluxes in the atmosphere-ocean system. The radiative transfer equation for the atmosphere is solved by the modified discrete ordinate method. Obtained angular distribution of radiance at the ocean surface serves as a boundary condition for radiative transfer equation in the ocean. The latter is solved by the quasi-single approximation. The comprehensive models are used for the spectral dependencies of optical properties of the atmosphere and the ocean. Calculations of spectral irradiance within the 290 - 400 nm region have been carried out for different solar zenith angles and total amount of ozone inherent to high latitude regions. By convolution of irradiance spectra for the ocean with the phytoplankton action spectrum the UV dose rates have been obtained for different depths.

Optimization of the polarization remote-sensing techniques of the ocean

A numerical code has been developed to calculate Stokes parameters of the visible solar radiation, scattered in the atmosphere-ocean system. Mathematical modeling is used to examine spectral and angular (azimuth and zenith angle) variations of degree of polarization at sea level and at different heights in the atmosphere above the sea surface. On the basis of a developed computer code the efficiency of the polarization measurements for different optical passive remote sensing techniques of the ocean has been investigated. For the passive spectral measurements of the water bio-productivity (chlorophyll-a, dissolved organic matter, concentration of suspended particles) the polarizer can improve signal-to-background ratio. The magnitude of this effect and optimum direction of the polarizer depend upon height, viewing direction, and solar zenith angle. Within the framework of polarization remote sensing technique the influence of the observation height and viewing direction on the results of water turbidity measurements is investigated. Optimal viewing directions in such polarization passive remote sensing technique are discussed.

Remote control of natural waters with use of space optoelectronic systems

This paper poses two important objectives of environmental monitoring and quality assessment of surface waters and describes approaches to their solution using multispectral optoelectronic space platforms. The first objective is closely connected with oil contamination of the water surface. This problem brings forward the issue of development of methods for remote detecting of oil spills based on data received from aerospace remote sensing platforms. An opportunity to improve the efficiency of oil contamination diagnostics at observation from the space platform at different angles is considered. This concept was provoked by the approaching launch of a French-Japanese `Polder system installed onboard the ADEOS satellite. The unique feature of this optoelectronic system is the ability to measure reflectivity of the underlying surface in the visible wavelengths at different viewing angles. The second objective refers to remote methods for determining the primary bio-productivity of sea waters by spectro- radiometric evaluations of the intensity of the radiation rising from below the `water-atmosphere boundary and transformed in the real aerosol-molecular atmosphere.

Solar ultraviolet radiation penetration and photochemical effect on colored dissolved organic matter in coastal waters

The photochemical effect of solar ultraviolet radiation on colored dissolved organic matter has been recently revealed. This effect consists in the change of spectral absorption of sea water in the UV band. In the present paper the experimental data are parameterized to include this effect into radiative transfer theory. A radiative transfer model is developed for calculation of the spectral UV flux at different depths in the sea. The resulting nonlinear radiative transfer equation is solved by the consequent iteration method using quasi-single scattering approximation. Calculations of the spectral irradiance within 290 - 400 nm band were carried out for different solar zenith angles and for different total amounts of ozone. A model of spectral optical properties of sea water inherent to the coastal zone was used. By convolution of irradiance spectra with the DNA action spectrum the biologically effective UV dose rates were calculated. It is shown that solar UV-light effect on the colored dissolved organic matter absorption leads to about 40% increase of the DNA dose rates at a depth of 4 m.

Estimation of the uncertainties of the retrieved seawater parameters

A method is proposed to estimate the accuracy of parameters retrieval from the spectral sea reflectance based on certain assumptions about errors distribution in the input data. Maximum likelihood method has been used to estimate the error of the retrieval stemmed from the random errors of experimental data. The method has been applied to the retrieval of chlorophyll-a concentration, coefficient of scattering in the backward direction of non-chlorophylous particles and absorption of yellow substance in the waters of north-east part of the Black sea. The retrieved chlorophyll concentrations are consistent with in situ measurements. The modeled values of scattering coefficient check well with measurements of extinction coefficient. Proposed accuracy estimates proved to be close to direct ones. The proposed method is general enough to be applicable to the accuracy estimation in various inverse problems of remote sensing of the ocean and atmosphere.

Airborne lidar remote sensing of vertical distribution of sea water scattering coefficient

It has been theoretically shown that vertical distribution of sea water scattering coefficient b(z) can be derived from temporal dependence of backscattered pulse polarization degree when sounding sea water by linear polarized light pulses. Experimental examination of the theoretical method has been carried out by using airborne lidar receiving two orthogonal polarized components of backscattered pulse. Measurements of the lidar signals are accompanied by shipboard measurements of the vertical distribution of the sea water attenuation coefficient c(z). Obtained data show that the backscattered pulse depolarization degree is small and doesnt exceed 6 - 8% at the time interval until 150 ns. The comparison of theoretical calculations and experimental results concerning depolarization degree allow us to evaluate the important parameter of sea water Mueller matrix- `depolarization factor describing the difference between the first and second diagonal elements of the scattering matrix.