Michael Sydor

Bio: Michael Sydor is an academic researcher from University of Minnesota. The author has contributed to research in topics: Scattering & Electron mobility. The author has an hindex of 15, co-authored 38 publications receiving 874 citations.

Optical scattering and backscattering by organic and inorganic particulates in U.S. coastal waters

Abstract: We present the results of a study of optical scattering and backscattering of particulates for three coastal sites that represent a wide range of optical properties that are found in U.S. near-shore waters. The 6000 scattering and backscattering spectra collected for this study can be well approximated by a power-law function of wavelength. The power-law exponent for particulate scattering changes dramatically from site to site (and within each site) compared with particulate backscattering where all the spectra, except possibly the very clearest waters, cluster around a single wavelength power-law exponent of -0.94. The particulate backscattering-to-scattering ratio (the backscattering ratio) displays a wide range in wavelength dependence. This result is not consistent with scattering models that describe the bulk composition of water as a uniform mix of homogeneous spherical particles with a Junge-like power-law distribution over all particle sizes. Simultaneous particulate organic matter (POM) and particulate inorganic matter (PIM) measurements are available for some of our optical measurements, and site-averaged POM and PIM mass-specific cross sections for scattering and backscattering can be derived. Cross sections for organic and inorganic material differ at each site, and the relative contribution of organic and inorganic material to scattering and backscattering depends differently at each site on the relative amount of material that is present.

Relationship between mercury accumulation in young-of-the-year yellow perch and water-level fluctuations.

Abstract: A three-year (2001−2003) monitoring effort of 14 northeastern Minnesota lakes was conducted to document relationships between water-level fluctuations and mercury bioaccumulation in young-of-the-ye...

Absorption, Scattering, and Remote-Sensing Reflectance Relationships in Coastal Waters: Testing aNew Inversion Algorithm

Abstract: In-water absorption and scattering coefficients, and above-water remote-sensing reflectance are tightly linked and, in coastal waters, exhibit unique spectral characteristics at near-infrared wavelengths. The surface-reflectance is uncoupled from the total, measured reflectance, the corrected remote sensing reflectance is calculated by difference, then the absorption and scattering coefficients are estimated using a new inversion algorithm. The surface correction and inversion algorithms are based on a reflectance difference at 715-735 nm. At these wavelengths, total absorption is due primarily to pure water absorption, and the reflected sky/cloud light and backscattering spectra are nearly flat. Required algorithm parameters, and ultimately the corrected remote sensing reflectance spectra and spectral absorption and scattering estimates, can be refined if in situ measurements of absorption at 412 nm and spectral scattering shape are available. The coupled surface correction/inversion algorithms were tested using data from 14 experiments at five U.S. coastal locations collected over a three-year period and representing a variety of absorption and scattering regimes. The average errors between measured and modeled absorption and scattering coefficients over the 400-700 nm wavelength range were 14.6% and 3.0%, respectively (without regard to sign).

Effect of suspended particulate and dissolved organic matter on remote sensing of coastal and riverine waters

Abstract: We use remote sensing reflectance (RSR) together with the inherent optical properties of suspended particulates to determine the backscattering ratio bb/b for coastal waters. We examine the wavelength dependence of bb(λ) and f(λ)/Q(λ) and establish the conditions when C(λ) in RSR(λ) ≅ C(λ)bb(λ)/a(λ) can be treated as a constant. We found that for case 2 waters, RSR was insensitive to the natural fluctuations in particle-size distributions. The cross-sectional area of the suspended particulate per unit volume, xg, showed an excellent correlation with the volume scattering coefficient.

Photoreflectance from GaAs and GaAs/GaAs interfaces.

Abstract: Photoreflectance from semi-insulating GaAs, and GaAs/GaAs interfaces, is discussed in terms of its behavior with temperature, doping, epilayer thickness, and laser intensity. Semi-insulating substrates show an exciton-related band-edge signal below 200 K and an impurity-related photoreflectance above 400 K. At intermediate temperatures the band-edge signal from thin GaAs epilayers contains a contribution from the epilayer-substrate interface. The interface effect depends on the epilayer's thickness, doping, and carrier mobility. The effect broadens the band-edge photoreflectance by 5--10 meV, and artifically lowers the estimates for the critical-point energy, ${E}_{\mathrm{CP}}$, obtained through the customary third-derivative functional fit to the data.

Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters

Abstract: For open ocean and coastal waters, a multiband quasi-analytical algorithm is developed to retrieve absorption and backscattering coefficients, as well as absorption coefficients of phytoplankton pigments and gelbstoff. This algorithm is based on remote-sensing reflectance models derived from the radiative transfer equation, and values of total absorption and backscattering coefficients are analytically calculated from values of remote-sensing reflectance. In the calculation of total absorption coefficient, no spectral models for pigment and gelbstoff absorption coefficients are used. Actually those absorption coefficients are spectrally decomposed from the derived total absorption coefficient in a separate calculation. The algorithm is easy to understand and simple to implement. It can be applied to data from past and current satellite sensors, as well as to data from hyperspectral sensors. There are only limited empirical relationships involved in the algorithm, and they are for less important properties, which implies that the concept and details of the algorithm could be applied to many data for oceanic observations. The algorithm is applied to simulated data and field data, both non-case1, to test its performance, and the results are quite promising. More independent tests with field-measured data are desired to validate and improve this algorithm.

Optical Properties of Snow

Abstract: Measurements of the dependence of snow albedo on wavelength, zenith angle, grain size, impurity content, and cloud cover can be interpreted in terms of single-scattering and multiple-scattering radiative transfer theory. Ice is very weakly absorptive in the visible (minimum absorption at λ = 0.46 µm) but has strong absorption bands in the near infrared (near IR). Snow albedo is therefore much lower in the near IR. The near-IR solar irradiance thus plays an important role in snowmelt and in the energy balance at a snow surface. The near-IR albedo is very sensitive to snow grain size and moderately sensitive to solar zenith angle. The visible albedo (for pure snow) is not sensitive to these parameters but is instead affected by snowpack thickness and parts-per-million amounts (or less) of impurities. Grain size normally increases as the snow ages, causing a reduction in albedo. If the grain size increases as a function of depth, the albedo may suffer more reduction in the visible or in the near IR, depending on the rate of grain size increase. The presence of liquid water has little effect per se on snow optical properties in the solar spectrum, in contrast to its enormous effect on microwave emissivity. Snow albedo is increased at all wavelengths as the solar zenith angle increases but is most sensitive around λ =1 µm. Many apparently conflicting measurements of the zenith angle dependence of albedo are difficult to interpret because of modeling error, instrument error, and inadequate documentation of grain size, surface roughness, and incident radiation spectrum. Cloud cover affects snow albedo both by converting direct radiation into diffuse radiation and also by altering the spectral distribution of the radiation. Cloud cover normally causes an increase in spectrally integrated snow albedo. Some measurements of spectral flux extinction in snow are difficult to reconcile with the spectral albedo measurements. The bidirectional reflectance distribution function which apportions the reflected solar radiation among the various reflection angles must be known in order to interpret individual satellite measurements. It has been measured at the snow surface and at the top of the atmosphere, but its dependence on wavelength, snow grain size, and surface roughness is still unknown. Thermal infrared emissivity of snow is close to 100% but is a few percent lower at large viewing angles than for overhead viewing. It is very insensitive to grain size, impurities, snow depth, liquid water content, or density. Solar reflectance and microwave emissivity are both sensitive to various of these snowpack parameters. However, none of these parameters can be uniquely determined by satellite measurements at a single wavelength; a multichannel method is thus necessary if they are to be determined by remote sensing.

Estimation of the remote-sensing reflectance from above-surface measurements.

Abstract: The remote-sensing reflectance Rrs is not directly measurable, and various methodologies have been employed in its estimation. I review the radiative transfer foundations of several commonly used methods for estimating Rrs, and errors associated with estimating Rrs by removal of surface-reflected sky radiance are evaluated using the Hydrolight radiative transfer numerical model. The dependence of the sea surface reflectance factor ρ, which is not an inherent optical property of the surface, on sky conditions, wind speed, solar zenith angle, and viewing geometry is examined. If ρ is not estimated accurately, significant errors can occur in the estimated Rrs for near-zenith Sun positions and for high wind speeds, both of which can give considerable Sun glitter effects. The numerical simulations suggest that a viewing direction of 40 deg from the nadir and 135 deg from the Sun is a reasonable compromise among conflicting requirements. For this viewing direction, a value of ρ ≈ 0.028 is acceptable only for wind speeds less than 5 m s-1. For higher wind speeds, curves are presented for the determination of ρ as a function of solar zenith angle and wind speed. If the sky is overcast, a value of ρ ≈ 0.028 is used at all wind speeds.

Atmospheric correction of satellite ocean color imagery: the black pixel assumption

Abstract: The assumption that values of water-leaving radiance in the near-infrared (NIR) are negligible enable aerosol radiative properties to be easily determined in the correction of satellite ocean color imagery. This is referred to as the black pixel assumption. We examine the implications of the black pixel assumption using a simple bio-optical model for the NIR water-leaving reflectance [rho(w)(lambda(NIR))](N). In productive waters [chlorophyll (Chl) concentration >2 mg m(-3)], estimates of [rho(w)(lambda(NIR))](N) are several orders of magnitude larger than those expected for pure seawater. These large values of [rho(w)(lambda(NIR))](N) result in an overcorrection of atmospheric effects for retrievals of water-leaving reflectance that are most pronounced in the violet and blue spectral region. The overcorrection increases dramatically with Chl, reducing the true water-leaving radiance by roughly 75% when Chl is equal to 5 mg m(-3). Relaxing the black pixel assumption in the correction of Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) satellite ocean color imagery provides significant improvements in Chl and water-leaving reflectance retrievals when Chl values are greater than 2 mg m(-3). Improvements in the present modeling of [rho(w)(lambda(NIR))](N) are considered, particularly for turbid coastal waters. However, this research shows that the effects of nonzero NIR reflectance must be included in the correction of satellite ocean color imagery.

Mercury Contamination in Forest and Freshwater Ecosystems in the Northeastern United States

Abstract: Eastern North America receives elevated atmospheric mercury deposition from a combination of local, regional, and global sources. Anthropogenic emissions originate largely from electric utilities, incinerators, and industrial processes. The mercury species in these emissions have variable atmospheric residence times, which influence their atmospheric transport and deposition patterns. Forested regions with a prevalence of wetlands and of unproductive surface waters promote high concentrations of mercury in freshwater biota and thus are particularly sensitive to mercury deposition. Through fish consumption, humans and wildlife are exposed to methylmercury, which markedly bioaccumulates up the freshwater food chain. Average mercury concentrations in yellow perch fillets exceed the Environmental Protection Agency's human health criterion across the region, and mercury concentrations are high enough in piscivorous wildlife to cause adverse behavioral, physiological, and reproductive effects. Initiati...