Particle Backscatter Ratio Research Articles

Bio-optical Properties of Cyanobacteria Blooms in Western Lake Erie

There is a growing use of remote sensing observations for detecting and quantifying freshwater cyanobacteria populations, yet the inherent optical properties of these communities in natural settings, fundamental to bio-optical algorithms, are not well known. Towards bridging this knowledge gap, we measured a full complement of optical properties in western Lake Erie during cyanobacteria blooms in the summers of 2013 and 2014. Our measurements focus attention on the optical uniqueness of cyanobacteria blooms, which have consequences for remote sensing and bio-optical modeling. We found the cyanobacteria blooms in the western basin during our field work were dominated by Microcystis, while the waters in the adjacent central basin were dominated by Planktothrix. Chlorophyll concentrations ranged from 1 to over 135 ${\mu}g/L$ across the study area with the highest concentrations associated with Microcystis in the western basin. We observed large, amorphous colonial Microcystis structures in the bloom area characterized by high phytoplankton absorption and high scattering coefficients with a mean particle backscatter ratio at 443nm greater than 0.03, which is higher than other plankton types and more comparable to suspended inorganic sediments. While our samples contained mixtures of both, our analysis suggests high contributions to the measured scatter and backscatter coefficients from cyanobacteria. Our measurements provide new insights into the optical properties of cyanobacteria blooms, and indicate that current semi-analytic models are likely to have problems resolving a closed solution in these types of waters as many of our observations are beyond the range of existing model components. We believe that different algorithm or model approaches are needed for these conditions, specifically for phytoplankton absorption and particle backscatter components. From a remote sensing perspective, this presents a challenge not only in terms of a need for new algorithms, but also for determining when to apply the best algorithm for a given situation. These results are new in the sense that they represent a complete description of the optical properties of freshwater cyanobacteria blooms, and are likely to be representative of bloom conditions for other systems containing Microcystis cells and colonies.

Re-examining the effect of particle phase functions on the remote-sensing reflectance.

Even though it is well known that both the magnitude and detailed angular shape of scattering (phase function, PF), particularly in the backward angles, affect the color of the ocean, the current remote-sensing reflectance (Rrs) models typically account for the effect of its magnitude only through the backscattering coefficient (bb). Using 116 volume scattering function (VSF) measurements previously collected in three coastal waters around the U.S. and in the water of the North Atlantic Ocean, we re-examined the effect of particle PF on Rrs in four scenarios. In each scenario, the magnitude of particle backscattering (i.e., bbp) is known, but the knowledge on the angular shape of particle backscattering is assumed to increase from knowing nothing about the shape of particle PFs to partially knowing the particle backscattering ratio (Bp), the exact backscattering shape as defined by β˜p(γ≥90°) (particle VSF normalized by the particle total scattering coefficient), and the exact backscattering shape as defined by the χp factor (particle VSF normalized by the particle backscattering coefficient). At sun zenith angle=30°, the nadir-viewed Rrs would vary up to 65%, 35%, 20%, and 10%, respectively, as the constraints on the shape of particle backscattering become increasingly stringent from scenarios 1 to 4. In all four scenarios, the Rrs variations increase with both viewing and sun angles and are most prominent in the direction opposite the sun. Our results show a greater impact of the measured particle PFs on Rrs than previously found, mainly because our VSF data show a much greater variability in Bp, β˜p(γ≥90°), and χp than previously known. Among the uncertainties in Rrs due to the particle PFs, about 97% can be explained by χp, 90% by β˜p(γ≥90°), and 27% by Bp. The results indicate that the uncertainty in ocean color remote sensing can be significantly constrained by accounting for χp of the VSFs.

Absorption correction and phase function shape effects on the closure of apparent optical properties.

We present a closure experiment between new inherent optical properties (IOPs: absorption a, scattering b, backscattering b_b) and apparent optical properties (AOPs: remote-sensing reflectance R_rs, irradiance reflectance R, and anisotropic factor at nadir Q_n) data of Ionian and Adriatic seawaters, from very clear to turbid waters, ranging across one order of magnitude in R_rs. The internal consistency of the IOP-AOP matchups was investigated though radiative transfer closure. Using the in situ IOPs, we predicted the AOPs with the commercial radiative transfer solver Hydrolight. Closure was limited by two unresolved issues, one regarding processing of in situ data and the other related to radiative transfer modeling. First, different correction methods of the absorption data measured by the Wetlabs ac-s produced high variations in simulated reflectances, reaching 40% for the highest reflectances in our dataset. Second, the lack of detailed volume scattering function measurements forces us to adopt analytical functions that are consistent with a given particle backscattering ratio. The analytical phase functions named Fournier-Forand and two-term Kopelevich presented reasonable angular shapes with respect to measurements at a few backward angles. Between these phase functions, induced changes were within 4% for R_rs, within 11% for R, and within 10% for Q_n. Additionally, closure of Q_n was generally not successful considering radiometric uncertainties. Simulated Q_n overestimated low values and underestimated high values, especially at 665 nm, where Hydrolight was unable to predict measured Q_n values greater than 6 sr. The physical nature of Q_n makes this mismatch almost independent of the measured IOPs, thus precluding Q_n tuning by varying the former. The non-closure of Q_n might be caused by an inaccurate phase function and, to a lesser extent, by the modeling of the incoming radiance. For the future, this remains the task of accurate absorption and phase function measurements, especially at red wavelengths.

Raman lidar observations of a Saharan dust outbreak event: Characterization of the dust optical properties and determination of particle size and microphysical parameters

The Raman lidar system BASIL was operational in Achern (Black Forest) between 25 May and 30 August 2007 in the framework of the Convective and Orographically-induced Precipitation Study (COPS). The system performed continuous measurements over a period of approx. 36 h from 06:22 UTC on 1 August to 18:28 UTC on 2 August 2007, capturing the signature of a severe Saharan dust outbreak episode. The data clearly reveal the presence of two almost separate aerosol layers: a lower layer located between 1.5 and 3.5 km above ground level (a.g.l.) and an upper layer extending between 3.0 and 6.0 km a.g.l. The time evolution of the dust cloud is illustrated and discussed in the paper in terms of several optical parameters (particle backscatter ratio at 532 and 1064 nm, the colour ratio and the backscatter Angström parameter). An inversion algorithm was used to retrieve particle size and microphysical parameters, i.e., mean and effective radius, number, surface area, volume concentration, and complex refractive index, as well as the parameters of a bimodal particle size distribution (PSD), from the multi-wavelength lidar data of particle backscattering, extinction and depolarization. The retrieval scheme employs Tikhonov’s inversion with regularization and makes use of kernel functions for randomly oriented spheroids. Size and microphysical parameters of dust particles are estimated as a function of altitude at different times during the dust outbreak event. Retrieval results reveal the presence of a fine mode with radii of 0.1–0.2 μm and a coarse mode with radii of 3–5 μm both in the lower and upper dust layers, and the dominance in the upper dust layer of a coarse mode with radii of 4–5 μm. Effective radius varies with altitude in the range 0.1–1.5 μm, while volume concentration is found to not exceed 92 μm3 cm−3. The real and imaginary part of the complex refractive index vary in the range 1.4–1.6 and 0.004–0.008, respectively.

Backscattering ratio variation and its implications for studying particle composition: A case study in Yellow and East China seas

Using in situ optical measurements collected during the 2003 spring cruise over the Yellow and East China seas, the particle backscattering ratio is calculated and spectral variability is analyzed by means of geometric mean regression. The analysis shows that the particle backscattering ratio can be regarded as wavelength‐independent in the range of 442–676 nm, given the measurement uncertainties associated with the backscattering and scattering data. The backscattering ratio and attenuation measurements are used to calculate the particle refractive index, which is related to the particle composition. The distributions of the particle refractive index and the water component concentration along two transects suggest the feasibility for studying the particle composition.

Ray scattering model for spherical transparent particles

We propose a model for the reflectance of a particle medium made of identical, large, spherical, and absorbing particles in a clear binder. A 3D geometrical description of light scattering is developed by relying on the laws of geometrical optics. The amount of light backscattered by a single particle is determined as a function of its absorbance and refractive index. Then, we consider a set of coplanar particles, called a particle sublayer, whose reflectance and transmittance are functions of the particle backscattering ratio and the particle concentration. The reflectance of an infinite particle medium is derived from a description of multiple reflections and transmissions between many superposed particle sublayers. When the binder has a refractive index different from that of air, the medium's reflectance factor accounts for the multiple reflections occurring beneath the air-binder interface as well as for the measuring geometry. The influences of various parameters, such as the refractive indices and the particle absorption coefficient, are examined.

Spectral beam attenuation coefficient retrieved from ocean color inversion

A reflectance inversion model in presented in which particle backscattering is parameterized by the particle backscattering ratio, attenuation and absorption coefficients rather than a hyperbolic function. This parameterization removes backscattering spectral constraints and yields independent estimates of the magnitude and spectral slope of the particle beam attenuation, which have been found to correlate with the particle concentration and size distribution, respectively. The model yields estimates of component absorption and backscattering consistent with observations and Mie models, particularly for environments dominated by phytoplankton and other strongly absorbing particles.

Erosion and mixing of filaments in the arctic lower stratosphere revealed by airborne lidar measurements

The extent of ozone depletion in the Arctic stratosphere is limited by the dynamical instability of the winter circulation. During January 1999, transport across the polar vortex boundary occurred with intrusions of midlatitude air into the vortex and via peeling of filaments off the vortex edge, followed by intensive stirring. Contour advection simulations follow the reduction of the scales. The structures are evident as heterogeneities in two‐dimensional (2‐D) airborne lidar sections of aerosol backscatter ratio, along flight paths inside the vortex during January/February 1999. Large gradients of the particle backscatter ratio indicate that microscopic mixing is slow in the presence of continuous intensive macroscopic stirring. The gradients persist several weeks after the vortex recovered and isentropic stirring started. The filament boundary layers typically extend over 100 m vertically and a few kilometers in the horizontal. Under typical stratospheric conditions, it takes about two weeks until 3‐D turbulence takes over and effectively mixes air masses at these scales. In late January 1999 only about 10–13% of the air probed during the flights inside the vortex are mixed microscopically, which, however, is a prerequisite for their chemical and microphysical interaction. As chemical transport models cannot distinguish subgrid‐scale stirring from molecular mixing, the incomplete mixing implies that homogenization would occur prematurely in a model run and interaction rates would be overestimated.