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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 9 — Apr. 25, 2011
  • pp: 7945–7959
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Effects of particle aggregation and disaggregation on their inherent optical properties

Wayne H. Slade, Emmanuel Boss, and Clementina Russo  »View Author Affiliations


Optics Express, Vol. 19, Issue 9, pp. 7945-7959 (2011)
http://dx.doi.org/10.1364/OE.19.007945


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Abstract

In many environments a large portion of particulate material is contained in aggregated particles; however, there is no validated framework to describe how aggregates in the ocean scatter light. Here we present the results of two experiments aiming to expose the role that aggregation plays in determining particle light scattering properties, especially in sediment-dominated coastal waters. First, in situ measurements of particle size distribution (PSD) and beam-attenuation were made with two laser particle sizing instruments (one equipped with a pump to subject the sample to aggregate-breaking shear), and measurements from the two treatments were compared. Second, clays were aggregated in the laboratory using salt, and observed over time by multiple instruments in order to examine the effects of aggregation and settling on spectral beam-attenuation and backscattering. Results indicate: (1) mass normalized attenuation and backscattering are only weakly sensitive to size changes due to aggregation in contrast to theory based on solid particles, (2) the spectral slope of beam-attenuation is indicative of changes in PSD but is complicated by instrument acceptance angle, and (3) the spectral shape of backscattering did not provide as clear a relationship with PSD as spectral beam attenuation, as is predicted by theory for solid spheres.

© 2011 OSA

1. Introduction

The scattering of light in aquatic environments is dominated by the effects of particulate material. The intensity and spectral characteristics of scattering depend strongly on the concentration, composition, and particle size distribution (PSD) of suspended matter. In many environments a large portion of suspended particulate material is packaged as aggregated particles [1

1. D. Eisma, “Flocculation and de-flocculation of suspended matter in estuaries,” Neth. J. Sea Res. 20(2-3), 183–199 (1986). [CrossRef]

], and the overall characteristics of the particulate matter pool are a result of multiple processes including resuspension, aggregation, and disaggregation [e.g., 2

2. I. McCave, “Particle size spectra, behavior, and origin of nepheloid layers over the Nova Scotian continental rise,” J. Geophys. Res. 88(C12), 7647–7666 (1983). [CrossRef]

,3

3. P. S. Hill and A. R. M. Nowell, “Comparison of two models of aggregation in continental-shelf bottom boundary layers,” J. Geophys. Res. 100(C11), 22,749–22,763 (1995). [CrossRef]

]. Aggregation and disaggregation affect changes in particle porosity and size, and the composition of an aggregate is remarkably dynamic, reflective of the heterogeneity of its physical, biological, and chemical environments, as well as to its role as a scavenger, gaining and losing material as it is transported throughout the water column [e.g., 4

4. A. B. Burd and G. A. Jackson, “Particle aggregation,” Ann. Rev. Mar. Scie. 1(1), 65–90 (2009). [CrossRef]

,5

5. I. G. Droppo, “Rethinking what constitutes suspended sediment,” Hydrol. Process. 15(9), 1551–1564 (2001). [CrossRef]

]. Components of marine aggregates include bacteria, organic and inorganic colloids, algal particles and associated detritus, mineral particles, as well as polymers, fibrils, and gels, originating biologically and abiotically. Despite the profound consequences of aggregation, there is no accepted framework to describe the effects of aggregation on the scattering properties of suspended particulate material, and the idealized model of the homogenous sphere remains dominant in the study of particle optical properties [6

6. W. R. Clavano, E. Boss, and L. Karp-Boss, “Inherent optical properties of non-spherical marine-like particles—from theory to observations,” Oceanogr. Mar. Biol. 45, 1–38 (2007). [CrossRef]

]. However, consideration of particle packaging is likely needed for the extension of optical methods into environments such as river plumes, bottom boundary layers, and phytoplankton blooms.

The optical properties of aggregates have attracted substantial attention in disciplines other than oceanography, mostly relating to aerosols, interstellar dust, and colloids. Such studies are usually concerned with loose (diffusion limited) fractal aggregates constructed of submicron monomer particles smaller than the wavelength of incident light. Some of these algorithms invoke a (relatively) simple superposition of Rayleigh-Debye-Gans scattering for each monomer, ignoring internal scattering [e.g., 7

7. C. Sorensen, “Light scattering by fractal aggregates: a review,” Aerosol Sci. Technol. 35, 648–687 (2001).

], while others consider a rigorous Mie-based multiple scattering solution [e.g., 8

8. Y. Xu and B. Gustafson, “Light scattering by an ensemble of small particles,” Recent Res. Dev. Opt. 3, 599–648 (2003).

]. Invariably, despite simplifying assumptions, these approaches are defeated by computational limitations for the particle sizes relevant to aquatic aggregates. In contrast, Latimer and Wamble [9

9. P. Latimer and F. Wamble, “Light scattering by aggregates of large colloidal particles,” Appl. Opt. 21(13), 2447–2455 (1982). [CrossRef] [PubMed]

] presented a model describing the scattering properties of aggregates whose component particles are somewhat larger than the wavelength of incident light. They hypothesize that light scattering due to a suspension of randomly-oriented aggregates caries only information about the overall size and porosity (void fraction) of the aggregates. Given this assumption, they then approximated the optical effects of aggregate structure on optical properties using models for randomly-oriented spheroids and coated spherical particles having equivalent gross volume and net mass as the aggregates. Results from their simple model and experimental data from suspensions of latex sphere aggregates agreed to first order, with some of the disagreement likely explained by inaccuracies inherent in the microscopic analysis of the aggregates [10

10. P. Latimer, “Experimental tests of a theoretical method for predicting light scattering by aggregates,” Appl. Opt. 24(19), 3231–3239 (1985). [CrossRef] [PubMed]

].

The mass-specific optical properties of aggregates will differ from solid particles as a result of the fractal nature of an aggregate, where the large fluid fraction within the aggregate results in a cross-sectional area that is larger than that of a solid particle of the same mass. Previously [16

16. E. Boss, W. H. Slade, and P. Hill, “Effect of particulate aggregation in aquatic environments on the beam attenuation and its utility as a proxy for particulate mass,” Opt. Express 17(11), 9408–9420, 420 (2009). [CrossRef] [PubMed]

], we examined the beam-attenuation of marine particles using Latimer’s model [9

9. P. Latimer and F. Wamble, “Light scattering by aggregates of large colloidal particles,” Appl. Opt. 21(13), 2447–2455 (1982). [CrossRef] [PubMed]

,10

10. P. Latimer, “Experimental tests of a theoretical method for predicting light scattering by aggregates,” Appl. Opt. 24(19), 3231–3239 (1985). [CrossRef] [PubMed]

] that approximates aggregate particle structure as an ensemble of hollow spheres and randomly-oriented ellipsoids, with aggregate porosity a function of size. Using a traditional homogenous sphere model, mass-specific beam-attenuation varied significantly as a function of changing PSD. However, with the aggregate model, we found mass-specific attenuation to be remarkably constant and consistent with observations of marine particles in the environment encompassing a wide range of particle sizes and composition [17

17. P. S. Hill, E. Boss, J. P. Newgard, B. A. Law, and T. G. Milligan, “Observations of the sensitivity of beam attenuation to particle size in a coastal bottom boundary layer,” J. Geophys. Res. 116(C2), C02023 (2011), doi:. [CrossRef]

].

Finally, two additional studies have considered potential effects of particle dynamic processes on bulk inherent optical properties. Boss et al. [18

18. E. Boss, W. S. Pegau, W. D. Gardner, J. R. V. Zaneveld, A. H. Barnard, M. S. Twardowski, G. C. Chang, and T. D. Dickey, “Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf,” J. Geophys. Res. 106(C5), 9509–9516 (2001). [CrossRef]

] examined the tight relationship between particulate beam-attenuation magnitude and spectral slope (an indicator of PSD slope), and found the two parameters to be consistent with resuspension and size-dependent settling in the bottom boundary layer for most of their data. Deviation from this tight relationship occurred on the sampling day following a passing hurricane, and the authors consider aggregation dynamics to be a possible explanation for their observations. More recently, Ackleson [19

19. S. Ackleson, “Optical determinations of suspended sediment dynamics in western Long Island Sound and the Connecticut River plume,” J. Geophys. Res. 111(C7), C07009 (2006), doi:. [CrossRef]

] used a simple model linking optical properties derived from Mie theory and changes in PSD expected from disaggregation and settling scenarios to examine Long Island Sound and Connecticut River plume data, finding that disaggregation was able to explain optical variability at the plume boundary. However, Ackelson also found that changes in spectral slope may also be explained by mixing between the two water masses, and concluded that the method of using spectral optical properties to examine particle dynamics requires additional research.

To further increase our understanding of the effects of particle aggregation on optical properties, we conducted an in situ manipulation experiment, measuring and comparing optical properties of the natural suspension and the natural suspension subjected to shear (in order to break aggregates). Using two Sequoia Scientific LISST-100 instruments (measuring near-forward scattering and beam-attenuation) [20

20. Y. C. Agrawal and H. C. Pottsmith, “Instruments for particle size and settling velocity observations in sediment transport,” Mar. Geol. 168(1-4), 89–114 (2000). [CrossRef]

], one open to the environment and the other employing a sample chamber and pump, this experiment allowed us to qualitatively examine the effect of aggregation on beam-attenuation.

A second experiment was conducted in the laboratory to further investigate the effects of packaging of particles into aggregates. In this experiment, clays were aggregated using salt and observed over time by a LISST-100X instrument, open-path WET Labs ac-9 [21

21. M. S. Twardowski, J. M. Sullivan, P. L. Donaghay, and J. R. V. Zaneveld, “Microscale quantification of the absorption by dissolved and particulate material in coastal waters with an ac-9,” J. Atmos. Ocean. Technol. 16(6), 691–707 (1999). [CrossRef]

] (measuring multi-spectral beam-attenuation), and a WET Labs ECO Triplet (measuring volume scattering function, VSF, at 117° at three wavelengths) [22

22. WET Labs, Inc., “ECO Triplet User’s Guide (triplet),” Revision P, 19 Jan. 2010. http://www.wetlabs.com/products/pub/eco/tripletp.pdf

], in order to examine the effects of increasing aggregate size on optical properties.

2. Experimental setup, procedures, and data processing

2.1 LISST PSD and beam-attenuation

The LISST-100 (Sequoia Scientific, Inc.) is an in-water instrument designed to measure PSD in the field. The LISST-100 infers PSD from the scattering of a red laser beam (670 nm) introduced into a sample volume (5-cm path-length). The beam is scattered by particulate material within the sample volume, and near-forward scattered light at angles ranging from approximately 0.075° to 14.9° is received by a Fourier lens and transformed onto a set of 32 logarithmically-spaced, co-planar, concentric photodetector rings. For large particles that scatter light more near-forward, the most inner of the concentric photodetectors respond. Conversely, as particle size increases, angular light scatter becomes less concentrated in the near-forward and response increases in photodetectors further from the center. In addition, photodetectors measure transmitted (0.0269° acceptance angle) and reference (beam-split) laser power in order to estimate beam-attenuation cpg(670), where “pg” refers to combined particulate and dissolved components. Before each field or laboratory experiment, blank measurements (zscat) were made with the manufacturer-supplied software using Barnstead NANOpure water in the small volume chamber insert or by filling the laboratory sink with reverse osmosis (RO) water. In both cases the water was allowed to sit to reduce the effects of bubbles, and blank measurements were repeated (with instrument cleaning) until our zscat scattering patterns were comparable in shape but lower in amplitude than those supplied by the manufacturer. Raw scattering due to particulates could then be calculated by subtraction of the zscat from measurements made during the experiments [20

20. Y. C. Agrawal and H. C. Pottsmith, “Instruments for particle size and settling velocity observations in sediment transport,” Mar. Geol. 168(1-4), 89–114 (2000). [CrossRef]

,23

23. W. H. Slade and E. S. Boss, “Calibrated near-forward volume scattering function obtained from the LISST particle sizer,” Opt. Express 14(8), 3602–3615 (2006). [CrossRef] [PubMed]

]. Data processing and PSD inversion for the LISST was performed on un-binned data, and subsequently binned to five-minute intervals or burst-averaged.

Sequoia Scientific supplies an algorithm [20

20. Y. C. Agrawal and H. C. Pottsmith, “Instruments for particle size and settling velocity observations in sediment transport,” Mar. Geol. 168(1-4), 89–114 (2000). [CrossRef]

] to invert the angularly-resolved scattering pattern into a volume PSD (V(Di) in units of μL/L) having 32 size classes with geometric mean diameters (Di,i={1,232}) from approximately 1 μm to 184 μm. We used an updated version of the inversion kernel based on scattering by randomly-shaped natural particles [24

24. Y. C. Agrawal, A. Whitmire, O. A. Mikkelsen, and H. C. Pottsmith, “Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction,” J. Geophys. Res. 113(C4), C04023 (2008), doi:. [CrossRef]

,25

25. Y. C. Agrawal and O. A. Mikkelsen, “Empirical forward scattering phase functions from 0.08 to 16 deg. for randomly shaped terrigenous 1-21 microm sediment grains,” Opt. Express 17(11), 8805–8814 (2009). [CrossRef] [PubMed]

]. In order to condense changes in PSD into a single parameter, we employ a weighted-average particle size calculated as
Davg=i=132A(Di)Di/i=132A(Di),
(1)
where A(Di) is the areal PSD in suspended cross-sectional area per volume (m2 m−3) for each LISST size class i, with mean diameter Di. Areal size distribution (m2/m−3) is calculated from volume size distribution (μL/L) by assuming spherical geometry:A(Di)=32V(Di)Di1.

2.2 ac-9 spectral attenuation measurements

The WET Labs, Inc. ac-9 [21

21. M. S. Twardowski, J. M. Sullivan, P. L. Donaghay, and J. R. V. Zaneveld, “Microscale quantification of the absorption by dissolved and particulate material in coastal waters with an ac-9,” J. Atmos. Ocean. Technol. 16(6), 691–707 (1999). [CrossRef]

] is a combination spectral beam transmissometer (0.93° acceptance angle) and reflecting-tube absorption meter, normally measuring absorption and beam-attenuation at nine illumination wavelengths in the visible spectrum (412–715 nm) by use of a rotating filter wheel in the light source. In the laboratory aggregation experiment we used a 10-cm path-length version and left the absorption tube sealed, but left the transmissometer-side open to the environment with no flow sleeve so that aggregates would fall through the illuminated sample volume undisturbed. The ac-9 was blanked in the laboratory sink using RO water, and particulate beam-attenuation was then calculated by difference of the experimental measurement and blank.

Particulate beam-attenuation spectra from the ac-9 were fit to a power-law function of the form cp(λ)=Aλγ by unconstrained nonlinear optimization (MATLAB “fminsearch”) [18

18. E. Boss, W. S. Pegau, W. D. Gardner, J. R. V. Zaneveld, A. H. Barnard, M. S. Twardowski, G. C. Chang, and T. D. Dickey, “Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf,” J. Geophys. Res. 106(C5), 9509–9516 (2001). [CrossRef]

,26

26. E. Boss, M. S. Twardowski, and S. Herring, “Shape of the particulate beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution,” Appl. Opt. 40(27), 4885–4893 (2001). [CrossRef]

] using all available wavelengths except 715 nm. For ac-9 data from the laboratory experiment, the percent difference of the fit residuals relative to measured data was in general less than 2%.

2.3 ECO Triplet volume scattering function measurements

A WET Labs, Inc. ECO Triplet BB-3 was used to measure the VSF at a fixed angle of 117°, β(117°,λ), at three wavelengths (λ = 532, 660, 880 nm), with a sampling rate of ~1 Hz. The BB-3 was calibrated at the factory with 2-µm polystyrene microspheres in order to determine a scaling factor and dark offset, S and D, respectively. The calibration values S and D are used to determine β(117°,λ) from raw instrument digital counts, C, according to β(117°,λ)=S(CD) [22

22. WET Labs, Inc., “ECO Triplet User’s Guide (triplet),” Revision P, 19 Jan. 2010. http://www.wetlabs.com/products/pub/eco/tripletp.pdf

,27

27. M. S. Twardowski, H. Claustre, S. A. Freeman, D. Stramski, and Y. Huot, “Optical backscattering properties of the 'clearest' natural waters,” Biogeoscie. 4(6), 1041–1058 (2007). [CrossRef]

,28

28. G. Dall'Olmo, T. K. Westberry, M. J. Behrenfeld, E. Boss, and W. H. Slade, “Significant contribution of large particles to optical backscattering in the open ocean,” Biogeosci. 6(6), 947–967 (2009). [CrossRef]

]; and particulate volume scattering is calculated by difference of the RO water blank, βblank(117°,λ), from measurements, βtot(117°,λ), made during the experiment:

βp(117°,λ)=βtot(117°,λ)βblank(117°,λ).

Thus the effects of the dark offset, D, were subtracted out. Path-length attenuation correction was not performed since absorption measurements were not available. For typical environmental measurements with absorption below 1 m□1, error is expected to be small, 4% [22

22. WET Labs, Inc., “ECO Triplet User’s Guide (triplet),” Revision P, 19 Jan. 2010. http://www.wetlabs.com/products/pub/eco/tripletp.pdf

,29

29. E. Boss, W. S. Pegau, M. Lee, M. S. Twardowski, E. Shybanov, G. Korotaev, and F. Baratange, “Particulate backscattering ratio at LEO 15 and its use to study particles composition and distribution,” J. Geophys. Res. 109(C1), C01014 (2004), doi:. [CrossRef]

]. Assuming a single scattering albedo for particles of ~0.95 and a maximum particulate attenuation of ~10 m□1, we expect our particulate absorption was less than 1 m□1, however this remains a potential error in our estimates of βp(117°,λ).

The spectral shape of un-binned βp(117°,λ)data was examined in a similar way as cp(λ). We found fits to the form βp(117°,λ)=Aλγbbto have percent differences of greater than 25%, with an obvious trend across the wavelength channels, indicating that a power-law fit is not suitable to our measurements. Therefore, ratios of individual wavelength pairs were also considered in order to reduce the possible influence of calibration (slopes, S) errors; the channel ratios were transformed to an equivalent γbb,

γbb(λ1,λ2)=log(βp(λ2)βp(λ1))/log(λ1λ2).
(2)

To help reduce noise in βp(117°,λ)data (likely due to separation of the sample volume of each wavelength channel), βp(117°,λ)was binned to 15-minute intervals for calculation of γbb.

2.4 In situ disaggregation experiment

The qualitative effect of aggregation on beam-attenuation was observed by comparing the measurements of two similar LISST-100 (Type B) instruments deployed side-by-side with one having a mechanism to break aggregates prior to the sample being measured. The instruments were deployed in the same package at ~1 m above bottom in the Damariscotta River Estuary (~10 m mean water depth), Walpole, ME over approximately 24 hours. The first of the instruments (LISST A) was open to the environment while the second (LISST B) sampled water that was introduced to a sampling chamber via a pump (SeaBird SBE 5T, 3000 rpm) intended to break aggregates through increased turbulent shear, denoted by the superscripts “(open)” and “(shear),” respectively. Note that the shear is not quantified, nor do we know what percent of aggregates were broken; thus the comparison of measurements between the two treatments provides only a qualitative indication of the effect aggregation has on the optical properties as measured by the LISST. During the last two hours of the deployment, the sample chamber and pump were removed from the second instrument so that both instrument sample volumes were open to the environment. Both instruments were configured to sample in bursts, timed at five minute intervals. LISST measurements were processed using standard methods and then burst-averaged.

2.5 Laboratory aggregation experiment

The laboratory experiment was performed in order to examine how aggregation affects optical properties as a function of increasing aggregate size. Two beam transmissometers (a LISST–100 Type B, acceptance angle 0.0269° and an ac-9, acceptance angle 0.93°) were arranged side by side with their sampling volumes open to the environment in the bottom of a large 100×40×45-cm sink. The LISST measured both beam-attenuation and near forward scattering, which was inverted to PSD as described earlier. While both sample volumes were open to allow aggregates to sink through them, we assume the contribution of dissolved materials that might be released by the clay to be negligible during the experiment and refer to attenuation as cp rather than cpg. A WET Labs ECO-BB3 was used to measure backscattering at a single angle in the backwards direction, at three illumination wavelengths (532, 660, 880 nm). Care was taken to position all instruments to sample at the same depth.

The tank was also outfitted with sampling tubes having inlets at the instrument sampling depth. Samples (100 mL, in triplicate) were pumped gently at regular intervals throughout the experiment. Suspended particulate mass measurements (SPM) were made gravimetrically, using dried and pre-weighed 0.8-μm polycarbonate filter pads, and included a 100-mL deionized water rinse to remove accumulated salts. All data were captured by a single PC during the experiment, and later processed (time-stamping, calibration, inversion, and time-binning) in MATLAB. Calibration, correction, and data processing were performed using standard methods, and subsequently measurements were time-binned to five-minute intervals. Davg was calculated according to Eq. (1) for the individual LISST measurements within each time bin.

On the day of the experiment, the sink and all instruments were first cleaned thoroughly. The sink was then filled with particle-free reverse-osmosis water, which was allowed to degas and was used to blank all instruments in the sink. A slurry of bentonite clay was disaggregated by vigorous stirring for ~30 minutes and then added and mixed into the water (4 g dry weight in 120 L of water, producing an environmentally relevant mass concentration of approximately 33 g m−3). A calcium chloride solution (0.4 g CaCl L−1) was then mixed into the sink to initiate particle aggregation. Note that this procedure was repeated with differing instrumentation before the specific experiment discussed here, and in each case results were very similar, differing slightly in the timing of aggregation and sinking. Sampling protocol for the SPM measurements are described in more detail in Russo et al. [30

30. C. R. Russo, E. S. Boss, W. H. Slade, and J. Newgard, “An investigation of the acoustic backscatter response to suspensions of clay aggregates and natural sediments,” Cont. Shelf Res. (submitted).

].

Mass-specific optical properties (cp,LISST,cp,ac9,βp) are calculated using the binned data nearest the SPM measurements M, for example c¯p=c¯p/M¯, where bar notation indicates bin or triplicate mean value. Uncertainty in the mass-specific optical properties is determined by standard propagation of uncertainty, for example,
δcp=c¯p((δMM¯)2+(δcpc¯p)2)1/2,
(3)
where δ denotes standard deviation of the triplicate or binned measurements.

3. Results and discussion

3.1 In situ disaggregation experiment

PSD inverted from the LISST scattering measurements reveal disappearance (destruction) of large particles by the pump and creation of smaller particles consistent with disaggregation (Fig. 1A
Fig. 1 – PSD for the two treatments: (A) during the experiment when LISST B was fitted with a sample chamber fed by a pump to subject aggregates to shear, and (B) during the control when both LISST sample volumes were open to the environment.
). During the control period, the size spectra for the two different instruments were very similar in shape (Fig. 1B). A time series of attenuation and Davg during the experiment (Fig. 2A,B
Fig. 2 – Time series of optical and particle size properties during the in situ disaggregation experiment. The experimental package was deployed in a bottom-mounted configuration thus tidal variability is shown as pressure in each plot. Unfilled symbols indicate the control data where the pump and sample chamber were removed from LISST B and both instruments were open to the environment. (A) Beam-attenuation for the open treatment. (B) Average particle size for both treatments. (C) The ratio of beam-attenuation from the two treatments. (D) The ratio of beam-attenuation to volume concentration as measured by the LISST, indicative of aggregates (see text).
) shows strong semi-diurnal tidal variability. Beam-attenuation for the open treatment cpg(open) exhibits 12-hour variability suggesting that overall concentration at the site is advection-dominated; cpg(open) minima correspond to high tide and cpg(open) maxima to low tide where stronger riverine input to the partially-mixed estuary is expected. Both the attenuation ratio cpg(shear)/cpg(open) and open treatment Davg(open) exhibit peaks every six hours when tidal currents (and consequently bottom stress and in-water shear) are strongest. This variability in Davg(open) suggests physical control on aggregate size, while Davg(shear) exhibits less variability during the experiment, suggesting that the PSD of the disaggregated population is more constant.

The beam-attenuation in the sheared treatment is ~30% higher (relative to control) compared to the open treatment (Fig. 2C), consistent with the idea that the smaller particles are more efficient attenuators per mass, though significantly less so than predicted by theory of solid particles [16

16. E. Boss, W. H. Slade, and P. Hill, “Effect of particulate aggregation in aquatic environments on the beam attenuation and its utility as a proxy for particulate mass,” Opt. Express 17(11), 9408–9420, 420 (2009). [CrossRef] [PubMed]

]. These results also suggest that a large fraction of the particles contributing to the beam-attenuation are aggregates and that aggregation and disaggregation affect the beam-attenuation measured with the LISST. The observed effects on the beam-attenuation (as opposed to PSD inversion) are due either directly from changes in attenuation efficiency between the two treatments, or indirectly by making less material scatter within the acceptance angle of the instruments [31

31. K. J. Voss and R. W. Austin, “Beam-attenuation measurements error due to small-angle scattering acceptance,” J. Atmos. Ocean. Technol. 10(1), 113–121 (1993). [CrossRef]

,32

32. E. Boss, W. H. Slade, M. Behrenfeld, and G. Dall’Olmo, “Acceptance angle effects on the beam attenuation in the ocean,” Opt. Express 17(3), 1535–1550 (2009). [CrossRef] [PubMed]

]. For the latter to be important, particles greater than ~400 µm would have to be broken, based on the acceptance angle of the LISST [32

32. E. Boss, W. H. Slade, M. Behrenfeld, and G. Dall’Olmo, “Acceptance angle effects on the beam attenuation in the ocean,” Opt. Express 17(3), 1535–1550 (2009). [CrossRef] [PubMed]

].

3.2 Laboratory aggregation experiment

The evolution of aggregation during the experiment can be illustrated by dividing the PSD into three distinct pools: (1) primary particles smaller than 6 μm, (2) small aggregates from 6 to 60 μm, and (3) large aggregates greater than 60 μm [36

36. O. A. Mikkelsen, P. S. Hill, and T. G. Milligan, “Single-grain, microfloc and macrofloc volume variations observed with a LISST-100 and a digital floc camera,” J. Sea Res. 55(2), 87–102 (2006). [CrossRef]

]. Area concentration in primary particles decreases throughout the experiment as they themselves form aggregates and/or are scavenged by larger aggregates settling. Initially, small aggregates are formed, leading to formation of larger aggregates, sweeping both primary particles and smaller aggregates from the water. The rate of decrease in primary particles is highest as large aggregates dominate, due to scavenging. Evolution of the area-weighted average size, Davg, can also be seen in Fig. 3B and shows rapid aggregation (increase in Davg until ~2 h after the start of the experiment), followed by settling and scavenging (slow reduction in Davg).

As particles aggregated and settled out of the water column (seen as a decrease in measured SPM, Fig. 5A
Fig. 5 – (A) Time series of average particle size, Davg, calculated from area PSD measurements derived from the LISST during the laboratory aggregation-settling experiment, along with measurements of suspended particulate mass (SPM). For SPM, small black dots and the large blue dots represent the individual measurements and mean of triplicate measurements, respectively. Horizontal error bars show the duration of sampling. (B) Time series of beam-attenuation measured using the two transmissometers with different acceptance angles, as well as volume scattering function measured at 117° in the backwards direction. Each optical property shows decrease concurrent with decrease in suspended mass. (C) Mass-normalized optical properties are relatively constant despite changes in PSD during experiment. Uncertainty has been propagated according with standard methods, as Eq. (3). Note that symbols in (C) are offset slightly for clarity.
), the values of the optical properties decreased rapidly (Fig. 5B). However, throughout the experiment, mass-specific cp and βp (cp* and βp*) remained remarkably constant (Fig. 5C) during changes in PSD (mean absolute deviations of 7.0%, 7.9%, and 6.1% relative to mean values of 0.43 m2 g−1, 0.36 m2 g−1, and 0.48 × 10−3 m2 sr−1 g−1, cp,LISST*(670nm), cp,ac9*(660nm), and βp*(660nm), respectively). Our proposed explanation for these observations is that aggregates are much more efficient scatterers per mass than Mie theory would suggest for solid particles of the same size, nearly conserving the cross-sectional area of their primary particles as they aggregate [16

16. E. Boss, W. H. Slade, and P. Hill, “Effect of particulate aggregation in aquatic environments on the beam attenuation and its utility as a proxy for particulate mass,” Opt. Express 17(11), 9408–9420, 420 (2009). [CrossRef] [PubMed]

]. We also observed an increase in the variability of the measured optical properties (Fig. 5B) within each time bin as aggregates formed and settled through the sample volumes, causing spikes in raw measured data. Spikes in optical data have been previously linked to “rare large particles” and aggregation of phytoplankton following blooms [12

12. D. K. Costello, K. L. Carder, and W. Hou, “Aggregation of diatom bloom in a mesocosm: Bulk and individual particle optical measurements,” Deep Sea Res. Part II Top. Stud. Oceanogr. 42(1), 29–45 (1995). [CrossRef]

,37

37. D. W. Townsend, M. D. Keller, M. E. Sieracki, and S. G. Ackleson, “Spring phytoplankton blooms in the absence of vertical water column stability,” Nature 360(6399), 59–62 (1992). [CrossRef]

,38

38. N. Briggs, “Analysis of optical spikes reveals dynamics of aggregates in the twilight zone,” University of Maine M.S. Thesis (2010). http://www.library.umaine.edu/theses/pdf/BriggsN2010.pdf

]; and a method has been proposed to estimate particle size from fluctuations in beam-attenuation measurements [39

39. K. S. Shifrin, Physical Optics of Ocean Water (American Institute of Physics, 1995).

].

Deviation between beam-attenuation measured by the ac-9 and LISST is also evident in Fig. 6A
Fig. 6 – (A) Time series of beam-attenuation spectral slope and ratio of beam-attenuation for instruments (ac-9 and LISST) with differing acceptance angle. (B) The ratio of beam-attenuation to volume concentration as measured by the LISST, indicative of aggregate density. (C) Spectral shape of volume scattering function at 117° for two wavelength pairs (circles, squares) as well as the power law fit to all three wavelengths (diamonds). Note that symbols in (A) and (C) are offset slightly for clarity.
. This deviation is consistent with differences in acceptance angles of the LISST and ac-9 since the larger acceptance angle of the ac-9 compared to the LISST collects more near-forward light scattered by large particles [31

31. K. J. Voss and R. W. Austin, “Beam-attenuation measurements error due to small-angle scattering acceptance,” J. Atmos. Ocean. Technol. 10(1), 113–121 (1993). [CrossRef]

,32

32. E. Boss, W. H. Slade, M. Behrenfeld, and G. Dall’Olmo, “Acceptance angle effects on the beam attenuation in the ocean,” Opt. Express 17(3), 1535–1550 (2009). [CrossRef] [PubMed]

]. We find that the ratio of beam-attenuations from the two instruments correlates well with the average particle size (Fig. 7
Fig. 7 – The ratio of beam-attenuations from instruments with different acceptance angle is strongly correlated with average particle size. As particle size increases, scattering is increased in the near-forward angles; resulting in a greater amount of light captured (transmitted rather than attenuated) by the wider acceptance angle of the ac-9.
). This relationship is not due to the optical peculiarities of aggregates, but rather to the effects of particle size on near-forward scattering. Departure from the correlation for large Davg is likely due to the presence of large aggregates beyond the size range resolved by the LISST inversion.

3.3 Spectral optics, particle dynamics, and particle packaging

Finally, an additional diagnostic of particle packaging is the ratio of the beam-attenuation to total volume concentration (ΣV), as measured and inverted by the LISST. The beam-attenuation has been shown to be a reliable proxy for SPM [17

17. P. S. Hill, E. Boss, J. P. Newgard, B. A. Law, and T. G. Milligan, “Observations of the sensitivity of beam attenuation to particle size in a coastal bottom boundary layer,” J. Geophys. Res. 116(C2), C02023 (2011), doi:. [CrossRef]

], and assuming that the volume distribution from the LISST is reflective of the enclosing volume of aggregates, this ratio cp,LISST/ΣV is proportional to the density of the aggregate. In the field experiment, we find cp,LISST/ΣV for the open treatment to be lower than the shear treatment, suggesting the particles in the open treatment are less dense (Fig. 2D), and furthermore, the ratio exhibits a six-hour periodicity, as in Davg(open), suggesting physical control on size and packaging. In the laboratory, cp,LISST/ΣV drops sharply as aggregate size increases, which is consistent with a decrease in aggregate density (Fig. 6B) [40

40. A. Khelifa and P. S. Hill, “Models for effective density and settling velocity of flocs,” J. Hydraul. Res. 44(3), 390–401 (2006). [CrossRef]

,41

41. F. Maggi, “Variable fractal dimension: A major control for floc structure and flocculation kinematics of suspended cohesive sediment,” J. Geophys. Res. 112(C7), C07012 (2007), doi:. [CrossRef]

].

4. Summary

The in situ and laboratory experiments described here suggest a greater role for aggregation and disaggregation in interpretation of optical properties and their variability than previously assumed. The in situ disaggregation experiment suggests large differences in beam-attenuation for the same mass with different packaging. This result also implies that the disturbance of samples due to pumping could introduce a bias in currently-employed field protocols, where turbulent shear from flow within an instrument or due to a sampling platform or vehicle could affect observations by breaking aggregates.

Other optical parameters we found to be sensitive to aggregation were (1) the ratio of beam-attenuation and total particulate volume as measured by the LISST, which is indicative of packaging (and/or size) parameter in both experiments, and (2) the ratio of the beam-attenuation of ac-9 to that of the LISST, indicative of proportional increase in particles larger than 20 μm, due to the different acceptance angles of the two instruments. For aggregating particles, we did not find in our lab experiment that the information from spectral measurements of the VSF in the back direction provided as clear a picture of changes in particle size as that of spectral particulate beam-attenuation. More work is needed to understand the spectral shape of backscattering for natural particles.

In all but the simplest environments, attempting to understand how particle dynamic processes such as settling and aggregation affect observed optical properties is difficult, as examining hypotheses is muddied due to the presence of other ongoing processes, such as advection or local resuspension. The results presented here reinforce the use of in situ manipulation and idealized lab experiments where processes can be studied in isolation, as frameworks to help interpret field observations. Indeed, while we think these experiments are useful in isolating the effects of individual processes on optical properties, it is important to recognize their limitations as well. In this work, our measurements have focused on sediment-dominated systems, first near-bottom in an estuary, and second laboratory clays aggregated in salt. We expect that many of our results will be widely applicable, but must also acknowledge that in other cases such as large highly-absorbing phytoplankton aggregates, our understanding is incomplete and additional research is needed.

Acknowledgements

References and links

1.

D. Eisma, “Flocculation and de-flocculation of suspended matter in estuaries,” Neth. J. Sea Res. 20(2-3), 183–199 (1986). [CrossRef]

2.

I. McCave, “Particle size spectra, behavior, and origin of nepheloid layers over the Nova Scotian continental rise,” J. Geophys. Res. 88(C12), 7647–7666 (1983). [CrossRef]

3.

P. S. Hill and A. R. M. Nowell, “Comparison of two models of aggregation in continental-shelf bottom boundary layers,” J. Geophys. Res. 100(C11), 22,749–22,763 (1995). [CrossRef]

4.

A. B. Burd and G. A. Jackson, “Particle aggregation,” Ann. Rev. Mar. Scie. 1(1), 65–90 (2009). [CrossRef]

5.

I. G. Droppo, “Rethinking what constitutes suspended sediment,” Hydrol. Process. 15(9), 1551–1564 (2001). [CrossRef]

6.

W. R. Clavano, E. Boss, and L. Karp-Boss, “Inherent optical properties of non-spherical marine-like particles—from theory to observations,” Oceanogr. Mar. Biol. 45, 1–38 (2007). [CrossRef]

7.

C. Sorensen, “Light scattering by fractal aggregates: a review,” Aerosol Sci. Technol. 35, 648–687 (2001).

8.

Y. Xu and B. Gustafson, “Light scattering by an ensemble of small particles,” Recent Res. Dev. Opt. 3, 599–648 (2003).

9.

P. Latimer and F. Wamble, “Light scattering by aggregates of large colloidal particles,” Appl. Opt. 21(13), 2447–2455 (1982). [CrossRef] [PubMed]

10.

P. Latimer, “Experimental tests of a theoretical method for predicting light scattering by aggregates,” Appl. Opt. 24(19), 3231–3239 (1985). [CrossRef] [PubMed]

11.

K. L. Carder, and D. K. Costello, “Optical effects of large particles,” in Ocean Optics, R. Spinrad, K. Carder, and M. J. Perry, eds. (Oxford University Press, 1994).

12.

D. K. Costello, K. L. Carder, and W. Hou, “Aggregation of diatom bloom in a mesocosm: Bulk and individual particle optical measurements,” Deep Sea Res. Part II Top. Stud. Oceanogr. 42(1), 29–45 (1995). [CrossRef]

13.

W. Hou, K. L. Carder, and D. K. Costello, “Scattering phase function of very large particles in the ocean,” Proc. SPIE 2963 (Ocean Optics XIII), 579–584 (1997).

14.

A. Hatcher, P. Hill, and J. Grant, “Optical backscatter of marine flocs,” J. Sea Res. 46(1), 1–12 (2001). [CrossRef]

15.

E. N. Flory, P. S. Hill, T. G. Milligan, and J. Grant, “The relationship between floc area and backscatter during a spring phytoplankton bloom,” Deep Sea Res. Part I Oceanogr. Res. Pap. 51(2), 213–223 (2004). [CrossRef]

16.

E. Boss, W. H. Slade, and P. Hill, “Effect of particulate aggregation in aquatic environments on the beam attenuation and its utility as a proxy for particulate mass,” Opt. Express 17(11), 9408–9420, 420 (2009). [CrossRef] [PubMed]

17.

P. S. Hill, E. Boss, J. P. Newgard, B. A. Law, and T. G. Milligan, “Observations of the sensitivity of beam attenuation to particle size in a coastal bottom boundary layer,” J. Geophys. Res. 116(C2), C02023 (2011), doi:. [CrossRef]

18.

E. Boss, W. S. Pegau, W. D. Gardner, J. R. V. Zaneveld, A. H. Barnard, M. S. Twardowski, G. C. Chang, and T. D. Dickey, “Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf,” J. Geophys. Res. 106(C5), 9509–9516 (2001). [CrossRef]

19.

S. Ackleson, “Optical determinations of suspended sediment dynamics in western Long Island Sound and the Connecticut River plume,” J. Geophys. Res. 111(C7), C07009 (2006), doi:. [CrossRef]

20.

Y. C. Agrawal and H. C. Pottsmith, “Instruments for particle size and settling velocity observations in sediment transport,” Mar. Geol. 168(1-4), 89–114 (2000). [CrossRef]

21.

M. S. Twardowski, J. M. Sullivan, P. L. Donaghay, and J. R. V. Zaneveld, “Microscale quantification of the absorption by dissolved and particulate material in coastal waters with an ac-9,” J. Atmos. Ocean. Technol. 16(6), 691–707 (1999). [CrossRef]

22.

WET Labs, Inc., “ECO Triplet User’s Guide (triplet),” Revision P, 19 Jan. 2010. http://www.wetlabs.com/products/pub/eco/tripletp.pdf

23.

W. H. Slade and E. S. Boss, “Calibrated near-forward volume scattering function obtained from the LISST particle sizer,” Opt. Express 14(8), 3602–3615 (2006). [CrossRef] [PubMed]

24.

Y. C. Agrawal, A. Whitmire, O. A. Mikkelsen, and H. C. Pottsmith, “Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction,” J. Geophys. Res. 113(C4), C04023 (2008), doi:. [CrossRef]

25.

Y. C. Agrawal and O. A. Mikkelsen, “Empirical forward scattering phase functions from 0.08 to 16 deg. for randomly shaped terrigenous 1-21 microm sediment grains,” Opt. Express 17(11), 8805–8814 (2009). [CrossRef] [PubMed]

26.

E. Boss, M. S. Twardowski, and S. Herring, “Shape of the particulate beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution,” Appl. Opt. 40(27), 4885–4893 (2001). [CrossRef]

27.

M. S. Twardowski, H. Claustre, S. A. Freeman, D. Stramski, and Y. Huot, “Optical backscattering properties of the 'clearest' natural waters,” Biogeoscie. 4(6), 1041–1058 (2007). [CrossRef]

28.

G. Dall'Olmo, T. K. Westberry, M. J. Behrenfeld, E. Boss, and W. H. Slade, “Significant contribution of large particles to optical backscattering in the open ocean,” Biogeosci. 6(6), 947–967 (2009). [CrossRef]

29.

E. Boss, W. S. Pegau, M. Lee, M. S. Twardowski, E. Shybanov, G. Korotaev, and F. Baratange, “Particulate backscattering ratio at LEO 15 and its use to study particles composition and distribution,” J. Geophys. Res. 109(C1), C01014 (2004), doi:. [CrossRef]

30.

C. R. Russo, E. S. Boss, W. H. Slade, and J. Newgard, “An investigation of the acoustic backscatter response to suspensions of clay aggregates and natural sediments,” Cont. Shelf Res. (submitted).

31.

K. J. Voss and R. W. Austin, “Beam-attenuation measurements error due to small-angle scattering acceptance,” J. Atmos. Ocean. Technol. 10(1), 113–121 (1993). [CrossRef]

32.

E. Boss, W. H. Slade, M. Behrenfeld, and G. Dall’Olmo, “Acceptance angle effects on the beam attenuation in the ocean,” Opt. Express 17(3), 1535–1550 (2009). [CrossRef] [PubMed]

33.

K. Kranck, “Experiments on the significance of flocculation in the settling of fine-grained sediment in still water,” Can. J. Earth Sci. 17, 1517–1526 (1980). [CrossRef]

34.

K. Kranck, and T. G. Milligan, “Grain size in oceanography,” in Theory, Methods and Applications of Particle Size Analysis, J. P. M. Syvitski, ed. (Cambridge University Press, 1991).

35.

K. J. Curran, P. S. Hill, and T. G. Milligan, “The role of particle aggregation in size-dependent deposition of drill mud,” Cont. Shelf Res. 22(3), 405–416 (2002). [CrossRef]

36.

O. A. Mikkelsen, P. S. Hill, and T. G. Milligan, “Single-grain, microfloc and macrofloc volume variations observed with a LISST-100 and a digital floc camera,” J. Sea Res. 55(2), 87–102 (2006). [CrossRef]

37.

D. W. Townsend, M. D. Keller, M. E. Sieracki, and S. G. Ackleson, “Spring phytoplankton blooms in the absence of vertical water column stability,” Nature 360(6399), 59–62 (1992). [CrossRef]

38.

N. Briggs, “Analysis of optical spikes reveals dynamics of aggregates in the twilight zone,” University of Maine M.S. Thesis (2010). http://www.library.umaine.edu/theses/pdf/BriggsN2010.pdf

39.

K. S. Shifrin, Physical Optics of Ocean Water (American Institute of Physics, 1995).

40.

A. Khelifa and P. S. Hill, “Models for effective density and settling velocity of flocs,” J. Hydraul. Res. 44(3), 390–401 (2006). [CrossRef]

41.

F. Maggi, “Variable fractal dimension: A major control for floc structure and flocculation kinematics of suspended cohesive sediment,” J. Geophys. Res. 112(C7), C07012 (2007), doi:. [CrossRef]

OCIS Codes
(010.4450) Atmospheric and oceanic optics : Oceanic optics
(120.5820) Instrumentation, measurement, and metrology : Scattering measurements
(290.2200) Scattering : Extinction
(290.5850) Scattering : Scattering, particles
(010.4458) Atmospheric and oceanic optics : Oceanic scattering
(010.1350) Atmospheric and oceanic optics : Backscattering

ToC Category:
Atmospheric and Oceanic Optics

History
Original Manuscript: January 28, 2011
Revised Manuscript: March 22, 2011
Manuscript Accepted: March 31, 2011
Published: April 11, 2011

Virtual Issues
Vol. 6, Iss. 5 Virtual Journal for Biomedical Optics

Citation
Wayne H. Slade, Emmanuel Boss, and Clementina Russo, "Effects of particle aggregation and disaggregation on their inherent optical properties," Opt. Express 19, 7945-7959 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-9-7945


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References

  1. D. Eisma, “Flocculation and de-flocculation of suspended matter in estuaries,” Neth. J. Sea Res. 20(2-3), 183–199 (1986). [CrossRef]
  2. I. McCave, “Particle size spectra, behavior, and origin of nepheloid layers over the Nova Scotian continental rise,” J. Geophys. Res. 88(C12), 7647–7666 (1983). [CrossRef]
  3. P. S. Hill and A. R. M. Nowell, “Comparison of two models of aggregation in continental-shelf bottom boundary layers,” J. Geophys. Res. 100(C11), 22,749–22,763 (1995). [CrossRef]
  4. A. B. Burd and G. A. Jackson, “Particle aggregation,” Ann. Rev. Mar. Scie. 1(1), 65–90 (2009). [CrossRef]
  5. I. G. Droppo, “Rethinking what constitutes suspended sediment,” Hydrol. Process. 15(9), 1551–1564 (2001). [CrossRef]
  6. W. R. Clavano, E. Boss, and L. Karp-Boss, “Inherent optical properties of non-spherical marine-like particles—from theory to observations,” Oceanogr. Mar. Biol. 45, 1–38 (2007). [CrossRef]
  7. C. Sorensen, “Light scattering by fractal aggregates: a review,” Aerosol Sci. Technol. 35, 648–687 (2001).
  8. Y. Xu and B. Gustafson, “Light scattering by an ensemble of small particles,” Recent Res. Dev. Opt. 3, 599–648 (2003).
  9. P. Latimer and F. Wamble, “Light scattering by aggregates of large colloidal particles,” Appl. Opt. 21(13), 2447–2455 (1982). [CrossRef] [PubMed]
  10. P. Latimer, “Experimental tests of a theoretical method for predicting light scattering by aggregates,” Appl. Opt. 24(19), 3231–3239 (1985). [CrossRef] [PubMed]
  11. K. L. Carder, and D. K. Costello, “Optical effects of large particles,” in Ocean Optics, R. Spinrad, K. Carder, and M. J. Perry, eds. (Oxford University Press, 1994).
  12. D. K. Costello, K. L. Carder, and W. Hou, “Aggregation of diatom bloom in a mesocosm: Bulk and individual particle optical measurements,” Deep Sea Res. Part II Top. Stud. Oceanogr. 42(1), 29–45 (1995). [CrossRef]
  13. W. Hou, K. L. Carder, and D. K. Costello, “Scattering phase function of very large particles in the ocean,” Proc. SPIE 2963 (Ocean Optics XIII), 579–584 (1997).
  14. A. Hatcher, P. Hill, and J. Grant, “Optical backscatter of marine flocs,” J. Sea Res. 46(1), 1–12 (2001). [CrossRef]
  15. E. N. Flory, P. S. Hill, T. G. Milligan, and J. Grant, “The relationship between floc area and backscatter during a spring phytoplankton bloom,” Deep Sea Res. Part I Oceanogr. Res. Pap. 51(2), 213–223 (2004). [CrossRef]
  16. E. Boss, W. H. Slade, and P. Hill, “Effect of particulate aggregation in aquatic environments on the beam attenuation and its utility as a proxy for particulate mass,” Opt. Express 17(11), 9408–9420, 420 (2009). [CrossRef] [PubMed]
  17. P. S. Hill, E. Boss, J. P. Newgard, B. A. Law, and T. G. Milligan, “Observations of the sensitivity of beam attenuation to particle size in a coastal bottom boundary layer,” J. Geophys. Res. 116(C2), C02023 (2011), doi:. [CrossRef]
  18. E. Boss, W. S. Pegau, W. D. Gardner, J. R. V. Zaneveld, A. H. Barnard, M. S. Twardowski, G. C. Chang, and T. D. Dickey, “Spectral particulate attenuation and particle size distribution in the bottom boundary layer of a continental shelf,” J. Geophys. Res. 106(C5), 9509–9516 (2001). [CrossRef]
  19. S. Ackleson, “Optical determinations of suspended sediment dynamics in western Long Island Sound and the Connecticut River plume,” J. Geophys. Res. 111(C7), C07009 (2006), doi:. [CrossRef]
  20. Y. C. Agrawal and H. C. Pottsmith, “Instruments for particle size and settling velocity observations in sediment transport,” Mar. Geol. 168(1-4), 89–114 (2000). [CrossRef]
  21. M. S. Twardowski, J. M. Sullivan, P. L. Donaghay, and J. R. V. Zaneveld, “Microscale quantification of the absorption by dissolved and particulate material in coastal waters with an ac-9,” J. Atmos. Ocean. Technol. 16(6), 691–707 (1999). [CrossRef]
  22. WET Labs, Inc., “ECO Triplet User’s Guide (triplet),” Revision P, 19 Jan. 2010. http://www.wetlabs.com/products/pub/eco/tripletp.pdf
  23. W. H. Slade and E. S. Boss, “Calibrated near-forward volume scattering function obtained from the LISST particle sizer,” Opt. Express 14(8), 3602–3615 (2006). [CrossRef] [PubMed]
  24. Y. C. Agrawal, A. Whitmire, O. A. Mikkelsen, and H. C. Pottsmith, “Light scattering by random shaped particles and consequences on measuring suspended sediments by laser diffraction,” J. Geophys. Res. 113(C4), C04023 (2008), doi:. [CrossRef]
  25. Y. C. Agrawal and O. A. Mikkelsen, “Empirical forward scattering phase functions from 0.08 to 16 deg. for randomly shaped terrigenous 1-21 microm sediment grains,” Opt. Express 17(11), 8805–8814 (2009). [CrossRef] [PubMed]
  26. E. Boss, M. S. Twardowski, and S. Herring, “Shape of the particulate beam attenuation spectrum and its inversion to obtain the shape of the particulate size distribution,” Appl. Opt. 40(27), 4885–4893 (2001). [CrossRef]
  27. M. S. Twardowski, H. Claustre, S. A. Freeman, D. Stramski, and Y. Huot, “Optical backscattering properties of the 'clearest' natural waters,” Biogeoscie. 4(6), 1041–1058 (2007). [CrossRef]
  28. G. Dall'Olmo, T. K. Westberry, M. J. Behrenfeld, E. Boss, and W. H. Slade, “Significant contribution of large particles to optical backscattering in the open ocean,” Biogeosci. 6(6), 947–967 (2009). [CrossRef]
  29. E. Boss, W. S. Pegau, M. Lee, M. S. Twardowski, E. Shybanov, G. Korotaev, and F. Baratange, “Particulate backscattering ratio at LEO 15 and its use to study particles composition and distribution,” J. Geophys. Res. 109(C1), C01014 (2004), doi:. [CrossRef]
  30. C. R. Russo, E. S. Boss, W. H. Slade, and J. Newgard, “An investigation of the acoustic backscatter response to suspensions of clay aggregates and natural sediments,” Cont. Shelf Res. (submitted).
  31. K. J. Voss and R. W. Austin, “Beam-attenuation measurements error due to small-angle scattering acceptance,” J. Atmos. Ocean. Technol. 10(1), 113–121 (1993). [CrossRef]
  32. E. Boss, W. H. Slade, M. Behrenfeld, and G. Dall’Olmo, “Acceptance angle effects on the beam attenuation in the ocean,” Opt. Express 17(3), 1535–1550 (2009). [CrossRef] [PubMed]
  33. K. Kranck, “Experiments on the significance of flocculation in the settling of fine-grained sediment in still water,” Can. J. Earth Sci. 17, 1517–1526 (1980). [CrossRef]
  34. K. Kranck, and T. G. Milligan, “Grain size in oceanography,” in Theory, Methods and Applications of Particle Size Analysis, J. P. M. Syvitski, ed. (Cambridge University Press, 1991).
  35. K. J. Curran, P. S. Hill, and T. G. Milligan, “The role of particle aggregation in size-dependent deposition of drill mud,” Cont. Shelf Res. 22(3), 405–416 (2002). [CrossRef]
  36. O. A. Mikkelsen, P. S. Hill, and T. G. Milligan, “Single-grain, microfloc and macrofloc volume variations observed with a LISST-100 and a digital floc camera,” J. Sea Res. 55(2), 87–102 (2006). [CrossRef]
  37. D. W. Townsend, M. D. Keller, M. E. Sieracki, and S. G. Ackleson, “Spring phytoplankton blooms in the absence of vertical water column stability,” Nature 360(6399), 59–62 (1992). [CrossRef]
  38. N. Briggs, “Analysis of optical spikes reveals dynamics of aggregates in the twilight zone,” University of Maine M.S. Thesis (2010). http://www.library.umaine.edu/theses/pdf/BriggsN2010.pdf
  39. K. S. Shifrin, Physical Optics of Ocean Water (American Institute of Physics, 1995).
  40. A. Khelifa and P. S. Hill, “Models for effective density and settling velocity of flocs,” J. Hydraul. Res. 44(3), 390–401 (2006). [CrossRef]
  41. F. Maggi, “Variable fractal dimension: A major control for floc structure and flocculation kinematics of suspended cohesive sediment,” J. Geophys. Res. 112(C7), C07012 (2007), doi:. [CrossRef]

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