Optical scattering and backscattering by organic and inorganic particulates in US coastal waters

100

Variability in the optical particle backscattering coefficient (b bp ) is investigated in oceanic waters from two sites, namely the BOUée pour l'acquiSition d'une Série Optique à Long termE site in the northwestern Mediterranean Sea and the Plumes and Blooms stations in the Santa Barbara Channel off Southern California. Data from these two sites span two orders of magnitude in b bp and likely cover typical open ocean values. A significant relationship is found between b bp at wavelengths of 442 and 555 nm and chlorophyll concentration. However the large spread in this relationship makes chlorophyll a poor predictor of b bp . The relationship between b bp and the particulate beam attenuation coefficient at 660 nm is tighter for both sites, indicating covariability of the particle size ranges that determine both coefficients. A detailed study of the seasonal changes of the b bp vs. chlorophyll relationship reveals that this bio-optical relationship might be best described as a succession of distinct regimes with rapid transitions from one to another. The backscattering ratio (b bp ; the ratio of b bp to total particulate scattering, b p ) ranges from about 0.2% to 1.5%, which is similar to previously reported values. The relationship between b bp and chlorophyll was not significant, while values of the backscattering ratio varied spectrally.

Section: Discussionsupporting

confidence: 68%

Section: Discussionmentioning

confidence: 99%

Variability in optical particle backscattering in contrasting bio‐optical oceanic regimes

Antoine¹,

Siegel²,

Kostadinov³

et al. 2011

100

“…We determined the mass-specific, beam attenuation spectrum of suspended sediments by regressing field measurements of spectral beam attenuation against simultaneously determined SPM concentrations (see below). The angular scattering dependence (volume scattering function) of suspended particles was approximated with a Fournier-Forand phase function with a spectrally invariant particulate backscatter fraction (b b /b) of 0.02-a typical value for suspended, mostly inorganic particles from this region (Snyder et al 2008). In EcolightH, an overcast sky model approximated the diffusing effect of the reflective inner walls of the solar simulator.…”

Section: Methodsmentioning

confidence: 99%

Rate and apparent quantum yield of photodissolution of sedimentary organic matter

Estapa

Mayer

Boss

2012

We quantified rates of photochemical dissolution (photodissolution) of organic carbon in coastal Louisiana suspended sediments, conducting experiments under well-defined conditions of irradiance and temperature. Optical properties of the suspended sediments were characterized and used in a radiative transfer model to compute irradiances within turbid suspensions. Photodissolution rate increased with temperature (T), with activation energy of 32 6 7 kJ mol 21 , which implicates indirect (non-photochemical) steps in the net reaction. In most samples, dissolved organic carbon (DOC) concentration increased approximately linearly with time over the first 4 h of irradiation under broadband simulated sunlight, after higher rates in the initial hour of irradiation. Four-hour rates ranged from 2.3 mmol DOC m 23 s 21 to 3.2 mmol DOC m 23 s 21 , but showed no relation to sample origin within the study area, organic carbon or reducible iron content, or mass-specific absorption coefficient. First-hour rates were higher-from 3.5 mmol DOC m 23 s 21 to 7.8 mmol DOC m 23 s 21 -and correlated well with sediment reducible iron (itself often associated with organic matter). The spectral apparent quantum yield (AQY) for photodissolution was computed by fitting DOC photoproduction rates under different spectral irradiance distributions to corresponding rates of light absorption by particles. The photodissolution AQY magnitude is similar to most published dissolved-phase AQY spectra for dissolved inorganic carbon photoproduction, which suggests that in turbid coastal waters where particles dominate light absorption, DOC photoproduction from particles exceeds photooxidation of DOC.

“…The spectral dependency of c p or b bp is beyond the scope of this paper and has been investigated by other authors (Snyder et al 2008). Instead, we focus on c p and b bp at a wavelength of 650 nm, where the attenuation by particles (c p 5 a p + b p ) is essentially determined by their scattering properties because a p , the absorption by living and nonliving particles, only makes a very small contribution (Loisel and Morel 1998).…”

mentioning

confidence: 99%

“…McKee and Cunningham (2006) observed good correlations between b bp and both [SPM] and [PIM] in mineral-dominated turbid waters in the Irish Sea, whereas much weaker correlations were found in clear, more organic-dominated waters. Snyder et al (2008) show evidence of spatial variability of b m bp along the U.S. coast and found significant differences between the massspecific coefficients b bp : [PIM] (Snyder et al 2008;Boss et al 2009a). The projected surface area concentration of suspended particles, [AC], is also known to correlate well with b bp .…”

mentioning

confidence: 99%

In situ variability of mass‐specific beam attenuation and backscattering of marine particles with respect to particle size, density, and composition

Neukermans

Loisel

Mériaux

et al. 2011

180

179

This study analyzes relationships between concentration of suspended particles represented by dry mass, [SPM], or area, [AC], and optical properties including particulate beam attenuation (c p ), side scattering (b s ), and backscattering (b bp ), obtained from an intensive sampling program in coastal and offshore waters around Europe and French Guyana. First-order optical properties are driven by particle concentration with best predictions of [SPM] by b bp and b s , and of [AC] by c p . Second-order variability is investigated with respect to particle size, apparent density (dry weight-to-wet-volume ratio), and composition. Overall, the mass-specific particulate backscattering coefficient, b bp remains unexplained. Possible causes are the limitation of the measured size distributions to the 2-302-mm range and effects of particle shape and internal structure that affect b bp more than c p and were not accounted for.