The objective of this project was to compare absorption (a), scattering (b), and beam attenuation (c) from East Sound measured by an AC-9 to a, b, and c calculated from Mie theory. Inputs to the Mie code were derived from Coulter Multisizer II particle size distribution (PSD) between 1 and 60 (m and spectrophotometric data.
Particulate a, b, and c were calculated in two ways using the equation:
EMBED Equation.3 ;
where j is a, b, or c; Qij is the efficiency factor for a, b, or c, for diameter of i (m; Ni is the number of particles/mL for a diameter of i (m; and d is the diameter of the particle. Three calculations of a, b, and c were made using Mie theory. The first calculation of particulate a, b, and c utilized one index of refraction (n) equal to 1.05 and complex indices of refraction (ní) calculated from particle absorption (ap) for each AC-9 wavelength. Ni was obtained from total PSD from the Coulter counter. The second calculation of a, b, and c utilized an n = 1.07, for more highly refringent particles and nís calculated from detrital absorption data for each AC-9 wavelength. An Ni for detritus was modeled by fitting slopes to the total PSD excluding humps which were assumed to be phytoplankton blooms. The third calculation used n = 1.05 and nís derived from absorption by phytoplankton data for each AC-9 wavelength. An Ni for phytoplankton was determined by taking the difference spectrum between total PSD and modeled detrital PSD. The calculations of detrital a, b, and c and phytoplankton a, b, and c were added to obtain a second particulate a, b, and c.
Both methods for calculating a, b, and c from Mie theory underestimated the magnitude of a, b, and c compared to AC-9 data taken in situ. The spectral shape of c and the ratio of a(440)/c(440) ( 0.3 suggested that absorption by phytoplankton is of greater importance than indicated by calculated values of a. The spectral shape of the difference spectrum of the AC-9 absorption spectrum and Mie theory derived absorption spectra suggested that the calculated ap neglected to account for adequate absorption by particles larger than 60 (m. The spectral shape of calculated and measured bp were similar supporting that, indeed, calculated a, b, and c neglected the influence of larger particles. Because the Coulter counter excluded particles smaller than 2 (m and larger than 60 (m further assessment of the particle sizes present in the water column will be necessary for more accurate calculations of particulate a, b, and c.
Figure captions for: Comparison of Measured and Calculated absorption, scattering, and beam attenuation coefficients
Figure 1: Particle size distribution measured using a Coulter Multisizer II with an aperature of 100 (m. Particle size distribution was used to determine the number of particles/mL (Ni) for calculations of ap, bp, and cp. Data were extrapolated to 1 (m. Water samples for analysis were taken from Eastsound from a depth of 1 m.
Figure 2: Particle size distribution as shown in figure one and estimated phytoplankton distribution. Phytoplankton distribution was estimated by fitting slopes to the particle size distribution and taking the difference spectrum between the Coulter Counter particle size distribution and the distribution calculated by the fitted slopes. The estimated phytoplankton distribution was used to determine Ni(. The distribution of detritus Nid was considered to be the particles/mL for the line calculated by fitting slopes to the actual Coulter counter data.
Figure 3: Spectral particulate absorption coefficients measured by AC-9 and calculated using Mie theory. Note than both calculated aps underestimate ap.
Figure 4: Spectral particulate scattering coefficients measured by AC-9 and calculated using Mie theory. Spectra are plotted to show differences in magnitude between the measured and calculated values. Both calculated bps underestimate bp.
Figure 5: Spectral particulate scattering coefficients measured by AC-9 and calculated using Mie theory. Spectra are plotted to emphasize similarities in shape between the spectra. The similarities in the shapes of the calculated and AC-9 bps suggest that the Coulter counter PSD has not grossly underrepresented the influence on scattering by small particles.
Figure 6: Spectral particulate beam attenuation coefficients measured by AC-9 and calculated using Mie theory. Note the spectral shape of cp measured by the AC-9 indicates the influence of ap on cp.
Figure 7: Spectral particulate a coefficients measured by AC-9 and calculated using Mie theory including the difference spectrum between ap from the AC-9 and calculated ap. Note the spectral shape of the difference spectrum suggests that calculations based on PSD data from the Coulter counter omitted absorption due to either the small or large large fractions of particles which the Coulter counter could not measure.
