A brief investigation into the performance of the Lisst-100 (Sequoia instruments) was undertaken to make a first approximation of its ability to measure beam attenuation, the forward volume scattering function-between ~0.1-17.37° -and the particle size distribution between ~1-230m m diameter. In order to achieve this, data gathered from a field deployment off the M/V Humpback August 4 and 5, 1998 in East Sound WA, from a time series collected off the Friday Harbor Laboratories (FHL) (WA) dock August 14, 1998 and measurements made of crushed glass in the laboratory August 16, 1998 were used. Comparison of the measured beam attenuation between the LISST and an AC-9 for August 4 and 5, 1998 show similar shapes with depth (figure 1), particularly for August 5. The absolute magnitude of the LISST beam attenuation is higher than that of the AC-9 but this is expected since it has a smaller acceptance angle (0.06° vs. 0.1° ). The volume scattering function (VSF) measured by the LISST was found to be frequently skewed upwards in the larger angles (see figures 4, 5 and 6). Although no explanation is immediately obvious, an upward skew suggests that the vignetting correction (see figure 13) applied by the LISST software may not be flawless. Figure 6 shows a plot of the VSF for 1-17m and 1-35m depth bins from the M/V Humpback August 4 and 5 respectively. Notice that all of the depths that deviate from the smooth VSF seen in the majority of depth bins are within the first 7m include the first four on both days. The potential for this to be a result of smearing or background light should be investigated for future field deployments since the first 10m are often of integral importance. In order to assess the shape of the VSF measured by the LISST, the ratio of the measured beta at 0.1° to that measured at ~8° was compared to the Junge exponent of the particle number distribution measured by Coulter Counter (CC) from discrete samples taken on the M/V Humpback August 4 and 5, 1998 and by the LISST. The Junge exponent is the slope of the linear line for the log-log plot of the particle size distribution. Since larger particles scatter more in the forward direction, we expect the beta ratio to be higher for less negative Junge exponents. Figure 7 shows the results of the comparison between the CC measured Junge exponent and the LISST measured Junge exponent. The results are relatively as expected. Plots of the measured particle number distributions for both the LISST and the CC during the lab experiment (figure 8), the FHL dock experiment (figure 9) and the M/V Humpback August 4 (figure 10) and 5 (figure 11), 1998 show that the LISST gives particle number per unit volume estimates that are between a factor of two and an order of magnitude higher than those measured by the CC. It is possible that this is in part due to the differences between how these instruments measure particles. The LISST arrives at a particle size distribution by measuring how the particles scatter a beam of light whereas the CC measures particles based on how they (do not) conduct electricity. Therefore a mitigating factor for the LISST is the index of refraction whereas the similar factor for the CC is electrical conductivity. Any changes in these two factors may change the magnitude of the measured particle size distribution in a way that is beyond the scope of the present investigation. It is unclear on a first approximation which instrument is doing a better job in estimating the magnitude. The LISST has an estimated particle size range of 1.36-230.14m m. However, none of the LISST particle size distributions measured in this investigation (see figures 8 through 11) contained reliable estimates for particle diameters less than 6m m or greater than 120m m. While the reliable CC size range is significantly less (typically 2-15m m during this investigation), it is disappointing that the LISST was not able to provide reliable in situ estimates of the particles less than 6m . There is a current dearth of knowledge about in situ particle size distributions less than 100m m but particularly less than 10m m, which is integral to our understanding of light in the ocean. Finally, in this investigation, the effects of filtering or letting stand for a few hours a CC sample were investigated. Figure 13 shows that although the filter seemed to add small particles to particle "free" (blank) water, neither filtering nor letting the sample stand for a few hours seemed to affect the measured distribution.
Figures and Tables
Figure 1: Beam attenuation with depth from the LISST versus that from the AC-9 on the M/V Humpback August 4 and 5, 1998. Notice that the shape of the curve on the 5th is fairly similar but the LISST gives higher values for c than does the AC-9 due probably to its smaller acceptance angle.
Figure 2: LISST versus AC-9 beam attenuation for measurements made on the M/V Humpback August 4/5, 1998 in East Sound. Notice that the 5th shows a general positive trend between the two however, the measurements made on the 4th are relatively clustered.
Figure 3: Beam attenuation with depth measured by the LISST off FHL dock on August 14, 1998. The curves represent two different background scattering files. The difference between the two is constant.
Figure 4: VSF for three additions of crushed glass as part of the FHL lab experiment August 16, 1998. Notice that for the third addition, as the number of small particles is increasing, the VSF has a slight upward trend for the largest measured angle.
Figure 5: VSF for the FHL dock experiment for the four depths and two background scattering measurements. Notice that they lie essentially on top of each other. Also included on this figure is the VSF from the M/V Humpback August 5, 1998 time series in East Sound.
Figure 6: Depth-binned VSF for the M/V Humpback August 4 and 5, 1998. Depths were binned over 1m intervals (such that 2m includes measurements made between 1.5-2.5m). The August 4 profile was from 1-17m and the August 5 from 1-35m. Notice that all the VSF are smooth, except for a few all found within the first 7m. The jagged VSF for 1m on both days is difficult to interpret or diagnose. The remaining deviancies show an upward shift in the largest measured angle-potentially an effect of the vignetting correction factor.
Figure 7: Ratio of the VSF beta at 0.1° and ~8° . A higher beta ratio is indicative of larger particles since larger particles are expected to scatter more forward than smaller. A less negative Junge exponent (slope of the linear line on the log-log particle size distribution) is also indicative of larger particles. Therefore we expect to see a positive correlation between the beta ratio and the Junge exponent. Points represent depths of discrete water samples for which CC measurements were made and binned LISST data were available. The correlation is relatively as expected although a lot of scatter is evident.
Table 1: Slopes of the log-log particle size distribution for all discrete samples run through the CC and depth bins from the LISST August 4/5, 1998 on the M/V Humpback.
Figure 8: Particle number distribution for the LISST and CC from the three additions of crushed glass in the FHL laboratory experiment August 16, 1998. Note that although particle fall out may have been a problem and the magnitudes are approximately a factor of two different, the general shape of the LISST and CC curves are the same between 6-16m m. Also of note is the conspicuous lack of reliable measurement of particles less than 6m m by the LISST. It is unclear if particles greater than 40m m were present and were not measured by the LISST or CC or were not present, statistically rare or fell out.
Figure 9: Particle number distribution for the LISST and CC for the FHL dock experiment August 14, 1998 and for the time series on the M/V Humpback August 5, 1998. Note that lines for the four depths and the two background scattering files of the LISST plot virtually on top of each other. Likewise for the filtered and unfiltered CC samples. Also interesting to note are the differences in magnitude between the LISST and the CC and the changes in slope evident between size ranges of the LISST.
Figure 10: Particle number distribution for the 1m depth bins (such that 2m represents depths from 1.5-2.5m) of the LISST profile and CC discrete samples for the M/V Humpback August 4, 1998. Note that there is over an order of magnitude difference between the measured numbers per unit volume of the LISST and the CC. Table 1 gives the Junge slopes (linear fit) for these lines and although they look similar they are fairly different.
Figure 11: Particle number distribution for the 1m depth bins (such that 2m represents depths from 1.5-2.5m) of the LISST profile and CC discrete samples for the M/V Humpback August 5, 1998. Note that there is over an order of magnitude difference between the measured numbers per unit volume of the LISST and the CC. Table 1 gives the Junge slopes (linear fit) for these lines and although they look similar they are fairly different.
Figure 12: Particle number distribution measured by the CC for the surface sample taken during the FHL dock experiment. This figure compares the effects of filtering and not filtering a raw sample as well as keeping it cool for a length of time (6 hours in this investigation) before measuring. It appears as though the effects of both of these treatments are negligible. Also shown in the figure is the blank distribution, the distribution of the blank filtered through 73-m m mesh and that of a blank filtered through a "clean" filter. Although the slopes are different between the three (see Table 1), filtering appears to have little effect on the raw samples.
Figure 13: How the LISST measures forward scatter. Also within this "figure" are calculations for the loss of angle detection along the 20cm beam pathlength. For example, 17.37° is no longer detected at a distance greater than 4cm from the detector face and likewise, 9.17° is no longer detected at a distance greater than 7.74cm from the detector face. The final conclusion is that all angles less than (and equal to) 3.42° are accurately measured by the LISST but angles greater than this must be corrected for vignetting.
