Filter Pad absorption measurements of suspended particulate matter - data from the BROKE-West voyage of the Aurora Australis, 2006

Particulates in the water were concentrated onto 25mm glass fibre filters. Light transmission and reflection through the filters was measured using a spectrophotometer to yield spectral absorption coefficients.Data Acquisition:Water samples were taken from Niskin bottles mounted on the CTD rosette. Two or three depths were selected at each station, using the CTD fluorometer profile to identify the depth of maximum fluorescence and below the fluorescence maximum. One sample was always taken at 10m, provided water was available, as a reference depth for comparisons with satellite data (remote sensing international standard). Water sampling was carried out after other groups, leading to a considerable time delay of between half an hour and 3 hours, during which particulates are likely to have sedimented within the Niskin bottle, and algae photoadapted to the dark. In order to minimise problems of sedimentation, as large a sample as practical was taken. Often so little water remained in the Niskin bottle that the entire remnant was taken. Where less than one litre remained, leftover sample water was taken from the HPLC group. Water samples were filtered through 25mm diameter GF/F filters under a low vacuum (less than 5mmHg), in the dark. Filters were stored in tissue capsules in liquid nitrogen and transported to the lab for analysis after the cruise. Three water samples were filtered through GF/F filters under gravity, with 2 30ml pre-rinses to remove organic substances from the filter, and brought to the laboratory for further filtration through 0.2micron membrane filters.Filters were analysed in batches of 3 to 7, with all depths at each station being analysed within the same batch to ensure comparability. Filters were removed one batch at a time and place on ice in the dark. Once defrosted, the filters were placed upon a drop of filtered seawater in a clean petri dish and returned to cold, dark conditions. One by one, the filters were placed on a clean glass plate and scanned from 200 to 900nm in a spectrophotometer equipped with an integrating sphere. A fresh baseline was taken with each new batch using 2 blank filters from the same batch as the sample filters, soaked in filtered seawater. After scanning, the filters were placed on a filtration manifold, soaked in methanol for between 1 and 2 hours to extract pigments, and rinsed with filtered seawater. They were then scanned again against blanks soaked in methanol and rinsed in filtered seawater. Data Processing:The initial scan of total particulate matter, ap, and the second scan of non-pigmented particles, anp, were corrected for baseline wandering by setting the near-infrared absorption to zero.This technique requires correction for enhanced scattering within the filter, which has been reported to vary with species. One dilution series was carried out at station 118 to allow calculation of the correction (beta-factor). Since it is debatable whether this factor will be applicable to all samples, no correction has been applied to the dataset. Potential users should contact JSchwarz for advice on this matter when using the data quantitatively.Not yet complete:Comparison of the beta-factor calculated for station 118 with the literature values.Comparison of phytoplankton populations from station 118 with those found at other stations to evaluate the applicability of the beta-factor.Dataset Format:Two files: phyto_absorp_brokew.txt and phyto_absorp_brokew_2.txt: covering stations 4 to 90 and 91 to 118, respectively. Note that not every station was sampled.File format: Matlab-readable ascii text with 3 'header' lines:Row 1: col.1=-999, col.2 to end = ctd numberRow 2: col.1=-999, col.2 to end = sample depth in metresRow 3: col.1=-999, col.2 to end = 1 for total absorption by particulates, 2 for absorption by non-pigmented particlesRow 4 to end: col.1=wavelength in nanometres, col.2 to end = absorption coefficient corresponding to station, depth and type given in rows 1 to 3 of the same column.This work was completed as part of ASAC projects 2655 and 2679 (ASAC_2655, ASAC_2679).

水体中的颗粒物被富集于25mm玻璃纤维滤膜（glass fibre filters）上。使用分光光度计（spectrophotometer）测量滤膜的光透射与反射特性，以获得光谱吸收系数。

数据采集：
水样取自安装于温盐深采水器组（CTD rosette）的尼斯金采水器（Niskin bottles）。每个站位选取2~3个水深，通过CTD荧光计（fluorometer）剖面确定荧光峰值深度及其下方水深。若有可用水样，始终在10m处采集一份样品作为参考深度，用于与卫星数据（遥感国际标准）进行比对。由于采样滞后于其他课题组作业，存在半小时至3小时的显著时间延迟，此期间颗粒物可能在尼斯金采水器内沉降，藻类也会在黑暗环境中发生光适应。为尽可能减轻沉降带来的问题，采集了尽可能多的水样；常因尼斯金采水器内剩余水量极少，故而采集全部剩余水样。若剩余水量不足1升，则从高效液相色谱（HPLC）组获取剩余样品水。水样在避光条件下，以低真空（低于5mmHg）通过25mm直径的GF/F滤膜（GF/F filters）过滤。滤膜置于组织封装盒内，保存在液氮（liquid nitrogen）中，并随科考航次（cruise）运回实验室进行分析。另有三份水样通过重力方式通过GF/F滤膜过滤，先用2次30ml预冲洗液去除滤膜上的有机物质，随后带回实验室，进一步通过0.2微米微孔滤膜（0.2micron membrane filters）过滤。

滤膜以3~7个为一批次进行分析，每个站位的所有深度样品均在同一批次内完成分析，以确保可比性。每次取出一批滤膜，置于避光冰面之上。待滤膜解冻后，将其放置于洁净培养皿（petri dish）中的一滴过滤海水中，再次放回低温避光环境中。依次将滤膜置于洁净玻璃板上，在配备积分球（integrating sphere）的分光光度计中扫描200~900nm波段的光谱。每批次新样品均使用同批次的2张空白滤膜（经过滤海水浸泡）进行基线校准。扫描完成后，将滤膜置于过滤歧管（filtration manifold）上，用甲醇（methanol）浸泡1~2小时以萃取色素，随后用过滤海水冲洗。之后再次以经甲醇浸泡并以过滤海水冲洗的空白滤膜为参比进行扫描。

数据处理：
对总颗粒物（total particulate matter）吸收系数$a_p$的初始扫描结果，以及非色素颗粒物（non-pigmented particles）吸收系数$a_{np}$的二次扫描结果，通过将近红外波段吸收值置零的方式校正基线漂移（baseline wandering）。该技术需对滤膜内增强的散射效应（enhanced scattering）进行校正，已有研究表明该效应会因物种不同而存在差异。本研究在118站位开展了一组稀释实验，以计算校正因子（beta-factor，β因子）。由于该因子是否适用于所有样品尚存争议，本数据集未进行该项校正。潜在使用者若需定量使用本数据，可联系JSchwarz获取相关建议。

待完善内容：
1. 将118站位计算得到的β因子与文献值进行比对；
2. 将118站位的浮游植物（phytoplankton）种群与其他站位的种群进行比对，以评估β因子的适用性。

数据集格式：
包含两个文件：phyto_absorp_brokew.txt与phyto_absorp_brokew_2.txt，分别对应站位4~90与91~118。请注意，并非所有站位均完成采样。
文件格式：Matlab可读的ASCII文本（ASCII text），包含3行表头：
第1行：第1列为-999，第2列及后续列为CTD编号；
第2行：第1列为-999，第2列及后续列为采样水深（单位：米）；
第3行：第1列为-999，第2列及后续列为标识：1代表颗粒物总吸收，2代表非色素颗粒物吸收；
第4行及后续行：第1列为波长（单位：纳米），第2列及后续列为对应站位、水深与类型的吸收系数，与第1~3行同列中给出的信息一致。

本研究作为ASAC项目2655与2679（ASAC_2655, ASAC_2679）的一部分完成。