海洋CDOM光吸收研究中若干问题的探讨

Discussion on some questions in the studies on CDOM light absorption

本文结合在厦门湾的现场调查和实验室分析,对目前海洋CDOM研究中的若干问题,从空白扣除、吸收系数和光谱斜率S值的估算方法、不同粒径滤膜对CDOM光学性质的影响等方面进行了探讨,并就河口混合过程中S值理论保守行为的判断进行了分析.结果表明,测定水样时,应以过滤后的纯水作为空白.CDOM的吸收系数应进行校正以消除样品中非CDOM信号的影响.进行CDOM的S值估算时,不论是高浓度的河流端还是低浓度的海端水样,当波长范围相同时,非线性回归所得S值均大于线性回归.当波长范围不同时,对任何一种回归方法,S值随长波区成分增多而降低,线性回归方法其降低程度尤其明显.总之波长范围对非线性回归处理的影响很小,S值通过对校正后的吸收系数进行非线性回归处理,同时波长范围选300～500nm较为适宜.决定河口混合时S值变化的因素不仅仅是河海两个端元CDOM的S值,还有它们CDOM浓度的相对大小.滤膜对近海Ⅱ类水域CDOM光吸收的影响是不可忽略的,以采用0.2μm左右的滤膜过滤水样为宜.

Based on the field investigation and laboratory experiment for CDOM （chromophoric dissolved organic matter） in seawater from Xiamen Bay, some issues in the current CDOM studies were discussed, including blank subtraction, estimation of spectral absorption coefficient and exponential slope coefficient（S） and the effects of different filter membrane on CDOM optical properties. How to identify a theoretical conservative mixing line of S during estuarine mixing process was also explained. In order to eliminate any artifacts on the absorbance spectra of water samples, pre-filtered （0.2μm） purified water was used as a blank. Absorption coefficients should be corrected for the influence of non-CDOM signals. For both seawater and river water samples, the linear regression technique gave consistently lower estimates of S compared with nonlinear regression technique over the same wavelength ranges. When estimated over the different wavelength ranges, S vales decreased as the components of longer wavelengths increased in the wavelength range, especially using the linear regression technique. On the whole, the difference in wavelength range did not prominently influence the S values when estimated by the non-linear regression technique. In this paper, S values were derived from the non-linear regression of the corrected absorption coefficients against wavelength from 300 - 500 nm. It is not only the initial S values of CDOM from river water and seawater endmembers, but also the relative magn/tude of CDOM concentration in each endmember that determine how S will behave during estuarine mixing. The influence of the difference of the filter pore size on the absorbance spectra can not be neglected in the coastal Case Ⅱ water and 0. 2μm filter was recommended.