川西高原河流水体CDOM的光化学降解特性

川西高原部分河流溶解性有机碳(DOC)含量极高,因有色溶解性有机质(CDOM)具有独特的光学特性,其光降解特征与规律对于分析高寒区天然水体DOC动态及区域水-陆碳循环具有重要的环境意义.本文选取川西高原两种主要地貌类型的河流共5条,即高山峡谷区河流杂谷脑河、抚边河及岷江,丘状高原区河流白河、黑河,对河流水体CDOM三维荧光光谱(EEMs)与紫外-可见光吸收光谱(UV-Vis)采用平行因子分析(PARAFAC)与二维相关光谱(2D-COS)进行分析.结果表明:①白河和黑河CDOM的5 d降解率分别为64. 85%和63. 43%,光降解速率常数分别为0. 167 d-1和0. 173 d-1;抚边河CDOM浓度低(0. 71 m-1)光降解现象不明显,杂谷脑河和岷江CDOM光降解行为较为复杂;②在光降解过程中,除岷江外其余4条河流荧光溶解性有机质(FDOM)内源特征(FI)逐渐减弱,黑河和白河CDOM芳香性、疏水性特征(SUVA254和SUVA260)及腐殖化程度(HIXa)逐渐降低;③川西高原5条河流中FDOM组分均呈2类4个组分,即C1(275/310 nm,类酪氨酸)、C2 [280(250)/400 nm,UVA类腐殖质]和C3(255/440 nm,UVA类腐殖质)、C4[270(360)/492 nm,UVA类腐殖质],类腐殖质FDOM较易光降解;④5条河流中UVA类腐殖质FDOM(尤其是500 nm发射波段)光降解反应先于类酪氨酸物质;丘状高原河流中UVA类腐殖质FDOM对于光照的敏感度大于高山峡谷区河流;⑤白河的主成分分析中识别出了2个因子,解释了这些参数变化的87. 28%,反映了光降解过程对于CDOM特征、荧光组分等方面的影响.

Characteristics of Chromophoric Dissolved Organic Matter (CDOM) in Natural Rivers of Western Sichuan Plateau

The concentration of dissolved organic carbon (DOC) in some rivers in the western Sichuan Plateau is extremely high. Due to the unique optical properties of colored dissolved organic matter (CDOM), a thorough understanding of the photodegradation characteristics of CDOM helps in revealing regional DOC dynamics and carbon cycle at the water-land interface in alpine natural waters. Surface water samples were collected from three rivers (the upper reaches of the Minjiang, Zagunao, and Fubian rivers) in the alpine-gorge region, and the Baihe and Heihe rivers in the plateau planation surface distributed among the watersheds in the western Sichuan plateau, southwest China. Ultraviolet-visible spectra(UV-vis) absorbance and excitation emission matrix (EEM) fluorescence spectroscopy with two-dimensional correlation spectrum (2D-COS) and principal component analysis (PCA) were used to characterize the environmental behavior responses to light irradiation. The first-order decay constants and photodegradation rates of the Baihe and Heihe rivers are 0.167 d^-1,0.173 d^-1 and 64.85%,63.43%, respectively. Due to the low concentration (0.71 m^-1) of the Fubian River CDOM, photodegradation behavior is limited. The photodegradation behavior of CDOM in the Zaogunao and Minjiang rivers is relatively complicated. Exposure to sunlight led to decreases in the chromophores and changes in origin of CDOM from the four rivers, except for the Minjiang River. The aromaticity, hydrophobicity, and humification degree of CDOM in the rivers from the plateau planation surface were decreased with increasing exposure time. PARAFAC produced a four-component model:C2[280(<250)/400 nm], C3 (255/440 nm), and C4[270(360)/492 nm] represented terrestrial UVA humic-like fluorophores, and C1 (275/310nm) belonged to a tyrosine-like substance, as humic-like FDOM was found to be more readily photodegraded. 2D-COS indicated that UVA humic-like FDOM showed a higher susceptibility in the rivers from plateau planation surface. The preferential removal of UVA humic-like FDOM (especially at 500 nm emission wavelength), and delayed response of tyrosine-like fluorescence, were revealed from the five rivers upon irradiation. Two factors were identified in the principal component analysis of the Baihe River, explaining 87.28% of these parameters, which comprehensively reflected the effects of the photodegradation process on CDOM characteristics and fluorescent components.