黄河冬、夏季节颗粒态有机碳和黑碳变化特征、来源及入海通量

Variations, sources and transport f luxes of particulate organic carbon and black carbon in the Yellow River during winter and summer

关键词:  黄河  有机碳  黑碳  季节性变化  通量

Key words:  the Yellow River  organic carbon  black carbon  seasonal variations  f luxes

摘要:

河流是连接陆地和海洋两大碳库的重要通道，伴随河流输运迁移的颗粒态有机碳和黑碳在全球碳循环中发挥重要作用。2021—2022年冬季和夏季在黄河下游利津水文站进行高频率水体样品采集，分析水体悬浮物中总悬浮颗粒物（TSS）、颗粒态有机碳（POC）、颗粒态黑碳（PBC）以及黑碳的两种亚型焦炭（char）和烟炱（soot）的浓度，研究它们的季节变化特征和来源，并对入海通量进行估算。结果表明：在采样阶段，黄河下游利津站的TSS、POC、PBC和char、soot浓度呈现较为一致的变化趋势，受调水调沙工程和汛期的影响，它们的季节差异明显。黄河悬浮物中PBC在POC中的占比高于世界河流平均值，且冬季PBC占比高于夏季，这可能与我国北方地区冬季人类活动排放更多的黑碳有关。Char/soot比值显示PBC可能主要来源于流域内的化石燃料燃烧。经估算，2021年11月至2022年10月黄河POC、PBC、char、soot输入渤海的通量分别约为2.55×10¹¹、7.04×10¹⁰、4.95×10¹⁰、2.09×10¹⁰ g，且呈现明显的季节变化。

Abstract:

Background, aim, and scope Rivers are important channels connecting land and marine carbon pools. Particulate organic carbon (POC) and black carbon (PBC) in riverine systems play important roles in the global carbon cycle. Dynamic changes in such carbon systems have an important impact on the global carbon cycle and carbon budget, reflecting not only natural and human activities in river basins but also, as the main source of particulate and dissolved organic matter in the ocean, significantly affecting the biogeochemical cycles of marginal seas. In this study, suspended particulate matter was collected from the Lijin Hydrology Station on the Yellow River for study of variations in characteristics, sources, and fluxes of POC and PBC. Materials and methods A total of 39 samples was collected from the Lijin section of the Yellow River during the winter and summer of 2021—2022 (December 2021—March 2022, June 2022—September 2022). Suspended particulate matter samples were analyzed using the thermal/optical reflectance method after drying and grinding, and concentrations of POC, PBC, char, and soot were determined. Results Concentrations of total suspended solids (TSS), POC, PBC, char, and soot displayed similar seasonal trends during the sampling period, with average concentrations in summer being around 5.2, 5.9, 5.3, 5.9, and 4.3 times higher, respectively, than those in winter due to the influence of water and sediment regulation and the flood season. PBC/POC ratios at Lijin Station displayed distinct seasonal differences, with an average of 0.31 in winter and 0.26 in summer. Char/soot ratios displayed no notable seasonal variations, with a range of 1.02—3.36 (average 2.04). It was estimated that POC, PBC, char, and soot fluxes from the Yellow River to the Bohai Sea during November 2021 to October 2022 were 2.55×10¹¹, 7.04×10¹⁰, 4.95×10¹⁰, and 2.09×10¹⁰ g, with summer fluxes being 10.95, 9.71, 10.56, and 8.16 times those of winter, respectively. Discussion During the study period, concentrations of TSS, POC, PBC, char, and soot at the Lijin Station on the Yellow River exhibited clear seasonal patterns, with higher concentrations in summer than in winter. The different concentrations were most likely due to water and sediment regulation and increased precipitation in the basin during summer, leading to increased surface runoff and intense erosion in coastal areas. PBC/POC ratios in the Yellow River were higher than the global average, indicating an above-average effect of human activities in the basin. The ratios also displayed seasonal variations, with higher values in winter than summer, possibly because of higher BC emissions in winter. The primary source of PBC in the Yellow River is the combustion of fossil fuels. Incomplete combustion by motor vehicles and coal-fired power plants in the basin leads to the release of BC to the river, where it is transported by river scour and atmospheric precipitation, resulting in lower char/soot ratios. Conclusions Our results for the lower reaches of the Yellow River indicate that concentrations of TSS, POC, PBC, char, and soot are similar, with significant seasonal variations showing higher concentrations in summer than in winter. It is likely that PBC in the Yellow River is sourced mainly from fossil-fuel burning within the basin, which is greater in winter than in summer due to increased human activity. During 2021—2022, the POC flux of the Yellow River to the sea was slightly lower than that of previous studies, whereas the PBC flux was much higher. Recommendations and perspectives Further study of POC and PBC fluxes from the Yellow River to the Bohai Sea over a longer period and with higher sampling frequency would provide valuable data for the evaluation of spatial patterns and the driving factors of river carbon fluxes in China and would improve our understanding of POC and PBC “source-sink” processes and geochemical behavior in the marginal sea area of the continental shelf.