富营养化淡水湖营养盐的大气沉降

【作者】 尹昊；

【导师】 陈莹； 罗胜；

【作者基本信息】 复旦大学 ， 环境工程， 2011， 硕士

【摘要】 太湖是中国第三大淡水湖,总面积36,500 km2,以不到全国0.4%的国土面积创造着约占全国12.5%的国民生产总值,周边地区城市化水平居全国之首,工业发达,粮食产量占全国的3%,淡水鱼业产值也有较高比重。太湖平原气候温和湿润,水网稠密,土壤肥沃,是我国重要的商品粮基地和三大桑蚕基地之一,近30年来由于沿湖地区的经济快速发展,湖水富营养程度日趋严重,导致蓝藻水华持续发生,特别是2007年太湖蓝藻大规模暴发造成几百万无锡市民生活用水困难,成为社会经济发展的巨大障碍。已有研究表明大气沉降是太湖营养盐的重要来源,本研究于2009年7月8日至2009年7月21日,分别用两个中流量总悬浮颗粒物(TSP)和PM2.5采样器以及8级分级采样器,在太湖梅梁湾总共采集53个气溶胶样品,分析了气溶胶质量、可溶性离子和元素的浓度。同步采集了雨水湖水样品进行分析,测定了太湖梅梁湾上空气态氨的浓度,并与邻近大都市上海的氨气数据进行了比较。采用大气颗粒物沉降模式与分级采样器采样结合的方式,较为准确的测定计算营养盐的大气干沉降通量；根据月平均降雨量和雨水样品测定获得氮盐的湿沉降通量,将干湿沉降量之和与太湖营养盐的河道点源污染输入量比较,证实了大气沉降的重要性；利用HYSPLIT (Hybrid Single-Particle Langrangian Integrated Trajectores)气象模式计算气团的后向轨迹,分析了太湖大气沉降营养盐的可能来源；指出了大气干沉降中的营养盐氮磷比对太湖水中氮磷比及浮游微生物生长的可能影响,为减缓太湖富营养化,控制蓝藻水华提供了重要科学依据。具体研究结果如下：大气气溶胶细颗粒中氮盐浓度高,达到1.8μgNm-3,是粗颗粒中含量的2倍以上；但由于粒径大小相差悬殊,颗粒物沉降速率最大可相差4个数量级,导致粗颗粒对于太湖无机氮盐的大气沉降通量起主要贡献,最高可达833μgNm-2d-1,是细颗粒的521倍。初夏时期太湖梅梁湾无机氮的日均干沉降量达34 t,假设有机氮沉降量占总氮沉降量的16%,该地区大气总氮的年均沉降量达8410 t,占到太湖年总氮输入量的29%-38%,成为太湖N的重要来源。无机氮中氨氮(NH4+-N)的比例占到58%,硫酸根(SO42-)和铵根(NH4+)的浓度呈显著正相关,R2为0.94。由后向轨迹分析得知,高浓度的NH4+和陆地气团的传输密切相关,在非农耕季节,外来污染的长程传输可能替代本地的农业排放成为NH4+的主要来源。已有研究指出淡水湖泊的N/P比值超过25时,随着N浓度的升高,附着藻类和浮游藻类都有明显增加,太湖湖水中的N/P比为23左右[1]。我们测得初夏时期太湖梅梁湾上空大气总磷的浓度为(0.0053 ug m-3),日均沉降通量达31.2 ugm-2 d-1,大气沉降物中的N/P比已经超过58,大气沉降一边促使湖水中的N/P比增加,一边增加N的浓度,可能成为诱发蓝藻水华的一个重要因子。更多还原

【Abstract】 Lake Tai is the third largest freshwater lake in China with an area of over 36,500 km-. The lake is a source of drinking water for 5 million people in Wuxi, and contributes to 12.5% of gross domestic product (GDP) of China. The water of Lake Tai is highly eutrophied due to rapid development of both industry and agriculture in the area around the lake in recent 30 years. Cynobacterial bloom has occurred in Lake Tai for many years causing a serious deterioration of water quality, and the intensive bloom led to a public water crisis in May 2007.Atmospheric deposition may be an important source for nutrients entering Lake Tai. We sampled aerosols using two mid-flow samplers (TSP and PM2.5 samplers) and an 8-stage cascade impactor in Meiliang bay of lake Tai between July 8th and 21st, 2009, and collected total 53 aerosol samples for analysis of soluble ions and total elements.A particle deposition model and cascade sample measurements were used to estimate dry deposition fluxes of nutrients to the Lake Tai. Rainwater samples were used for the calculation of wet deposition flux of nitrogen. Air-mass backtrajectories were calculated from the National Oceanic and Atmospheric Administration (NOAA) GDAS data base using HYSPLIT program, and showed the synoptic situation and general sources of air masses sampled at Lake Tai. This study indicated the potential effects of atmospheric dry deposition on the N/P ratio and phytoplankton growth rate in the Lake Tai water, and provided useful information to policy-makers on how to reduce the eutrophication and cynobacterial bloom of the lake.High concentrations of dissolved inorganic nitrogen (DIN) were observed in fine particles with an average of 1.8μg m-3, about twice as much as that in coarse particles. However coarse particles had much higher deposition velocities compared to fine particles, and the maximum difference could be 4 orders of magnitude. Thus coarse particles were the main contributor to the dry deposition flux of DIN with the highest flux of 833μg N m-2 d-1, over 500 times than fine particles. The average daily dry flux of DIN was about 34 ton with 58% of NH4+-N over Lake Tai in July 2009. Assuming deposition flux of dissolved organic nitrogen (DON) was 16% of the atmospheric flux of total nitrogen (TN), the average annual atmospheric flux of TN would be 8410 t, which accounted for 29%-38% of total nitrogen input to the Lake Tai and became an important source of nitrogen to the lake. There was significantly positive relationship between NH4+and SO42- (R2=0.94). High concentrations of NH4+may be derived from the transport of non-local sources according to air mass backtrajectories. Local agricultural activities were not a significant source of NH3 during our sampling period.The average ratio of DIN to total phosphorus was approximately 58 in the aerosols sampled over Lake Tai, which was higher than the N/P ratio of 23 in the lake water. It was suggested that the increasing nitrogen may lead to an increase of phytoplankton growth in a freshwater lake as the water N/P ratio is greater than 25[1]. Atmospheric dry deposition not only adds DIN but also increases the N/P ratio towards 25 in Lake Tai, which may be one of important factors for inducing cynobacterial bloom.更多还原