典型农田大气活性气态汞的干沉降速率研究

The wet and dry deposition is the major path for the atmospheric mercury (Hg) to the earth’s surface. Recent studies have revealed that approximately 60% of atmospheric Hg is from ‘re-emissions’ of previously released Hg that has built up in surface soils and oceans, and the dry deposition flux of atmospheric Hg is comparable to or higher than that of wet deposition flux, and reactive gaseous mercury (RGM) plays an important role in the dry deposition of atmospheric Hg. However, How to accurately determine the dry deposition velocity of RGM is one of the hotspots and difficulties. In order to determine the dry deposition velocity of RGM over cropland, we plan to observe the atmospheric RGM, nitric acid, meteorological and vegetation parameters at Luancheng Agro-Ecosystem Experimental Station (Chines Academy of Sciences), and then calculate the dry deposition velocity of RGM based on the resistance model. In addition, we will also calculate the dry deposition velocity of gaseous nitric acid, which can provide some reference for the study of the dry deposition velocity of RGM. The objectives are to summarize and analyze the seasonal and diurnal variation of RGM dry deposition velocity, and to explore the key factors that influencing the RGM deposition velocity. The results not only can improve our knowledge on the dry deposition of RGM over cropland but also can provide scientific evidence for the dry deposition models of RGM on regional scale.

干湿沉降是大气汞迁移至地表的主要过程。最新研究表明，大气中有60%左右的汞是先前通过沉降方式进入地表又重新释放到大气中的汞，在某些地区大气汞的干沉降通量和湿沉降通量相当甚至高于湿沉降通量，活性气态汞是大气汞干沉降的重要因素。如何更精准测定大气活性气态汞的干沉降速率是目前研究的热点和难点。为测定农田大气活性气态汞的干沉降速率，本研究拟以“栾城站（典型农田）”为现场试验点，通过对活性气态汞、硝酸气体、气象参数和植被参数等的长时间序列观测，采用大气污染物干沉降阻抗模型计算出活性气态汞的干沉降速率；通过计算硝酸气体的干沉降速率为确定农田大气活性气态汞的干沉降速率提供参考；总结分析活性气态汞干沉降速率的日变化和季节变化特征，揭示关键参数对活性气态汞干沉降速率的影响。本研究一方面可丰富对农田活性气态汞干沉降的认识，另一方面可为区域大气活性气态汞干沉降模型提供参数及科学依据。

我们首先在华北平原典型农田站点（即栾城站）开展了四个季节的大气形态汞及气象数据的观测，从而探究他们的季节变化特征、大气汞的来源以及活性气态汞（GOM）的干沉降速度和速率。结果表明：冬季气态元素汞（GEM）的浓度（3.66±2.62 ng m-3）是北半球平均值的两倍左右，也显著高于其它季节的浓度；颗粒态汞的浓度（92.4±59.6 pg m-3）和一些城市地区的浓度接近，但GOM的浓度（5.7±5.6 pg m-3）显著低于城市地区的浓度；整体上看，GEM和HgP2.5均表现出明显的季节变化特征（冬>春>秋>夏），而GOM正好相反。模型模拟的结果显示：大气中的汞主要来自观测点及周边区域（如河北南部山东西部）的排放而不是长距离传输。GOM干沉降速度的季节变化特征不明显，春、夏、秋、冬四个季节Vd的平均值分别为0.88±0.44、0.65±0.40、0.88±0.50、0.72±0.37 cm s-1，春秋季的平均值略大于夏季和冬季的值。而GOM的干沉降速率却表现胡了明显的季节变化特征，春季的值（299.6±351.4 pg m-2 h-1）是其它季节平均值（126.8±121.7-162.4±159.1 pg m-2 h-1）的两倍左右。白天GOM的平均干沉降速度均是对应季节夜间值的1.5-2.0倍；另外，白天GOM的干沉降速率显著大于夜间的干沉降速率，这与白天GOM的浓度较高有关。