基于“质心”迁移的典型城市群氮排放-干沉降输送过程研究

含氮化合物的排放、输送和沉降是生物地球化学氮循环的关键过程,对生态系统和气候变化具有重要影响.其中输送过程的定量化表征一直是困扰沉降来源识别的突出难点之一.为此,本研究采用WRF-EMEP模型模拟2015年我国大气氮干沉降时空分布特征,通过标准差椭圆（SDE）"质心"迁移法定量分析京津冀地区（BTH）、长江三角洲（YRD）和珠江三角洲（PRD）3大典型城市群的氮排放-干沉降输送过程,评估不同区域大气氮干沉降输送过程的季节差异和影响因素.研究结果表明,我国典型城市群氮干沉降通量呈现出散布于城市群周边50~200 km地区的空间分布格局,其中氧化态氮（NO_y）和还原态氮（NH_x）沉降的年平均通量水平分别为8.5 （6.0~22.0） kg ·hm^-2·a^-1和10.2 （6.0~31.0） kg ·hm^-2·a^-1.氧化态氮和还原态氮的输送方向均受大气环流运动主导：在同一城市群,各季节氧化态氮和还原态氮的输送方向保持一致;在不同城市群,二者的输送方向具有季节性差异：京津冀地区氮沉降春秋冬三季多向东及东南方向迁移,夏季向南迁移;受西太平洋副热带反气旋环流影响,长三角地区春夏季氮沉降向西迁移;珠三角地区氮沉降在春夏季往偏北方向迁移,秋冬两季向西南方向迁移.氧化态氮和还原态氮的输送距离受其化学性质主导：在相同气象条件下,氧化态氮传输距离约为还原态氮传输距离的1~2倍,使得氧化氮多沉降在城市群的域外,而还原态氮主要沉降在排放源周边区域.其中,不同城市群氮沉降的输送距离有一定差异,除夏季外,京津冀地区（127~541 km）和长三角地区（108~374 km）的输送距离高于珠三角地区（57~285 km）,其中京津冀地区和珠三角地区沉降主要在域外,即两地的氮排放更易输送到周边地区.因此,在开展氮沉降生态效应相关研究时,应关注周边地区对本地氧化态氮输送的影响.

Nitrogen emission and dry deposition transportation process in typical city clusters based on standard deviation ellipse

It has been known that emission, transportation and deposition of nitrogenous compounds, as the key processes of the biogeochemical nitrogen cycle, show important impacts on the natural ecosystem and climate change. However, the quantitative characterization of the transport process has been one of the obvious difficulties in identifying the source of deposition. This study employed the WRF-EMEP to simulate and assess the dry nitrogen deposition in China for the year of 2015. The standard deviation ellipse (SDE) center migration method was applied to quantitatively analyze the nitrogen emission-dry deposition transportation process in the three typical city clusters, Beijing-Tianjin-Hebei region (BTH), Yangtze River Delta (YRD) and Pearl River Delta (PRD), as well as to evaluate the seasonal differences and influencing factors. The results indicate that the nitrogen dry deposition flux presents a spatial distribution pattern that could be distributed around 50~200 km of city clusters, in which the average annual flux for oxidized nitrogen (NO_y) and reduced nitrogen (NH_x) deposition is estimated to be 8.5 (6.0~22.0) kg ·hm^-2·a^-1 and 10.2 (6.0~31.0) kg ·hm^-2·a^-1, respectively. The transport directions of oxidized nitrogen and reduced nitrogen are dominated by the movement of atmospheric circulation: the transport directions of oxidized nitrogen and reduced nitrogen in each season are consistent in the specific city clusters; while the transport directions show seasonal differences. Nitrogen deposition in the BTH migrates to the east and southeast in spring, autumn and winter, and to the south in summer. Due to the circulation of the subtropical anticyclone in the western Pacific, nitrogen deposition in the YRD migrates to westward in the spring and summer. However, nitrogen deposition in the PRD migrates north in spring and summer and southwest in autumn and winter. Interestingly, the transport distance of oxidized nitrogen and reduced nitrogen is characterized by its chemical properties and emission properties. The transport distance of oxidized nitrogen is about 1~2 times higher than that of reduced nitrogen under the same meteorological conditions, which leads to more oxidized nitrogen deposits in the outside of the city clusters, while the deposition of reduced nitrogen mainly distributed in the surrounding area of the emission source. In general, the transport distance of nitrogen deposition in the BTH (127~541 km) and the YRD (108~374 km) is higher than that in the PRD (57~285 km) with an exception in the summer season. In other words, nitrogen emissions from the BTH and the YRD are more easily transported to the surrounding region. Therefore, future research on the ecological effects of nitrogen deposition and the corresponding attention should be given to the transportation of oxidized nitrogen from surrounding areas.