多轴差分吸收光谱技术测量NO2对流层垂直分布及垂直柱浓度

摘要

The inversion method of the vertical profile and vertical column density (VCD) of tropospheric NO2 using multi-axis differential optical absorption spectroscopy (MAX-DOAS) is investigated in this paper. An inversion method of two-step procedure is operated. In this method firstly the aerosol vertical profile is retrieved. Then the vertical distribution of trace gases is retrieved based on the corresponding aerosol status. Nonlinear optimal estimation algorithm is extended to acquire NO2 profile to reduce the dependence of the inversion on priori information. It is more advantageous to automatically obtain trace gases profile. At first we investigate how to calculate some parameters (weighting function, the covariance matrices of measurement, and a priori information) of the algorithm and design nonlinear iteration strategy suited to the region where NO2 vertical distribution usually shows rapid variation. Then this inversion algorithm is verified by computer simulation in the cases of box profile and elevated profile of NO2. It is indicated that the distribution of NO2 below 2 km could be well rebuilt and the retrieval accuracy of surface-near NO2 volume mixing ratio is 0.6%. The study of how accurately this algorithm can rebuild the same true profile in three aerosol status of low aerosol, high aerosol and elevated aerosol indicates that similar retrieval results could be acquired. In addition, the effect of wrong aerosol status on the retrieving of NO2 profile and the error sources of this algorithm are analyzed. After that a continuous observation is reported in the city of Hefei. NO2 VCDs derived from MAX-DOAS are compared with those from satellite observations, and the correlation coefficient is 0.85. The surface-near NO2 concentrations measured by MAX-DOAS are compared with those from LP-DOAS, and the correlation coefficient is 0.76. In addition, the simplified MAX-DOAS inversion method of obtaining the trace gas profile usually uses invariable typical aerosol status as input. The comparison with the tropospheric NO2 VCD from simplified method indicates that the using of invariable typical aerosol status would cause large deviation of NO2 VCD, and its maximum relative deviation is about 112%. So exactly acquiring aerosol status, aerosol optical density especially, is necessary to exactly retrieve tropospheric NO2 vertical column density.%研究了多轴差分吸收光谱技术(MAX-DOAS)的对流层NO2垂直廓线及垂直柱浓度反演方法.该方法采用了先反演气溶胶廓线，然后在此基础上反演痕量气体垂直分布的两步反演方法.其中痕量气体廓线反演时采用了非线性最优估算法，使反演更少地依赖于先验信息，更有利于自动获取痕量气体廓线.首先研究了应用非线性最优估算法的痕量气体垂直廓线反演算法中权重函数、先验廓线及其协方差矩阵的计算方法，设计了适合于痕量气体垂直分布变化剧烈地区的迭代方案.通过计算机仿真，研究了算法重建盒子型和抬高型NO2廓线的效果，研究表明两种典型分布下算法都可以较好地重建2 km以下的NO2分布，在近地面的反演精度达到0.6%.然后在低气溶胶、高气溶胶和抬高型气溶胶三种典型条件下，研究了算法重建同一NO2廓线的效果，研究表明不同气溶胶条件下反演算法都可以得到相似的结果.分析了错误的气溶胶状态对于NO2廓线反演的影响以及反演算法的误差来源.在合肥地区开展连续观测实验，并将观测的NO2垂直柱浓度与卫星对比，相关性系数达到了0.85.将MAX-DOAS反演的近地面NO2浓度与长程DOAS结果对比，相关性系数达到0.76.此外简化的MAX-DOAS痕量气体垂直柱浓度反演方法中常采用固定典型的气溶胶状态，将两步法结果与简化方法结果进行对比，两者的最大相对偏差为112%.因此准确获取气溶胶状态，尤其是气溶胶光学厚度，对准确反演对流层NO2垂直柱浓度十分必要.