MAX-DOAS重建NO_(2)竖直平面分布的方法研究

Study on the Reconstructing the NO_(2)Gas Distribution in a Vertical Plane Using MAX-DOAS

多轴差分吸收光谱仪(MAX-DOAS)结合计算机断层重建算法可获取目标痕量气体的空间分布情况。为研究在具有背景浓度的条件下,如城市背景下某个竖直截面上重建NO_(2)空间分布的可行性,设计了气体浓度可控条件下的验证性实验;证明了利用MAX-DOAS在竖直平面重建NO_(2)气体分布的可行性。将充入标准气体的JGS1石英玻璃样品池作为研究对象,使用两台MAX-DOAS采集光谱数据。将气体浓度的梯度作为先验信息,利用经典的ABOCS算法和Barzilai-Borwein算法重建了竖直平面内的NO_(2)气体分布,验证了利用MAX-DOAS在竖直平面内重建NO_(2)气体空间分布的可行性,同时确定了背景浓度对重建结果的影响。研究结果表明,以天空为背景的光谱作为参考谱和以空样品池为背景作为参考谱,反演得到的NO_(2)浓度非常接近,因此研究对象中的样品池容器在NO_(2)竖直平面分布重建方法中对实验结果的影响可以忽略。实验中以市区为背景的MAX-DOAS具有较高的背景浓度,特别是在仰角较低的情况下NO_(2)背景浓度几乎达到6×10^(16)molec·cm^(-2),以城市郊区没有明显的污染源为背景的MAX-DOAS,背景浓度较低可以忽略。重建结果显示,当仰角为28°时,气体沿光路的平均分子数密度为3.9327×10^(15)molec·cm^(-2),且在样品池内下部密度大,上部密度小;重建得到的SCD和测量得到的SCD符合比较好,计算结果显示重建得到的气体分子数密度的峰值为5.77×10^(15)molec·cm^(-2),与以城市郊区为背景的MAX-DOAS反演结果较为接近,而以市区为背景时,特别是仰角较小时,NO_(2)背景浓度特别明显,重建结果比测量结果的值小很多。结果表明,背景浓度在重建图像中表现为伪影,影响对气体分布的观察,而如果在重建算法时加入利用样品池内外气体存在浓度突变这一先验信息,能够减轻背景浓度对重建结果造成的影响。

The multi-axis differential absorption spectrometer(MAX-DOAS)combined with computed tomography(CT)reconstruction algorithm can be used to obtain the spatial distribution of the target trace gases.In order to study the feasibility of reconstructing the spatial distribution of NO_(2)on a vertical cross-section under the condition of background concentration(such as the urban background),a confirmatory experiment was designed under the condition of controllable gas density.The feasibility of using MAX-DOAS to reconstruct the distribution of NO_(2)in the vertical plane is proved.The JGS1 quartz glass sample cell filled with standard gas was used as the research object,and two MAX-DOAS were used to collect spectral data.Taking the gradient of gas density as a priori information,and using the classical ABOCS algorithm and the Barzilai-Borwein algorithm,the NO_(2)distribution in the vertical plane is reconstructed.The feasibility of using MAX-DOAS to reconstruct the spatial distribution of NO_(2)in the vertical plane was verified,and the influence of background density on the reconstruction results was determined.The results show that the NO_(2)density obtained by the retrieving is very close to that by using the sky as the reference spectrum and the empty sample pool as the reference spectrum.Therefore,the influence of the sample pool container on the experimental results can be neglected in the reconstruction method of NO_(2)vertical plane distribution.The background density of MAX-DOAS with the urban background was high,especially when the observation angle was low,the background density of NO_(2)was almost 6×10^(16)molec·cm^(-2),the MAX-DOAS,whose background density has no obvious pollution source in the city suburb,is low enough to be ignored.The reconstruction results show that the average molecular number density along the optical path is 3.9327×10^(15)molec·cm^(-2)when the observation angle is 28°,and the density in the lower region of the sample pool is higher than that in the upper region.The reconstructed SCD is in good agreement with the measured SCD.The calculated results show that the peak value of molecular number density is 5.77×10^(15)molec·cm^(-2),which is close to the MAX-DOAS retrieving results with the background of the urban suburbs.However,it is not close to the MAX-DOAS retrieving results with the background of the urban areas,especially when the elevation angle is small,whose background density of NO_(2)is especially obvious,and its reconstructed result is much lower than the measured one.To sum up,background density is an artifact in the reconstructed image,which affects the observation of gas distribution.If the prior information of gas density mutation in and out of the sample pool is added to the reconstruction algorithm,the effect of background density on the reconstruction results can be reduced.