车载多轴差分吸收光谱探测对流层NO2分布研究

Dectection and distribution of tropospheric NO_2 vertical column density based on mobile multi-axis differential optical absorption spectroscopy

对流层NO2垂直柱浓度在水平分布上具有较大不均匀性,研究对流层NO2分布特征对于研究污染的形成具有重要作用.本文在国内首次采用车载多轴差分吸收光谱技术探测对流层NO2的水平分布,着重研究了基于车载移动平台上的多轴差分吸收光谱技术反演对流层NO2垂直柱浓度的方法.采用低阶多项式拟合扣除夫琅禾费参考谱和平流层对对流层NO2的贡献,反演得到移动平台上对流层NO2垂直柱浓度.结合大气辐射传输模型,通过设置不同气溶胶光学厚度及层高、NO2层高、方位角等对反演误差进行分析,得出对流层NO2垂直柱浓度的总误差小于25%.在合肥开展观测实验,获取观测时间段内合肥市对流层NO2垂直柱浓度的水平分布特征.并将观测结果与OMI卫星过顶数据比对,在洁净和车载观测点较多的像元内,两者结果符合较好;在污染区域,两者结果有一定差别.研究显示,采用车载多轴差分吸收光谱技术能较好的探测区域对流层NO2的分布特征,这对模型验证、卫星校验及研究输送过程具有重要意义.

The distribution of tropospheric NO2 vertical column desity shows a characteristic of inhomogeneity. Such information is important for the study of pollution formation. A horizontal distribution of tropo-spheric NO2 vertical column desity based on mobile MAX-DOAS is studied in this paper, especially for a retrieval method of tropospheric NO2 with mobile MAX-DOAS. Using a low-order polynomial fitting can remove the conbtibuiton of the Frauenhofer and stratosphere, and then the tropospheric NO2 vertical column desity can be detected on the mobile platform. The total tropospheric NO2 error is lower than 25%with the model simulation by setting the different aerosol optical densities, aerosol layer heights, NO2 layer heights and azimuths. The mobile MAX-DOAS system is designed by ourself and the pattern of scanning sequentially is selected for this system. On the other hand, using electronic compass sensors, inclinometer, and software control method, the system can determine the elevation, the azimuth angle drift due to unstability of mobile platform during measurement, as well as the elevation and azimuth angle acquisition exactly, and automatically refer to the north and reduce measurement errors. In addition, the observation of tropospheric NO2 is carried out in Hefei city based on the mobile MAX-DOAS. The horizontal distribution of tropospheric NO2 across Hefei ring expressway and the 2nd ring in Hefei city is obtained during the measurement period. Furthermore, the tropospheric NO2 vertical column density from the mobile DOAS is compared with those from ozone monitoring instrument （OMI）. Three pixels are covered by OMI in Hefei city during the measurement period of mobile MAX-DOAS, reprsenting “clean area”,“more mobile MAX-DOAS data area”and“polluted area”respectivley. A good agreement is found for “clean area” and the pixel including more data of mobile MAX-DOAS with 3.34 x1015 molec/cm2 from mobile MAX-DOAS and 3.00 x 1015 molec/cm2 from OMI for “clean area” as well as 5.10 x 1015 molec/cm2 from mobile MAX-DOAS and 5.60 x 1015 molec/cm2 from OMI for“more mobile MAX-DOAS data area”. While there is a small difference between the two results for polluted area with 9.16x 1015 〈br〉 molec/cm2 from mobile MAX-DOAS and 4.50x 1015 molec/cm2 from OMI. The unsensitivity of OMI to sources near surface may be accounted for by this difference. These results indicate that the mobile MAX-DOAS can well detect the regional distribution of tropospheric trace gas rapidly. This is important for validation of the model and satellite and study of transport process.