Dectection and distribution of tropospheric NO2 vertical column density based on mobile multi-axis differential optical absorption spectroscopy

Abstract

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 tropospheric 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 3.34×1015 molec/cm2 from mobile MAX-DOAS and 3.00×1015 molec/cm2 from OMI for “clean area” as well as 5.10×1015 molec/cm2 from mobile MAX-DOAS and 5.60×1015 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 9.16×1015 molec/cm2 from mobile MAX-DOAS and 4.50×1015 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.