A New Method for Ground-Based Sun Photometer Radiometric Calibration Using AERONET Data inside the Same City

The ground-based hyperspectral radiometer DMAX-DOAS, developed by the Space Science and Application Research Center of the Chinese Academy of Sciences, has dual functions of multi axis differential absorption spectrometry and hyperspectral direct solar irradiance measurement. The system consists of an optical fiber - prism optical path, can realize the analysis of NO2, SO2 in tropospheric and O3 total column density. It could be used for local pollution gas monitoring and satellite data validation. Regional pollution and transportation measurement could also be realized under reasonable distribution, combined with the wind speed information. The spectral range of the spectrometer is 290-420nm with <0.6 nm resolution. The angle of scattered light includes 0, 5, 10, 15, 20 and 90 degrees and the quantitative level is 14bits.Because of the huge energy of the sun, the radiometric calibration of the direct measurement mode is difficult in the laboratory. In order to validate the laboratory calibration accuracy, whole day solar direct radiation measurement was carried out in Chaoyang district of Beijing Meteorological Bureau (Latitude: 39.95 North, Longitude: 116.51 East) through November, 2016 to February, 2017. The Langley traditional calibration method is used to derive the calibration coefficient of the direct mode. The accuracy of Langley calibration depends strongly on the stability of atmospheric for the whole days. Observation data in the winter Beijing is very difficult to reach the ideal status. The Langley calibration coefficients calculated with different observation date changes obviously. The validation could not achieve by the traditional method.A new method for ground-based sun photometer radiometric calibration using Aerosol Robotic Network (AERONET) data inside the same city is designed. AERONET Aerosol Optic Depth (AOD) product observed in Atmospheric Physics Research (Latitude: 39.97689 North, Longitude: 116.38137 East) in the same period were downloaded. It is the nearest data of AERONET from Chaoyang district of Beijing Meteorological Bureau, within the distance of 15 km. Calibration coefficient of DMAX-DOAS could be calculated according to the difference of AOD between DMAX-DOAS and AERONET.The most difficult problem for the algorithm is that the two instruments are 15 km away, the atmospheric environment maybe different at the same time. The difference between these dynamic changes is unknown. To solve this problem, the concept of the minimum AOD was put forward. Regardless of how the atmosphere was changing by time and space, from the perspective of statistics, the minimum AOD observed by two instrument in the same city should be equal, in the range within a certain space (for example, 15 km), and certain time period (such as a month). Why the maximum AOD can’t work? Because when the clouds are thick, the maximum AOD changes so much that it's difficult to transfer the calibration. According to the idea mentioned above, the radiation calibration coefficient of DMAX-DOAS is recalculated and compared to the laboratory one.The AERONET AOD product is used to validate the accuracy of DMAX-DOAS AOD product by the new calibration coefficient. Influence of dynamic changes of the atmosphere in the time and space range also exists. In order to eliminate this influence, half of day’s stable atmosphere was required and the difference of observation time should be less than 5 minutes. Reason for these criteria is based on: in the same period of time, if the AOD from two instruments which is 15 km away are both stable, have little temporal changes, we could believe that the difference of the two AOD is caused by the uncertainty of calibration and algorithm rather than the atmospheric turbulence. Accordingly, the observation data during 10:00 – 15:00, November 16, 2016 were chosen to validate the accuracy. Results show that the relative error of AOD product between DMAX-DOAS and AERONET is <10%.