Retrieval Of Aerosol Optical Depth And Total Surface Radiative Forcing Over Lanzhou Areas

The retrieval of aerosol optical depth (AOD) over Lanzhou areas and the corresponding aerosol radiative effect were studied in this paper. The AOD of 11 days over Lanzhou areas within 10,000 km~2 was retrieved by using the Moderate Resolution Imaging Spectroradiometer (MODIS) data, Kaufman's extended dark target method and the Second Simulation of Satellite Signal in the Solar Spectrum (6S) model. The retrieval result was compared with the ground-based measurement result, and the error was analyzed. Based on the retrieved AOD results, total surface radiation fluxes were simulated by 6S. First of all, the total surface radiation fluxes at different times in the 11 days at two sites, one in the main campus of Lanzhou University in Lanzhou downtown and the other in the Yuzhong campus of Lanzhou University located in rural areas, were simulated and compared with the measured results respectively. Then, the total surface radiative forcing was simulated. Through the research of this paper, more experiences for retrieving AOD in northwest China were accumulated, and an new effort to investigate the aerosol radiative forcing was made.(1) In 10 of the 11 days, the errors of the retrieved AOD were reasonable compared with ground-based measurements, in only one day the error was a little bigger. The mean AOD value of the 11 days at the main campus site measured by photometer was 0.355, and the retrieved one was 0.366. The mean relative error was 3.1%.(2) According to the distribution of AOD at 550nm in 11 days in Lanzhou city, there was always a relatively bigger center of AOD. In Linxia city and some other towns, there were AOD bigger centers. The retrieved results over the surface of water and snow were not reasonable.(3) The diurnal variation trends of the total surface radiation fluxes simulated at two sites, main campus and Yuzhong campus, were consistent with the measurements, and the simulation error was small.(4) The AOD of main campus was bigger than that in Yuzhong campus at different times in a day, and the corresponding total surface radiation flux was on the contrary. During a day, the maximal absolute value of the difference is 112.7 W/m2, and the minimum is 10.8 W/m .(5) The distribution of total surface radiative forcing was consistent with the distribution of AOD at 550 nm. The bigger centers of AOD were also the bigger centers of total surface radiative forcing. The total surface radiative forcing was negative. The maximal absolute value of total surface radiative forcing during the 11 days at the main campus was 92.0 W/m2, it was reduced by 10.6% compared with that there was no aerosol. The minimum was 51.6 W/m2, it was reduced by 5.5%. The simulated total surface radiation flux was well consistent with the measured one in four days.