Retrieval Of Aerosol Optical Depth In Different Seasons Over Lanzhou Areas Using MODIS Data

The climate and environmental effects of atmospheric aerosol is a hot issue for current atmospheric sciences and related fields. Through ground observation, more accurate optical characteristic information of aerosol can be gained. But this approach only can executed in limited ground stations, and it is hard to get the information of good 3D spatial coverage and spatial distribution. However, the means of satellite remote sensing can make up the insufficiency of ground observations. Based on the depth understanding of the importance of characteristics of aerosol in global change and regional environmental research, the international community launched a series of satellites for earth observation during last century, including EOS, to acquire optical characteristics of the global distribution of aerosol and the seasonal and year change of aerosol direct and indirect radiative forcing. MODIS is a key instrument on the board the Terra (EOS AM) and Aqua (EOS PM) satellites, and Terra MODIS and Aqua MODIS are viewing the entire Earth's surface every 1 to 2 days, and it provides a wealth of data for retrieval of aerosol and so on. NASA has issued aerosol optical depth (AOD) and other optical characteristics products of global distribution for the 10 km resolution, it provides a wealth of information for climate research and regional scale atmospheric pollution studies, but the resolution is far inadequate for particles pollution studies for the urban scale.The Aerosol Optical Depth (AOD) over Lanzhou city and surrounding areas are retrieved by using MODIS data during May 2005 to April 2006 in clear days, and the spatial distribution and seasonal changes of aerosols are analyzed. The results of AOD retrieved are verified by using the ground-based observation results, and the error sources are analyzed. Some retrieval experimental investigations are made with Walthall BRDF model and 2006 spring MODIS data, in which we taken into account the surface BRDF. The main purpose of this work is to accumulate some experience and conduct some exploration for further study of the nature of aerosol in northwest China. The main results are as follows:1. The results of the retrieved AOD indicate that: 1) normally, there are two AOD great value zones over Lanzhou city, located over Xigu district and Chengguan district separately. In some cases, a great value zone also will appear over Qilihe district. And the maximum AOD usually appears over Xigu district, it is consistent with the actual distribution of polluting sources. In spring, the AOD are vulnerable impacted by the weather of dust;from the retrieval results over more large range, the AOD over Lanzhou city is obviously much larger than the surrounding areas.2. From April 2005 to April 2006, we had a sustained ground observation by a multiband solar photometer at the main Campus of Lanzhou University. The results ofthe AOD at 550nm wavelength show: from the end of April 2005, the AOD begins to decrease, during May to September 2005, the value of AOD is relatively smaller;and from October 2005, the AOD began to increase, and till early April 2006, the AOD was relatively larger. The value of AOD is the largest during the end of November 2005 to December 2005. The air pollution over Lanzhou city is the most serious in winter;in spring (March-April), to a large extent, the AOD is vulnerable impacted by the weather of dust.3. There are some difference between the retrieval results of the AOD and the results of ground observation. The differences in summer and Winner are larger, and are smaller in spring and autumn;from mid-March to early April in 2006, the value of the, difference is the lest. It is the best period for AOD retrieving over Lanzhou city during a year.4. The results of experiment for retrieving AOD with BRDF correction indicate: 1) the Bidirectional reflecting property has a certain impact on the estimation of surface reflectance;2) The results of retrieved AOD after BRDF correction is compared with the results without BRDF correction, it shows that the average relative error of AOD over the main campus of Lanzhou University increases by 6.11%, and the average relative error of AOD over Yuzhong Campus reduces by 6.24%. In a large range, the retrieved results are improved after BRDF correction. 3) The improved effect for surface reflectance with Walthall BRDF model for homogeneous surface region is better than that for urban area.