Retrieval Of Aerosol Optical Depth Over Urban Area Using MODIS Data

Atmospheric aerosols play an important role in the Earth's radiation balance and global climate change, aerosol optical thickness is one of the most important parameters which can not only reflect the characterization of atmospheric turbidity, but also identify the climate effects of aerosols. So the precise detection of aerosol optical thickness is very important for the study of the climate change. The satellite remote sensing retrieval method has much more advantages in comparing with the traditional detection, such as wide area coverage, convenient accessing to information, fast detection etc. It can also be more efficient in accessing information of atmospheric aerosols. The traditional detection method can only access to the point of space data, but it can't reflect the large area of the spatial and temporal distribution of aerosols, so it is possible for people to understand the regional changes of aerosol in real time over large areas.In this paper, many domestic and foreign remote sensing aerosol optical thickness retrieval algorithms have been systematic summarized, at last the extended dense dark vegetation algorithm is selected to retrieval the aerosol optical thickness using 6S radiative transfer model based on Terra/MODIS data over the Lanzhou city and its surrounding areas in four different seasons. We can know the distribution of trend expansion from the maps, in most of the cases, there are two high-value areas of aerosol optical thickness which are located in Xigu and Chengguan district, Qilihe district will also appear high-value areas in some cases, and the aerosol optical thickness maximum value usually appears in Xigu district, which is consistent with the actual distribution of sources.The dust aerosol optical thickness of spring is influenced by weather over Lanzhou city and its surrounding areas. The urban area of the aerosol optical thickness is significantly larger than the surrounding areas from a wide range of retrieval results. Because of sand and dust weather or high winds, the aerosol optical thickness changes in the spatial distribution of the larger spring is not entirely due to the end of heating period, together with the industrial production of emissions of air pollutants and the effects of dust particles, aerosols by natural sources and anthropogenic effects of complex composition, so air pollution is serious.The summer aerosol optical thickness distribution is relatively stable, mainly including the industrial emissions which shows that the main pollution characteristics.From September to October in autumn, due to the harvesting of crops, the increase in bare soil, the dust aerosol components of natural increase, combined with the gradual approach of winter, the city is also a gradual increase in soot emissions, the aerosols vertical thickness increased, the quality of the atmosphere declined. By the end of October, urban aerosol optical thickness is beginning to increase because of winter heating,.The results showed that the air pollution was very serious for one year in the winter in Lanzhou City, because of the increasing in emissions of air pollutants and the spread of adverse conditions and other reasons.From May 2005 to April 2006, the ground-based observations were carried out in Lanzhou University by the multi-band sun photometer at the band of 550 nm, the aerosol optical thickness is beginning to reduce by the end of April 2005, and it maintain a relatively small state from May to September. The aerosol optical thickness is beginning to increase in October, until early April 2006, aerosol optical thickness is relatively large, it achieved in the largest from November 2005 to December 2005, and it is corresponding to winter air pollution in Lanzhou City which is the most serious period. The aerosol optical thickness is reflected by the dust weather in 2006 spring (March-April).