Aerosol Optical Properties And Its Radiative Effects Based On Satellite Observations In China

This paper investigated a comprehensive evaluation of satellite aerosol products in China, and then explored aerosol optical properties and its direct radiative forcing on the basis of satellite data retrieved by MISR, MODIS and CERES and ground observations derived from the Aerosol RObotic NETwork (AERONET) and the Chinese Sun Hazemeter Network (CSHNET) that included 25 stations distributed across China representing different ecological and geographic regions.Firstly, MISR aerosol optical depth (AOD) products over China were comprehensively validated by the ground observations obtained from CSHNET and AERONET. Furthermore, the spacial and temporal distribution characteristics of aerosol over China and dust aerosol in north china were studied by MISR and MODIS aerosol products, meanwhile, the relationships between AOD and pollutant concentration (PM10) were discussed over four cities of northern China. In final, aerosol optical properties and its radiative effects on the energy budget during two different pollution episode controlled by anthropogenic and dusty aerosol, respectively, were investigated combining the ground and satellite-based data with radiative transfer model. The main conclusions are as follows:1. AOD products retrieved by MISR are suitable for quantitative analysis of aerosol loading over mostly China (except northeast and Lanzhou), especially over bright deserts, which is an outstanding advantage for MISR. MISR tends to systematically underestimate when AOD values are larger than 1. For the different regions, the apparently systematic errors appear in northern, eastern and southwest China; for the different surface types, the MISR AOD values at deserts are very well in agreement with the observations, but smaller than the ground observations at the agriculture and forest; for seasonal variations, the errors are minimal in summer and maximal in winter. The MISR algorithm errors over China might be induced by the choice of aerosol model, for instance, the single scattering albedo values derived from MISR are about 0.96, much higher than those from ground-based observations. It is indispensable to further investigate and revise the aerosol model for China in the MISR algorithm.2. The spatial distributions of AOD retrieved by MISR over China are same as those by MODIS. The high AOD values occur in northern, eastern China and Sichuan basin, especially in northern China. The dust aerosols in north of China show a decreasing trend from 2001 to 2007 in general, and the descendant trend in the west is larger than that in the east. The days of good air quality increase according to the tendencies of AOD and PM10 parameters. Note that the worse air quality go with the increment of the days of higher aerosol loading , which suggests that the coarse aerosol particles might be dominant in the severe air pollution.3. CERES radiation flux products are validated by the simulations of SBDART model and ground observations at Xianghe site. When it is cloudless sky and the observation zenith angles of CERES are smaller than 50 degree. the upward flux products at the top of the atmosphere (TOA) are well agree with the simulations, with the errors of 80 percent of products within the range of±10 Wm^-2, But the downward surface flux products are disagree with in situ observations, requiring a further improvement to the CERES radiation algorithm.4. Using aerosol and radiation data obtained from ground-based and satellite, the dramatic variation of aerosol loading and the significant effects on the energy budget are explored during a heavy pollution episode occurred in northern China. Aerosol optical depth increases gradually from the background level to tens times six days later and to more than 3.0 ten days later. During the peak of the AOD value, the diurnal aerosol direct radiative forcing at the TOA, the surface and within the atmosphere, are -12 Wm^-2, -50 Wm^-2, 38 Wm^-2, respectively. This redistribution of shortwave irradiance induced by aerosols could alter atmospheric stability, thus influencing local wind circulation patterns and cloud dynamics and ultimately, affecting regional climate.5. The dust aerosol properties and its radiative effects in spring 2006 are investigated in northern China. The results show that the scattering capability of aerosol during dust episode from red to infrared wavelength is higher than that during non-dust episode, while at blue wavelength consists with that of anthropogenic aerosol. However, the absorption of aerosol is a half of that during non-dust days. Due to the strong scattering and weak absorption of dust aerosol compared to other aerosols, the more reflection of shortwave radiation into space occurs at the top of the atmosphere, and a weaker warming effect is within the atmosphere, resulting that the overall effect of dust aerosols is towards a cooling of the Earth's system. Owing to the strong absorption of the fine aerosols in non-dust days, the main roles of aerosol radiative forcing are warming interior atmosphere.