| dc.description.abstract | Measurements of optical properties of ambient aerosol particles were carried out from September 2002 to April 2003 at Shi-Men, Taipei. In order to understand the influence of aerosol size distribution and chemical composition on aerosol optical properties. Light scattering coefficient and aerosol particle number size distribution were made by Nephelometer and PMS, filter samples of aerosols were collected and analyzed for mass and chemical species simultaneously. In order to understand the effect of long-term transport to aerosol optical properties, this study use backward air trajectory and wind direction to categorize the aerosol optical properties into different pollutant. Because of aerosol particles will influence the Earth’s radiative balance and cause climate change, computing the radiative forcing at Shi-Men finally.
The results show that aerosols size between 0.4 and 1?m contributed the light scattering coefficient mainly. Light scattering coefficients will increase following 0.4 to 1?m particle number increasing. Backscatter fraction and total light scattering coefficient have an inverse relationship, in other way, light scattering coefficient increasing as backscatter fraction decreasing. The reason of this is particle number at this active size is sensitive to forward scattering than backscattering, this implies forward scattering increasing more than backscattering. Moreover, backscatter fraction increasing following the decreasing in aerosol size.
The predominant wind direction can be divided into two types, one is the airflow from the sea, and the other is from the land. The airflow from the sea and from the land have the quite same aerosol volume, when the predominant airflow from the sea the aerosol consisted of coarse mode ; on the contrary, as the wind shifted from the sea to the land, the aerosol size distribution shifted to the fine mode; therefore, the wind from the land have highly light scattering coefficient, the mean value and standard deviation are 124±45Mm-1,91±64-1 Mm-1,respectively. With the different sources of air mass, aerosols have different properties, our research categorize the backward air trajectory into 8 types. When Air mass from the coastal of Mainland China, the aerosol consists of fine mode and the predominant chemical species is sulfate, so it can cause the high light scattering coefficient; when air mass from the ocean , their size distribution is characterized by coarse mode, sea salt is the predominant species, having lower light scattering coefficient.
A critical value of aerosol albedo at Shi-Men is 0.86, aerosol single scattering albedo will greater than this value during the Yellow Dust period, having negative radiative forcing(cooling). | en_US |