| dc.description.abstract | Dust Storm from Mainland China brings a great amount of aerosols to degrade air quality, reduce visibility, and modify aerosol chemical properties. From time to time, the most frequent occurrence of dust storm is from March to May each year. To investigate aerosol optical property, size distribution, and chemical properties at normal days and dust storm event, this study collected atmospheric aerosols continuously at Shi-Men site in Taipei County for 45 days starting from March to May in 2001.
The results show the light scattering coefficient (ssp) and size distribution of aerosols are related to local wind direction. For predominant airflow from the sea, aerosol volume is mainly contributed from super-micrometer particles with an average ssp at 0.083km-1. As the wind shifted from the sea to from the land, aerosol volume is turned to the control of sub-micron particles having an average ssp at 0.101km-1. In addition, aerosol ssp, number concentration, and volume concentration are distinguished between holidays and non-holidays.
During the intensive study period, Taiwan Environmental Protection Administration announced three dust storm events. The average ssp during the dust storm event was 0.344km-1 in March, 0.128km-1 in April, and 0.168km-1 in May. Although the dust storm in April was the strongest, the aerosol ssp was not correspondingly high which was due to the majority existence of low scattering efficiency super-micron particles.
Owing to its various chemical compositions, atmospheric aerosol is capable of both scattering (cooling) and absorbing (heating) solar radiation. The cooling effect is related to the backscattering of radiation to the upper atmosphere. A critical value of aerosol albedo (defined as the ratio of aerosol light-scattering coefficient to extinction coefficient) to have negative radiative forcing (and thus cooling) is found at 0.85. For the historical data and date from this study, the average albedo was calculated at 0.87 in Kaohsiung City, 0.82 in Taipei, and 0.92 at Shi-Men, respectively. Consequently, a radiative cooling by the atmospheric aerosol in Kaohsiung City and Shi-Men is evidentable. | en_US |